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Title: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 19, 2013, 09:44:26 AM
Why does a guitar amp buzz when you're not touching the strings, and why does it stop when you touch the strings or anything metal on the guitar? I'll provide the answer and explanation next week.  8)

Thanks to Al Keltz for telling me about this.  ;)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Steve M Smith on September 19, 2013, 01:30:58 PM
Can we post our own answers here to see how they compare to the 'official' answer?


Steve.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 19, 2013, 01:35:12 PM
You mean like none of the above? Or a little of this and a little of that?



JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 19, 2013, 01:35:54 PM
Can we post our own answers here to see how they compare to the 'official' answer?

Steve.
Yes, please do....
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 19, 2013, 01:41:33 PM
You mean like none of the above? Or a little of this and a little of that?

JR

There's one primary reason for this effect, and certainly a few smaller ones. So please, post whatever mix of this and that you like. Nobody here's going to judge you for a crazy answer. In fact, a lot of my fundamental ideas on power and sound systems seemed a little crazy at first read (RPBG's for instance) but have since been validated by peer review.

No little green Martians, though.... as I'm pretty sure they have nothing to do with guitar buzz. 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Steve M Smith on September 19, 2013, 01:55:13 PM
My thoughts on the subject.

The input to the amplifier consists of the centre core of the lead in series with the pick up which is then in series with the outer shield of the lead.

Any mains hum picked up by any of these will present itself as an input signal to the amplifier.  The central (hot) core is obviously shielded so shouldn't cause a problem.  The pickups might add a bit but are generally o.k.

However,  the outer core of the cable will pick up any noise in the area.  The higher it's resistance (relative to the pickup) ,the worse the problem.

When you touch the strings you are grounding the guitar end.  Hopefully the amp end is already grounded. By grounding the guitar end, you are putting both ends of the shield at the same potential so it no longer forms part of the input signal.

With a good lead this isn't usually a problem as the noise picked up by the shield is generally of a much lower level than the guitar signal but if there is a fault with the ground shield or a high resistance part somewhere e.g. dry joint or oxidisation on a plug to socket contact, its relative level will increase.


Steve.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 19, 2013, 02:34:49 PM
There's one primary reason for this effect, and certainly a few smaller ones. So please, post whatever mix of this and that you like. Nobody here's going to judge you for a crazy answer. In fact, a lot of my fundamental ideas on power and sound systems seemed a little crazy at first read (RPBG's for instance) but have since been validated by peer review.

No little green Martians, though.... as I'm pretty sure they have nothing to do with guitar buzz.
You don't know me if you think I worry about how my answers look.

I see this as a combination of grounding and damping an antenna, but neither is literally precise as we technically define grounds or antennas. The human body appears "ground-like" or loosely coupled to ground just enough, when the noise source is very high impedance. It's all relative so the effective path to ground can be weak and still work to suppress even weaker noise.

Several years ago there was an interesting discussion about using a capacitor to couple the guitar's metal parts to the guitar cable ground (amp's input ground-to chassis-to wall), so that the hum would be suppressed. However if the now "capacitor coupled to ground" musician is exposed to mains voltage, that current must also be small enough that it doesn't disrupt his heart's electrical system.   

I don't recall the magic cap value, but there was a range of values that satisfied both human safety and noise shielding constraints (100-240V, 50-60Hz).

JR

Title: Re: Guitar stops buzzing when I touch the strings
Post by: Scott Helmke on September 19, 2013, 02:55:45 PM
What JR said - grounding/damping the human body so that it doesn't act as an antenna. When grounded to the guitar the body starts acting more like a shield instead.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 19, 2013, 04:57:34 PM
I'm going to stay out of this until we get some more answers to the poll. But somebody is getting very warm... 8)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jonathan Johnson on September 20, 2013, 01:30:53 AM
Since I have nearly no experience with guitar amps, and I've never inspected an electric guitar to see how it's made, I'll throw out a wild guess.

Why does a guitar amp buzz? There is current flowing through the pickups, which causes the strings to vibrate.

Why does it stop when you touch the strings? You stop the strings from vibrating, which creates a damping effect on the pickups.

(Maybe not. I really don't know.)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 20, 2013, 07:22:20 AM
The guitar is an extension of the amplifiers pre-amplifier circuit. The pickup, mainly single coil pickups, exhibit the same functionality and properties as an antenna would, and are susceptible to  noise generated by nearby EMI and RFI sources. Placing your hand on the strings, which are grounded to the guitars circuit through the tailpiece provides a path to ground.

A properly grounded amplifier will not hum, however, single coil pickups will induce noise in the same amplifier.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jerome Malsack on September 20, 2013, 08:14:44 AM
I will say that this was mentioned at the Get Together in Shenandoah University, and I too will allow the conversation to continue.   
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 20, 2013, 08:49:12 AM
Some very thoughtful answers so far, and one that's correct. But the poll is going exactly in the wrong direction. Let me clarify one point. The buzz is happening when you're holding the guitar, but not touching the strings.

The hint is that the cause of the buzz is as close as your belly button.  :-X

I'll post the answer on Monday....
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Steve M Smith on September 20, 2013, 10:03:33 AM
Let me clarify one point. The buzz is happening when you're holding the guitar, but not touching the strings.

Then cancel my previous answer (although it has some merits).

I think in this case your body is picking up the buzz and the coil of the pickup is picking it up from your body.  Grounding your body through the strings dissipates the buzz your body has picked up.


Steve.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 20, 2013, 10:20:05 AM
Then there's hum bucking pickups? But the tone.... :-(

@ Bob are all tailpieces grounded**...? I suspect many are (Peavey's were).  I recall some long involved discussions about cap coupling between guitar ground system and amplifier ground. 

JR

** I suspect most are. The kid who was killed by the mis-wired mains outlet that made one of their guitar amps chassis hot, was exchanging guitars with his friend so touching both guitars at the same time. Those two guitars were both hard connected to the chassis ground putting 120v through the kid. Using a cap inside even one of the two guitars would have saved his life.   

PS: Does it matter if the player is barefoot on a cement floor?
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Scott Wagner on September 20, 2013, 11:40:20 AM
I believe the (single coil) pickup is "picking up" (or reacting to) the potential in your body.  By touching the strings, you are bringing the pickup to the same potential as your body - thereby eliminating the hum.

John, is there something you don't like about a humbucker's tone?  Personally, I love it.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Art Welter on September 20, 2013, 11:42:50 AM

@ Bob are all tailpieces grounded**...? I suspect many are (Peavey's were).  I recall some long involved discussions about cap coupling between guitar ground system and amplifier ground. 

The strings on a steel string electric should be connected to the sleeve of the 1/4", but the connection may break off or become oxidized (and non conductive), in which case the guitar (or bass) will buzz regardless of whether one touches the strings or not.
Sometimes the connection has been burnt off from contact with a microphone which has 120 or 240 volts difference in voltage.
Gotta love that polarity switch on old amps...

The lack of connection between the sleeve and the strings makes the guitar safe from an electrical standpoint, but generally untenable from a noise standpoint.
At any rate, Bob's antenna explanation is good, as anyone with a Stratocaster with a lot of gain can tell you- there is often only one direction you can point the Stratenna where it won't pick up noise, even with your hands on the strings.

Art

Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 20, 2013, 12:34:40 PM
The strings on a steel string electric should be connected to the sleeve of the 1/4", but the connection may break off or become oxidized (and non conductive), in which case the guitar (or bass) will buzz regardless of whether one touches the strings or not.
Sometimes the connection has been burnt off from contact with a microphone which has 120 or 240 volts difference in voltage.
Gotta love that polarity switch on old amps...

The lack of connection between the sleeve and the strings makes the guitar safe from an electrical standpoint, but generally untenable from a noise standpoint.
At any rate, Bob's antenna explanation is good, as anyone with a Stratocaster with a lot of gain can tell you- there is often only one direction you can point the Stratenna where it won't pick up noise, even with your hands on the strings.

Art

As i've already posted there is a range of capacitances that can be used between the cord shield and the guitar ground system that both provides a low enough impedance to shunt noise, while high enough impedance at mains frequency to limit current to below lethal levels.

While a GFCI is more practical to prevent killing the talent. I wouldn't suggest messing around inside a valuable axe.

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 20, 2013, 08:15:22 PM
Although some people may call it the negative side the complete guitar circuit is, as I said above, merely an extension of the amplifiers pre-amplifier circuit. No, your body does not generate an EMF or RF signal large enough to excite the circuit regardless of pickup type. Yes, in order to work, and with the exception of piezo or microphonic pickups, ALL  guitar tail pieces are grounded.

@ Art - It would take a full 20 amp load to melt or burn the circuit wiring used to wire most guitars. Loose maybe, but not melted or burnt unless a direct connection to 120/220v occurs.
 
Fender preamp circuit below where I work most of my magic.
 
Fender death cap. Note center tap for filament voltage. This center tap was eliminated in most models after 1970. The proper mod for eliminating hum is a pair of 100 ohm resistors, one from each side to ground.
 
Can you spot the tailpiece ground wire in the bottom picture?
 

 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 20, 2013, 08:37:42 PM
It would take a full 20 amp load to melt or burn the circuit wiring used to wire most guitars. Loose maybe, but not melted or burnt unless a direct connection to 120/220v occurs.

I saw this at a trade show last year, but never played with one. Taylor makes a "string ground" fuse that's supposed to "blow" before the musician is injured. See https://www.taylorguitars.com/taylorware/Item.aspx?itemid=1275&categoryid=1011&page=1# and the text below.

Hmmmmm..... The cost is $16 for a pair of 10 mA replacements fuses from Taylor. Seems a little pricey. An interesting idea, but I'm sure it would be inconvenient if a fuse blew and you had a humming/buzzing guitar for the rest of the set. I agree that the shunt capacitor could work if sized correctly, but I doubt that many guitar players would let you hack up their ax. There's got to be a million guitars out there with all the metal strings and hardware "grounded" to the shield.
   
===========================

String Ground Replacement Fuses. The fused string ground for Taylor Expression System® electronics and electric guitars features a fuse that’s designed to blow in the event of improper electrical ground, in order to protect the player. If the fuse blows, the guitar’s electronics will continue to work, but the strings will no longer be grounded, and the guitar could be more susceptible to electrical hum, so a replacement should be installed. Replacement fuses are compatible with all Taylor fused string grounds, as well as the Taylor universal string ground, designed for other electric guitar brands. (10mA Replacement Ground Fuses, set of 2, #84925, $16.00)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 21, 2013, 01:01:59 AM
Heard of those fuses but tend to think that by the time the fuse blows the damage will have been done.

The picture showing the guitar electronics is from a 75' - 85' Gibson SG. Note the full brass shield / ground plate the pots, selector switch, and 1/4" jack are mounted to. This was a Norlin mod which has since been dropped by Gibson. Other clues to the age are the ceramic caps, and non Switchcraft jack. The ground wire is the single wire attached to the bridge pickup volume pot (lower left pot in picture), which is routed through the pickup cavities to the tail piece.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 21, 2013, 01:13:21 AM
Here is the R0 (1960 historic) wiring. Note the location of the ground wire and the use of "bumble bee" caps. All of the Gibson historics are accurate including the Switchcraft switches, jacks, and capacitor type.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 21, 2013, 01:14:39 AM
1957 historic. Note the "Black Beauty" caps.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 21, 2013, 01:15:54 AM
1954 historic. Note the green caps.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 21, 2013, 11:57:26 AM
Heard of those fuses but tend to think that by the time the fuse blows the damage will have been done.


Perhaps... My experience with fuses in the context of design is that they are not precision wrt trip current, also the current that causes the fuse to open is not a sharp knee at exactly the trip current, but soft and could blow slightly lower after a long time, and to make it blow quickly depends on how much overload above the trip current for how long.

10mA fuses are expensive because they are not widely used like higher value fuses to prevent fires from overheating, but seem uniquely about human safety. If I was a fuse maker I would be apprehensive about selling such fuses but I guess they have experience and liability insurance.

My recollection was that human safety standards were set down around 3 mA, but apparently GFCI target 5 mA so that is in the same ball park.  I did find a few 5mA fuses on google but they probably make the 10mA fuses look cheap, and only one was in a normal fuse package. I am not even sure how to make a reliable 5-10mA trip current mechanical fuse, but apparently they have. 

OK enough about fuses, would a 10mA fuse protect the talent? Probably yes. From multiple sources, it appears that 10mA is the threshold of painful shock, 50-100mA ventricular fibrillation can occur (disturbed heartbeat that can lead to death), 100-200 fibrillation definitely occurs. So he would definitely feel the shock. !0mA is 1/5th the current where fibrillation "can" occur and 1/10 the current where it "does" occur, so a fair overload margin for fuse action, while there could be a momentary stress. 

One last factor is how does the actual current travel through our body? Our core or innards have significant water content so a pretty good conductor of electricity (say around 100 ohms). While our skin resistance is orders of magnitude larger, up to tens of K or higher for dry skin. The wetter the skin (with salt from sweat) the lower the resistance.  Since electricity follows the path of least resistance, it is highly unlikely that current will harmlessly travel across your skin, but will quickly sink down into your core. The old warning for tube gear technicians to keep one hand in your pocket when poking around a hot chassis has validity since the current path between two hands is likely right through your chest and heart.

FWIW Amp makers could include the equivalent of GFCI built into the preamp input to open up that ground when it detects a few mA of current flowing there but this seems to contradict most conventions about safety ground practice, that want labelled grounds to sink amps long enough to trip a breaker. Such a feature, would only protect if no pedals are involved, and would make a faulty pedal even more dangerous.

JR 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Kevin Maxwell on September 22, 2013, 01:08:50 PM
If you were talking about hum than the answer is very simple.

It hums because it doesn’t know the words. ;-)
 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 22, 2013, 05:44:22 PM
Mike Sokol,

So what is the answer in your book?
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 10:54:15 AM
Mike Sokol,

So what is the answer in your book?

I have a diagram to post later, but according to my logic the answer is The Strings Are Grounding You. When you're holding the guitar but not touching the strings (or anything else grounded) then your own body is ungrounded and picking up all sorts of noise from the environment. This is the same buzzing noise you'll hear if you touch the tip of a 1/4" phone plug that's connected to the input of your guitar amp. Since your belly is close to the back of the guitar, this noise voltage is coupled into the unshielded, high-impedance circuity of the guitar (including the pickups). The reason it's a high-frequency "buzz" and not a low frequency "hum" is that there's high-pass filter circuit formed by the capacitor formed by distance of your "belly" to the internal guitar wiring.

When you touch the guitar strings with your hand, that provides a low-impedance earth-ground path from your body, through the strings/bridge/tailpiece of your guitar, via the shield of your phone plug, to the chassis of the stage amp, then finally through the power cord to the (hopefully) grounded wall outlet. So any of the ambient electrical noise your body was picking up via a capacitive coupling to the wiring in the walls (and more) has now been shorted to ground, and the noise stops.

If you ask guitar players and sound engineers how this works, I would guess that 99% of them think the opposite, that your body is grounding the strings. But being that the strings are always grounded (should be), then that can't be what's happening.

However, I do like the discussion on decoupling caps and 10 mA fuses which could probably stop musicians from being electrocuted. But very few guitars have these features, with the vast majority being wired with strings and all metal pieces being "grounded" to the sleeve of the 1/4" phone jack. I'm sure it's 99.999% that way in the guitar world unless someone knows something I don't know.

What do you think? This all makes perfect sense to me, though I've not personally measured inter-belly/guitar capacitance and calculated the high-pass filter effect. Would be easy enough to do as an experiment and run the calculations though...
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 23, 2013, 11:12:06 AM
A simple observation that reinforces this, we all know the loud hum that occurs if we touch just the tip of a guitar cord plugged into an amp. while if we touch the tip and sleeve at the same time we are far less noisy.

Unless you are standing barefoot on a conductive surface.

JR

PS: I still prefer cap coupling the ground at the guitar, VS a fuse, but either seems like an improvement wrt safety. This could easily be built into a cable end so the guitar doesn't need to be messed with. Wireless mics also reduce the risk of exposing the meat to a hard ground (or hot) via the wired mic.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 11:18:35 AM
PS: I still prefer cap coupling the ground at the guitar, VS a fuse, but either seems like an improvement wrt safety. This could easily be built into a cable end so the guitar doesn't need to be messed with. Wireless mics also reduce the risk of exposing the meat to a hard ground (or hot) via the wired mic.
I agree there could be safer ways to "ground" a guitar without exposing the musician to shock treatment. Also in reference your wireless comment, I've run into a few guitar players who use a belt-pack RF transmitter for just that very reason. They won't play through a standard guitar cord for fear of getting shocked. And you know, I really don't blame them for the paranoia. We sound technicians have done too little to protect our stage musicians. I believe that discussions like this are a great way to educate everyone as to the importance of proper grounding and the dangers of electrical shock on stage. 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 11:44:25 AM

PS: I still prefer cap coupling the ground at the guitar, VS a fuse, but either seems like an improvement wrt safety. This could easily be built into a cable end so the guitar doesn't need to be messed with. Wireless mics also reduce the risk of exposing the meat to a hard ground (or hot) via the wired mic.

Here's another thought. While a 10 mA fuse is a pain to replace if it blows, and a 10 mA circuit breaker is probably very expensive (or next to impossible) to build, how about a simple current sensing circuit consisting of an op-amp and a Normally Open reed relay. For fail-safe use, the relay would be latched in grounding mode by pressing a momentary switch to reset the op-amp. When the 5 or 10 mA danger threshold is reached, the op-amp does it's thing and releases the reed relay which "un-grounds" the guitar strings. It could also turn on a red LED or something similar to show the guitar was in "ungrounded" mode. You could make it come on in "grounded" or "un-grounded" mode depending on which way you feel is more user friendly. 

What do you all think? This is simple op-amp 101 design stuff, and would only cost a couple of bucks to try out. I can draw up the design if someone wants to build one. Could even be put into a project box on the amp side of the power cord so you don't have to hack up an expensive ax.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jason Glass on September 23, 2013, 12:06:34 PM
Since your belly is close to the back of the guitar, this noise voltage is coupled into the unshielded, high-impedance circuity of the guitar (including the pickups). The reason it's a high-frequency "buzz" and not a low frequency "hum" is that there's high-pass filter circuit formed by the capacitor formed by distance of your "belly" to the internal guitar wiring.

Hi Mike,

When my Strat is resting on its stand and I place my hand on the strings near the nut, with my body a couple of feet away from the axe, the buzzing stops and the results are exactly the same as when I'm playing it.  How can we account for that?
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 12:33:46 PM
Hi Mike,

When my Strat is resting on its stand and I place my hand on the strings near the nut, with my body a couple of feet away from the axe, the buzzing stops and the results are exactly the same as when I'm playing it.  How can we account for that?

Strats have single-coil pickups, and make great antennas for any electrical noise in the room. In fact, my one guitar player would have to carefully align himself on stage to find the position of "least hum and buzz" when we were playing close to wiring that fed the lights.

When you touch the guitar strings up near the nut, those strings are indeed grounded to the guitar's output jack. And your body is then grounded and creating a bit of a ground-plane shield near the guitar, which quiets down the noise. If you could wrap yourself entirely around the guitar and ground your own body, you would then form a Faraday Cage, and it would be REALLY quiet. See http://en.wikipedia.org/wiki/Faraday_cage for Faraday Fun...
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jason Glass on September 23, 2013, 01:31:55 PM
Strats have single-coil pickups, and make great antennas for any electrical noise in the room. In fact, my one guitar player would have to carefully align himself on stage to find the position of "least hum and buzz" when we were playing close to wiring that fed the lights.

When you touch the guitar strings up near the nut, those strings are indeed grounded to the guitar's output jack. And your body is then grounded and creating a bit of a ground-plane shield near the guitar, which quiets down the noise. If you could wrap yourself entirely around the guitar and ground your own body, you would then form a Faraday Cage, and it would be REALLY quiet. See http://en.wikipedia.org/wiki/Faraday_cage for Faraday Fun...

Hi Mike,

See my signature.  I know the Faraday cage, antennas, and antenna theories very well.  You are dead-on correct that the system is an antenna, but I must disagree with the ground plane theory as the sole reason at those low frequencies.  As one demonstration, I can walk around the guitar on its stand while touching it and hear no change in the effect as I do so.  Same is true when not touching it.  You got my gears turning, though, and I suspect that the body (salt water bag) is simultaneously acting as a ground plane, inductor, and capacitor, all contributing to detuning the antenna formed by the pickup and cable.  If any low-pass, band-pass, or band-stop filtering occurs in the audio path (inner coaxial conductors), it must be minimal because the tone of the guitar does not change significantly whether touching the strings or not.  However, it doesn't exclude the possibility of making the antenna (mostly outer shield conductors) less sensitive to frequency bands filtered by the body's L-C effect.  What do you think?
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 23, 2013, 01:43:42 PM
Here's another thought. While a 10 mA fuse is a pain to replace if it blows,
Of course less pain than killing the axe player.  :o
Quote
and a 10 mA circuit breaker is probably very expensive (or next to impossible) to build,
Mechanically not a lot of energy available to trip a mechanical breaker, but roughly a half a watt for 5mA @ 120v, 1W @240V (total energy, not energy available to trip a breaker). Not much of a market to justify the development cost to tool a standard component. 
Quote
how about a simple current sensing circuit consisting of an op-amp and a Normally Open reed relay. For fail-safe use, the relay would be latched in grounding mode by pressing a momentary switch to reset the op-amp.
Where do you propose putting this? If this is built into the guitar, or a box between guitar and amp, it will require a battery or power supply to hold the relay closed. If built into the amp, it could make a rouge pedal even more dangerous.

Quote
When the 5 or 10 mA danger threshold is reached, the op-amp does it's thing and releases the reed relay which "un-grounds" the guitar strings. It could also turn on a red LED or something similar to show the guitar was in "ungrounded" mode. You could make it come on in "grounded" or "un-grounded" mode depending on which way you feel is more user friendly. 

What do you all think? This is simple op-amp 101 design stuff, and would only cost a couple of bucks to try out. I can draw up the design if someone wants to build one. Could even be put into a project box on the amp side of the power cord so you don't have to hack up an expensive ax.
Not to encourage you, but perhaps a small shunt resistance and LM339 quad comparator with trip thresholds. Unless you plan to use dual +/- supplies you will want to level shift the current sense voltage up to v/2.

This might make some sense built into a floor pedal, perhaps with a preamp/buffer/DI, since you already have a PS and enclosure, and jacks, and 99% of the cost already spent. As a manufacturer this might open you up for liability should it fail to protect, but it could save a life in the right circumstance. I have a friend who owns a stomp box company, maybe i should ask him what he thinks? I don't expect much interest.

I have given this some thought and while i still like the KISS approach (capacitor** if the shielding is adequate), I can imagine configuring some high voltage mosfet switches in a normally on mode, with a current sense circuit (discrete not opamp) that trips a scr to turn the mosfets off. This could latch off/open as long as the voltage risk is present, and automatically reset when voltage is removed.  I have not reduced this to an actual design (that would resemble circuit design work and I'd have to bill myself), and I am not 100% certain of the MOSFET path ground quality (note any current sense technique involves measuring across a shunt resistance, or worse an active current sink.)

I keep coming back to KISS... A cap or if that doesn't provide enough shielding a fuse, while a 10mA fuse can still sting you since it will typically source more than 10mA in the process of blowing..

JR

*** My back of the envelope math suggests a 0.1uF cap should limit 120v 60Hz  to 5 mA, for 240V 50Hz the cap needs to be smaller, something like 0.068uF.  Just to double check I just tested this with a VOM on the bench. Since I didn't have any 200V caps laying around I wired two .15uF in series (making 0.075uF) and measured 3 mA so we are in the right ball park.  These caps need to be good quality and capable of withstanding not only mains voltage but spikes on the mains. A quick search of Y1 and Y2 (safety agency rated for this kind of application), shows a largest value .022uf (Y2). The risk from using a cheaper, not self healing cap is that a voltage spike could cause the cap to fail as a short.

This is arguably over-engineering since the only time this cap should ever be exposed to any voltage stress is during a mains distribution or guitar amp fault event, but human safety engineering needs to be conservative. If I was a guitar player, and/or serious about this I might source a few 0.022uF Y2 caps and see if they keep the hum under control. They should effectively limit any fault current to sub lethal levels.   

Note: These Y caps are often used between mains and chassis ground so routinely dump AC current into system grounds. Common values are small so probably a fraction of a mA per rack unit when present.       
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 02:02:46 PM
Hi Mike,

See my signature.  I know the Faraday cage, antennas, and antenna theories very well.  You are dead-on correct that the system is an antenna, but I must disagree with the ground plane theory as the sole reason at those low frequencies.  As one demonstration, I can walk around the guitar while touching it and hear no change in the effect as I do so.  Same is true when not touching it.  You got my gears turning, though, and I suspect that the body (salt water bag) is simultaneously acting as a ground plane, inductor, and capacitor, all contributing to detuning the antenna formed by the pickup and cable.  If any low-pass, band-pass, or band-stop filtering occurs in the audio path (inner coaxial conductors), it must be minimal because the tone of the guitar does not change significantly whether touching the strings or not.  However, it doesn't exclude the possibility of making the antenna (mostly outer shield conductors) less sensitive to frequency bands filtered by the body's L-C effect.  What do you think?

Jason, good comments. I'm doing some actual "work" today, but I'll assign some thought time to this tonight when I'm in relaxing mode with a glass of good Bourbon. You might just have some valid points. Do you have a video of this effect to post.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jason Glass on September 23, 2013, 02:17:23 PM
Jason, good comments. I'm doing some actual "work" today, but I'll assign some thought time to this tonight when I'm in relaxing mode with a glass of good Bourbon. You might just have some valid points. Do you have a video of this effect to post.

Hi Mike,

I forgot to mention that the body would also act as a resistor, so a very complex L-R-C network would be more accurate.  The impedance of the antenna could change drastically at 60Hz (and its harmonics) with small changes to L, R, C, and the location where those loads connect to the antenna element.

Sorry, I don't have any videos to post.  I can recommend 4nec2 as a good freeware antenna modeling software, though.  It would be a huge undertaking to model the pickup+cable+human+amplifier antenna system, but you can see the effects I'm describing very easily with a simple 1/4 wave wire whip antenna model.  Just attach a second wire to the main element and mess with its location, length, resistance, etc., and you can see how it all affects the antenna's performance at different frequencies.

Now you've really got my gears turning, because it just occurred to me that modeling pickup+cable+human+amplifier antenna systems is fertile ground for a doctoral thesis!  Any takers out there?
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 02:27:58 PM

*** My back of the envelope math suggests a 0.1uF cap should limit 120v 60Hz  to 5 mA, for 240V 50Hz the cap needs to be smaller, something like 0.068uF.  Just to double check I just tested this with a VOM on the bench. Since I didn't have any 200V caps laying around I wired two .15uF in series (making 0.075uF) and measured 3 mA so we are in the right ball park.  These caps need to be good quality and capable of withstanding not only mains voltage but spikes on the mains. A quick search of Y1 and Y2 (safety agency rated for this kind of application), shows a largest value .022uf (Y2). The risk from using a cheaper, not self healing cap is that a voltage spike could cause the cap to fail as a short.   

Perhaps a 0.1 uF cap in parallel with a 100K Resistor (NOT 10K as I said originally, which was a dumb arithmetic mistake on my part) would do the trick. The 100K resistor would pull the ground plane of the guitar down close to earth potential at low frequencies, while limiting 60-Hz fault current to around 1.2 mA. Then the 0.1 uF cap would effectively act as a ground shunt for any high frequency trash picked up by the guitar or your human body. More to think about...
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Matt Edmonds on September 23, 2013, 02:41:37 PM
I'm not afraid to throw some caps/resistors in my guitars to try out. I had always wondered how safe it was plugging into a tube amp. Now I know. Not as safe as I'd like. And if I'm understanding correctly this just needs to be placed between the ground of the output jack and the internal grounding of the guitar? I've modded the pickups/switches in just about ever guitar I've ever owned. So def not afraid of doing this.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 23, 2013, 02:45:18 PM
Perhaps a 0.1 uF cap in parallel with a 10K Resistor would do the trick. The 10K resistor would pull the ground plane of the guitar down close to earth potential at low frequencies, while limiting 60-Hz fault current to around 1.2 mA. Then the 0.1 uF cap would effectively act as a ground shunt for any high frequency trash picked up by the guitar or your human body. More to think about...

!0k ohm @ 120v is 12 mA (24mA for 240V), well above painful shock and into possible fibrillation region. The crux of this biscuit both shock and nose abatements is all about the impedance "at" 50-60Hz so a shunt resistance is not helpful and mainly useful for discharging static charge build up, so could be several M ohm. There is some noise at higher harmonics of mains frequency but energy is generally lower and caps lower impedance at higher frequency.

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 02:50:03 PM
!0k ohm @ 120v is 12 mA (24mA for 240V), well above painful shock and into possible fibrillation region. The crux of this biscuit both shock and nose abatements is all about the impedance "at" 50-60Hz so a shunt resistance is not helpful and mainly useful for discharging static charge build up, so could be several M ohm. There is some noise at higher harmonics of mains frequency but energy is generally lower and caps lower impedance at higher frequency.

JR

My bad.... That should be a 100K resistor which is 1.2 mA at 120 volts. I forget a zero. Like I said, I'm actually supposed to be working on business stuff today, so I'm not paying attention to the details... Yikes!!!
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 23, 2013, 03:20:49 PM
I'm not afraid to throw some caps/resistors in my guitars to try out. I had always wondered how safe it was plugging into a tube amp. Now I know. Not as safe as I'd like. And if I'm understanding correctly this just needs to be placed between the ground of the output jack and the internal grounding of the guitar? I've modded the pickups/switches in just about ever guitar I've ever owned. So def not afraid of doing this.

This veer is a bit off topic, more like back-line than sound system engineering. As I mentioned this has been discussed at length on another forum.... It took me a while but I found it, discussion was back in 2007.

Any DIY luthiers interesting in experimenting I will provide the like via PM.

From that other thread...
Quote
FDA recognizes IEC 60601-1 for non patient attached equipment. 60601 allows 500 µA max leakage,


I am not sure a cap that small will be an effective shield, but you do not need to pass FDA leakage specs. Note: we are talking about mitigating a potentially lethal fault condition, so anything better is an improvement.

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jonathan Johnson on September 23, 2013, 03:50:44 PM
...if we touch the tip and sleeve at the same time we are far less noisy.

That would explain a lot of guitar players. They aren't touching both the tip AND the sleeve.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 23, 2013, 04:29:09 PM
You guys need to stop looking at the guitar circuit by itself and look at the entire preamp circuit as a whole. Toss out the theory based on capacitive reactance, it doesn't apply. All you're doing is supplying a path to ground when you touch the strings.  In the case of an amplifier, fender for example, the "ground" is more one side of the preamp circuit, the most sensitive circuit in the amplifier.


Does the pickup coil act as an antenna? This HAM say's absolutely, and just like any antenna for any type receiver, if the antenna is grounded to earth the will be little or no reception through the antenna.

why doesn't a humbucker (Gibson) react in the same manner? It does, however the second coil, wound in the opposite direction, "bucks" the hum.

would I put anything else in the guitar circuit? No, a capacitor would change the tone and that is not acceptable.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 04:40:35 PM
No, a capacitor would change the tone and that is not acceptable.

Yup, I think you're right. A cap there would definitely change the tone of the guitar.

So that's a good reason for my reed relay circuit for un-grounding the guitar using a comparator to trip at some specified mA of leakage, say 5 mA like a GFCI. It's not passive and would definitely need power to operate (from a pedal board, perhaps) but I think it would work very nicely.

Just a thought, but I seem to remember latching reed relays from my past. Wonder if 5 mA of AC current could "latch" a relay into "open ground" mode. More to play with.   ;D


Title: Re: Guitar stops buzzing when I touch the strings
Post by: Steve M Smith on September 23, 2013, 04:44:38 PM
Yup, I think you're right. A cap there would definitely change the tone of the guitar.

Where?  Between the bridge/strings and ground?  That won't affect the tone.


Steve.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 05:01:16 PM
Not to encourage you, but perhaps a small shunt resistance and LM339 quad comparator with trip thresholds. Unless you plan to use dual +/- supplies you will want to level shift the current sense voltage up to v/2.

How about a current transformer like this feeding the input a comparator circuit with a latch: http://www.newark.com/jsp/search/productdetail.jsp?SKU=05R6130&CMP=KNC-GPLA&mckv=|pcrid|27090075741|plid|

This one is pretty large at an inch across, but I think a small spool with a few hundred turns of pickup wire might do the trick. The shield/ground wire would feed through it and monitor 60 Hz current without changing the guitar's ground impedance path which would avoid changing the tone. This circuit could be the input box of a pedal board which is powered by a 9-volt DC supply. Since the current transformer would be floating you could relax the plus & minus power supply requirements for the comparator IC by biasing it with a voltage divider. Or that's what I'm seeing in my head. I'll have to look in my IC/Op-Amp cookbook and draw this out to be sure.

BTW: I do have a cool way to vary the ground voltage on a guitar while monitoring the fault current, so we could test to see if this works. I want to be able to say "No guitar players were harmed in the making of this circuit". So please don't go building something and trying it by grabbing a live wire and your guitar at the same time. This is serious electrocution theory.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 23, 2013, 05:47:52 PM
As I have already mentioned this (adding a cap) is not a new idea, in fact it isn't even my idea. I repeated it because IMO it is not a crazy idea.

For any certain about how much it will affect the tone of the guitar (without actually trying it) never mind.

To give any who's mind is still open a factoid about circuit design, components connected in series, can be rearranged into a different order without altering the combined series impedance. This added ground leg cap can be considered equivalent to being placed in series with the hot side of the pick-up. Since most(?) guitar amps use very high input impedance circuits. This is like putting that added cap in series with the existing input cap, which will shift the LF input pole "slightly" higher.

My speculation (guess) is that this could raise the LF pole from 8Hz to 16Hz, or something like that. Not really a problem for a lead guitar (probably not a problem for most bass guitars. Note these numbers are still brown from where I pulled them, but I trust my general judgement on this, and the fact that people report that this modification works.

PM me for a link... if interested in a lot more (old) discussion.  I am starting to repeat myself here.

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 23, 2013, 06:07:31 PM
How about a current transformer like this feeding the input a comparator circuit with a latch: http://www.newark.com/jsp/search/productdetail.jsp?SKU=05R6130&CMP=KNC-GPLA&mckv=|pcrid|27090075741|plid|

This one is pretty large at an inch across, but I think a small spool with a few hundred turns of pickup wire might do the trick. The shield/ground wire would feed through it and monitor 60 Hz current without changing the guitar's ground impedance path which would avoid changing the tone. This circuit could be the input box of a pedal board which is powered by a 9-volt DC supply. Since the current transformer would be floating you could relax the plus & minus power supply requirements for the comparator IC by biasing it with a voltage divider. Or that's what I'm seeing in my head. I'll have to look in my IC/Op-Amp cookbook and draw this out to be sure.

BTW: I do have a cool way to vary the ground voltage on a guitar while monitoring the fault current, so we could test to see if this works. I want to be able to say "No guitar players were harmed in the making of this circuit". So please don't go building something and trying it by grabbing a live wire and your guitar at the same time. This is serious electrocution theory.

Why don't you tell me how well it works, after you build it? For the record wrapping 100 turns around this sense transformer core will not leave the ground path unchanged. Instead you will create a series inductance (just like a transformer winding), instead of a capacitance or resistance.

Yes, there are a number of ways to skin this cat... That not all believe needs skinning.

I still prefer KISS but amuse yourselves. 

JR.

PS: Maybe get your test subjects to sign a damages waiver before live voltage testing on them. The medical 500uA limit is based on perhaps stressing an already weak heart. Even a few mA might kill a walking heart attack victim.   
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 23, 2013, 07:20:39 PM
PS: Maybe get your test subjects to sign a damages waiver before live voltage testing on them. The medical 500uA limit is based on perhaps stressing an already weak heart. Even a few mA might kill a walking heart attack victim.

FYI: I don't use test subjects. I use test meters...

PS: I once taught a seminar where I simply shouted into a mic with the dynamic compressor disabled to show the peak level difference. Well, two of my attendees in the front row grabbed their chest and ran out of the room. I went to see what was wrong and found they both had recent pacemakers implanted with built-in defibrillators. Since my shout caused their hearts to skip a beat, their pacemakers assumed the worse and started shocking them internally to reset their hearts. They said the doctors warned them the sensitivity might need to be tweaked a bit. After that I would always ask if anyone had a pacemaker before doing that demonstration. Talk about scary... You can almost kill somebody without any voltage being involved.  :o
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 24, 2013, 05:00:51 PM
Where?  Between the bridge/strings and ground?  That won't affect the tone.


Steve.

And it won't do anything either.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 24, 2013, 06:19:14 PM

And it won't do anything either.

I am not sure which is the right page in the ARRL handbook (my 1961 copy is long gone), but this is about relative impedances, voltage dividers and simple one pole filters..

A few 10k pF can be a relatively low impedance to the hum coming from a nearby meat antenna, and low relative to the megohm  input impedance of a guitar amp.
------
Cap coupling just the strings and external metal parts, while hard grounding the pick-up seems OK if the pickup does not present exposed metal (like a grounded can), but that still seems like a lesser risk than having all the metal parts hard connected to ground.   

I still wouldn't mess around inside any axe that may have some legacy value. A cap wired into a guitar cable connector shell seems less invasive. and if your audience complains about the tone, you can blame it on the cable, and swap it out.  8) 8)

One of you guitar boys could experiment with how small of a cap value you can get away with before you hear the low frequency fall off (this can depend somewhat on the amp input Z), and shielding effectiveness diminish. I was suggesting a 0.1uF (for 120VAC). One experienced guy on that other forum says that he typically uses 0.02uF. If a .02uF works, my .1uf is 5x over-kill.

Ok this is not apples to apples, but i just did another little experiment. With me sitting here ungrounded touching the center pin of an RCA cable connected to my computer speaker amp. As expected this makes a bunch of hum... Then I tried grounding myself through my trusty old capacitor diddle box while touching the pin. Grounding myself through 100 pF made almost no different in the hum level. 1000pF made a very audible reduction in hum. .01uF knocked it way down, with only slight (but audible) differences between .02uF and .22 uF. I also noticed a difference from how tightly I gripped the diddle box lead, so my skin resistance was a factor too. While a guitar amp has more voltage gain than my computer speakers, you are not literally touching the input lead of your guitar amp, only standing near the pickup coil so my little experiment may be representative.   

I'm sure somebody will claim to hear a 10 uF cap in series..  :o :o :o

JR
 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 24, 2013, 10:08:53 PM
JR,
Send me the schematic for the gadget you were talking about in a PM. I have an SG we use for a test bed.

I have a couple of other methods for killing hum in guitars that are unique. I'll post them when I get time.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 24, 2013, 10:48:43 PM
JR,
Send me the schematic for the gadget you were talking about in a PM. I have an SG we use for a test bed.

I have a couple of other methods for killing hum in guitars that are unique. I'll post them when I get time.

I do not have a schematic... just open the ground in the cord going to the guitar and solder a cap in series with that ground. My calculations suggest a 0.1uF will limit current from exposure to 120V 60Hz to around 5 mA, that should be sub-lethal. Other people who have actually performed this mod say 0.02uF is adequate...

My crude testing suggests that .02uF may work OK and will be only 1 mA of fault current.

Ideally this cap should be 200V or more but for noise testing any old cap will do.

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Steve M Smith on September 25, 2013, 03:03:07 AM
I find it strange when people are suggesting that having exposed metal parts connected to ground could be dangerous.

In the UK, all exposed metal parts are supposed to be connected to ground/earth for safety.

Surely it's only a problem if your ground connection isn't really grounded.

This seems to be quite common in the US from what I have read on this forum but almost unheard of here.


Steve.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 25, 2013, 07:44:41 AM
I do not have a schematic... just open the ground in the cord going to the guitar and solder a cap in series with that ground. My calculations suggest a 0.1uF will limit current from exposure to 120V 60Hz to around 5 mA, that should be sub-lethal. Other people who have actually performed this mod say 0.02uF is adequate...

My crude testing suggests that .02uF may work OK and will be only 1 mA of fault current.

Ideally this cap should be 200V or more but for noise testing any old cap will do.

JR

OK, I've tried that in the past with no success, but I'll try it again. I have another SG with P-90 single coils that makes all kinds of noise, especially in my basement under fluorescent lights. I'll try with that first.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 25, 2013, 08:21:39 AM
I find it strange when people are suggesting that having exposed metal parts connected to ground could be dangerous.

In the UK, all exposed metal parts are supposed to be connected to ground/earth for safety. Surely it's only a problem if your ground connection isn't really grounded. This seems to be quite common in the US from what I have read on this forum but almost unheard of here.

Steve.

Steve, you are correct. I work with UK sound crews at times and they're typically aghast at US power and grounding. We do tend to have loosey-goosey grounding here.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jerome Malsack on September 25, 2013, 10:04:14 AM
I have also seen someone on the internet who setup a guitar as balanced to the amp.  Would the CMRR help with this??
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 25, 2013, 10:08:39 AM
I find it strange when people are suggesting that having exposed metal parts connected to ground could be dangerous.

In the UK, all exposed metal parts are supposed to be connected to ground/earth for safety.

Surely it's only a problem if your ground connection isn't really grounded.

This seems to be quite common in the US from what I have read on this forum but almost unheard of here.


Steve.

To be logical about this there only needs to be a solidly grounded chassis, when there is dangerous power circuitry inside that chassis that could fail and energize the metal chassis. Grounding the chassis to the mains system trips the over-current breaker in the case of such a fault.  For the guitar there is no mains power inside that guitar, so little risk of wiring or insulation failure energizing those metal parts. Safety agency rules stipulate that exposed metal parts do not present elevated temperature or voltage to consumers. 

Ironically perhaps, most musician shocks related to guitars come from the guitar amps being dicey. Many old ungrounded line cord guitar amps used stinger caps to cap couple the chassis to the neutral pin of the two wire line cord for less hum. Musicians often get a shock from touching the real ground connected microphone while holding the guitar now energized by the stinger cap (if stinger cap connected to hot side of mains).   

The metal parts of guitars are routinely grounded to the amp to reduce hum noise pickup, not for safety. Many old guitar amps were not grounded themselves, so the guitars are just connected to the amp chassis, whatever potential that may be.   

Note: The stinger caps are mainly delivering a painful shock not stopping hearts, but musicians have fallen off stage or suffered incidental injuries after getting shocked. This may seem like much ado about nothing since deaths from this is rare, but they aren't zero. The sub-lethal shocks are unpleasant so less of that is better too.

I consider the amps with stinger caps a self inflicted wound that is mostly harmless, but venues with dangerous power distribution should not be tolerated.

JR

PS: Yes, the US is a little flaky about mains and outlet wiring. We need to be proactive about our home and work place wiring. 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 25, 2013, 10:19:18 AM
I have also seen someone on the internet who setup a guitar as balanced to the amp.  Would the CMRR help with this??

Not likely. By balanced I ASSume you mean 3 wire, +/-/ground. Floating the pickup and reading it differentially will not diminish the  hum that looks like valid signal to the pickup.

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 26, 2013, 06:29:40 PM
Unless your name is Les Paul. Read on.....

http://www2.gibson.com/News-Lifestyle/Features/en-us/les-paul-the-recording-model-1113-2012.aspx

http://www.youtube.com/watch?v=n2zitOYZqnU
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 26, 2013, 07:22:40 PM
Unless your name is Les Paul. Read on.....

http://www2.gibson.com/News-Lifestyle/Features/en-us/les-paul-the-recording-model-1113-2012.aspx

http://www.youtube.com/watch?v=n2zitOYZqnU

You can hear it start buzzing around 1:45 when he takes his hands off the strings.

And now we know why... Hah 8)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Craig Hauber on September 29, 2013, 12:03:31 PM
We sound technicians have done too little to protect our stage musicians.

Maybe because we've been beaten over the head with the "always have a licensed electrician do things for us or fear liability" concept that we no longer wish to open our mouths!

Sure would hate to recommend a proper fix only to have someone electrocuted shortly afterwards due to some other totally unrelated backline issue that you now get the blame for.




(-don't get me wrong, I always try to fix any safety problems no matter who's fault it really is, but the current litigious nature of american society does make me concerned)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 29, 2013, 12:47:31 PM
Maybe because we've been beaten over the head with the "always have a licensed electrician do things for us or fear liability" concept that we no longer wish to open our mouths!
In these times liability is a valid concern. If you are uncertain about a remedy, DO NOT DO IT. That said common sense and self preservation suggests not ignoring an obviously hazardous situation.
Quote
Sure would hate to recommend a proper fix only to have someone electrocuted shortly afterwards due to some other totally unrelated backline issue that you now get the blame for.

me too... Peavey was sued because a player was killed by properly designed and built guitar amps. When push comes to shove, it helps to follow safety agency standards.
Quote


(-don't get me wrong, I always try to fix any safety problems no matter who's fault it really is, but the current litigious nature of american society does make me concerned)
There is a difference between rewiring a venue's outlet(s) that could open you up for liability, and taping over a dangerous outlet so it can't be used.

I can't imagine anyone incurring personal liability from adding a GFCI in series with their portable mains power distribution. These are available in power strips or other configurations. If I was a performing musician I'd be tempted to buy my own as cheap shock insurance.

When in doubt do what's right.

JR

[edit] Regarding the safety implications of grounds in the context of a guitar, I am of the strong opinion that a ground there that provides a low impedance path to mains ground puts the musician at higher risk of harm. Grounded chassis provide human safety when internal mains power faults get loose inside the chassis. The grounded chassis, shorts out the power fault to first protect the human from high voltage, and secondly to trip the mains breaker and remove the fault. In a guitar there is no such human safety benefit from a low impedance ground path, just noise abatement.

Ironically the old stinger cap if sized properly is arguably less dangerous to the player, than the low impedance ground path if the fault is an energized microphone ground (or hot ground in back line distro). However if a power fault occur inside a floor pedal that energizes the ground lead the hard grounded guitar amp input will be more protective. 

In my judgement adding a cap in series with the ground lead "at the guitar" is prophylactic against energized microphone, energized floor boxes, or energized amp grounds. If the cap is sized to deliver less than lethal current while still adequate for shielding, why not?   [/edit]
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 30, 2013, 08:55:11 AM
[edit] Regarding the safety implications of grounds in the context of a guitar, I am of the strong opinion that a ground there that provides a low impedance path to mains ground puts the musician at higher risk of harm. Grounded chassis provide human safety when internal mains power faults get loose inside the chassis. The grounded chassis, shorts out the power fault to first protect the human from high voltage, and secondly to trip the mains breaker and remove the fault. In a guitar there is no such human safety benefit from a low impedance ground path, just noise abatement.

That's probably correct, but it's really a legacy issues as there must be millions of guitar with "grounded" strings in existence.  I doubt that's going to change quickly.

Quote

Ironically the old stinger cap if sized properly is arguably less dangerous to the player, than the low impedance ground path if the fault is an energized microphone ground (or hot ground in back line distro). However if a power fault occur inside a floor pedal that energizes the ground lead the hard grounded guitar amp input will be more protective. 

In my judgement adding a cap in series with the ground lead "at the guitar" is prophylactic against energized microphone, energized floor boxes, or energized amp grounds. If the cap is sized to deliver less than lethal current while still adequate for shielding, why not?   [/edit]

I think a cap in series between the strings and the guitar output jack shield would be reasonable and safe. But if the cap is in series with only the cable shield, that could inject 120-volts AC into the input stage of the guitar amp itself. While a tube amp might be able to withstand this fault, I'm pretty sure your average transistor guitar amp would have its input stage destroyed. You would probably need to add series caps in both the shield and signal paths of the unbalanced guitar cable, and providing clamping diodes across the input of the amp. All sorts of tonal issues could be involved if not designed carefully, but I'm sure that problem could be overcome.
Quote

I can't imagine anyone incurring personal liability from adding a GFCI in series with their portable mains power distribution. These are available in power strips or other configurations. If I was a performing musician I'd be tempted to buy my own as cheap shock insurance

Interestingly, a backline power fed from RPBG (Reverse Polarity Bootleg Ground) wiring will stop plug-in GFCI's from working. What will happen is that while the GFCI may "trip" due to the current imbalance, since the GFCI can't' disconnect the safety ground, the backline amps plugged into it won't have their chassis disconnected from the "hot ground" outlet condition. The amp chassis (and guitar strings) will be connected directly to the 120-volt line with a low-impedance path, the most dangerous of all shock conditions.

As I've noted here, this RPBG wiring condition most likely occurs in houses of worship originally built before 1970. Since old wiring didn't have safety grounds, and modern performance stages need safety grounds, many of these buildings have been "upgraded" to grounded receptacles by church volunteers who sometimes perform bootleg grounds instead of running all new grounded wiring back to the service panel.

With that in mind, perhaps the e-guitar industry needs to revisit the idea of guitar string grounding and come up with a solution (such as in-line caps) that will protect musicians from electrocution while not interfering with the tone. Much more to think about. 
 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 30, 2013, 10:47:09 AM
That's probably correct, but it's really a legacy issues as there must be millions of guitar with "grounded" strings in existence.  I doubt that's going to change quickly.
Yup, but a guitar cable maker could actually do something useful. 
Quote
I think a cap in series between the strings and the guitar output jack shield would be reasonable and safe. But if the cap is in series with only the cable shield, that could inject 120-volts AC into the input stage of the guitar amp itself. While a tube amp might be able to withstand this fault, I'm pretty sure your average transistor guitar amp would have its input stage destroyed. You would probably need to add series caps in both the shield and signal paths of the unbalanced guitar cable, and providing clamping diodes across the input of the amp. All sorts of tonal issues could be involved if not designed carefully, but I'm sure that problem could be overcome.
Equipment failure IM0 is a secondary concern if the shock is being delivered through a human's body. Clamping the guitar input depending on how it is done, could return us to the low impedance path scenario where human safety is at risk. FWIW The guitar amp input is probably already cap coupled and many solid state products have input clamps already to protect against static damage, and any current delivered through the pickup has the pickup impedance in series with it which "may" (?) be enough to limit it to sub lethal range. 
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Interestingly, a backline power fed from RPBG (Reverse Polarity Bootleg Ground) wiring will stop plug-in GFCI's from working. What will happen is that while the GFCI may "trip" due to the current imbalance, since the GFCI can't' disconnect the safety ground, the backline amps plugged into it won't have their chassis disconnected from the "hot ground" outlet condition. The amp chassis (and guitar strings) will be connected directly to the 120-volt line with a low-impedance path, the most dangerous of all shock conditions.
There should be a special room in hell for electricians who do that. Bootleg ground is dangerous enough, reverse bootleg is a killer.
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As I've noted here, this RPBG wiring condition most likely occurs in houses of worship originally built before 1970. Since old wiring didn't have safety grounds, and modern performance stages need safety grounds, many of these buildings have been "upgraded" to grounded receptacles by church volunteers who sometimes perform bootleg grounds instead of running all new grounded wiring back to the service panel.
+1 and any place where DIY electricians are involved. The case I am aware of was in a residence.
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With that in mind, perhaps the e-guitar industry needs to revisit the idea of guitar string grounding and come up with a solution (such as in-line caps) that will protect musicians from electrocution while not interfering with the tone. Much more to think about. 
 
I suspect there just aren't enough deaths from this to cause manufacturers to mess with working guitar designs. From my experience talking with guys who design and manufacture guitars, they are far more concerned about tone, and probably consider potential shock issues as somebody else's problem.  I am pleasantly surprised that one added a fuse. I can't imagine the safety agency ever condoning lifting a safety ground, despite my personal opinion, while UL does not approve guitars.

JR

PS: While this is a little odd, a special GFCI outlet strip with it's internal ground cap-coupled could protect the meat puppets from lethal shock in case of RPBG. I can not imagine UL ever blessing that, but perhaps a GFCI with only a two wire line cord? Again a rather odd duck, and no I am not seriously suggesting this. This 2-wire outlet strip could break out a terminal for ground that the user would have to tie into a ground (or float). This may have some merit for managing back-line noise, as long as the ground is connected to something safe. I can not imagine UL blessing this.

PPS: Perhaps a non-contact voltage sensor built into a GFCI outlet strip with a relay or Solid state switch  disconnect for all three connections, could sniff the ground and not connect if that ground is energized. (Sounds expensive). UL might actually bless this.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on September 30, 2013, 10:57:34 AM
This is a very nice video from Taylor guitar describing the problem at hand.

http://www.taylorguitars.com/media/audio-noise-and-grounding-david-hosler
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 30, 2013, 11:35:44 AM
I can not imagine UL ever blessing that, but perhaps a GFCI with only a two wire line cord? Again a rather odd duck, and no I am not seriously suggesting this. This 2-wire outlet strip could break out a terminal for ground that the user would have to tie into a ground (or float). This may have some merit for managing back-line noise, as long as the ground is connected to something safe. I can not imagine UL blessing this.

Actually, according to code the proper way to install a "grounded" receptacle in ungrounded wiring is to install a GFCI outlet WITHOUT the ground screw connected to anything. However, the outlet is supposed to be marked as "No Ground" that's visible from the outside. I've never seen one marked this way, but that's how code describes it. GFCI receptacles don't need the safety ground wire to operate, so the "meat puppets" (my son's description) are still protected from shock.

I think you could probably take a standard $10 GFCI receptacle, disconnect the internal ground screw from the incoming Edison plug, and put a "stinger" capacitor in parallel with a high-ohm resistor between the incoming safety ground wire and the GFCI ground screw. That way your guitar's ground plane potential would be held close to earth ground voltage by the 100K resistor, the capacitor would shunt any RF frequencies to the backline safety ground (if it existed), and any fault currents between the guitar strings and external grounds or hot-ground mics would be limited to below 5 mA when the GFCI trips, and less than 2 mA after the GFCI trips, even if you were plugged into an RPBG mis-wired backline receptacle with a hot ground.

I know a couple of code monkeys who could probably tell us if this sort of gizmo would be a code violation or not, but I suspect it would pass all code requirements, as long as the proper label was affixed to the outside of the receptacle.

Seems like an interesting experiment, and something that could save lives WITHOUT affecting guitar tone. Plus it would be cheap to build from off-the-shelf components. Less than $20 even using new parts.

What do you think? Worth building and gathering some data? If you think it's worth a try, I'll draw up a pretty diagram and post it here tomorrow.

And no, I won't try this out on a meat puppet. I'll use meters instead...
   
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on September 30, 2013, 11:55:13 AM
If it is legal to wire grounded outlets without the ground as long as you identify and label them that sounds like a possible combination (with GFCI) that could protect muso's against hot grounds. As I mentioned it might be useful for noise abatement to break out a separate ground terminal.  Perhaps a ground lift switch, per outlet.

If I was smart I'd just do this instead of talking about it, but I like the idea of automatically sensing for a hot ground, and just saying no mas... The ground lifted GFCI outlet strip would allow the show to go on, which may not be the best thing in the big picture. Any killer outlets need to be condemned and fixed. Enabling them is just perpetuating the human risk.

   JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jonathan Johnson on September 30, 2013, 04:33:11 PM
However, the outlet is supposed to be marked as "No Ground" that's visible from the outside.

Design-conscious people (AKA "women") will balk at the presence of any label on an outlet, regardless of it's necessity for safety. If the coverplate had "NO EQUIPMENT GROUND" molded into the plastic, that would be more design-sensitive in that it would be the same color as the rest of the plate and blend in with the decor easier. But what big-box home improvement store is going to stock something that they'll never sell? The price would be significantly more than a standard coverplate (due to economy of scale), and Joe "Weekend" Warrior isn't going to buy such a coverplate because they have no idea why they need it. And besides, it won't be available in the style that his wife wants.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 30, 2013, 04:57:42 PM
Design-conscious people (AKA "women") will balk at the presence of any label on an outlet, regardless of it's necessity for safety. If the coverplate had "NO EQUIPMENT GROUND" molded into the plastic, that would be more design-sensitive in that it would be the same color as the rest of the plate and blend in with the decor easier. But what big-box home improvement store is going to stock something that they'll never sell? The price would be significantly more than a standard coverplate (due to economy of scale), and Joe "Weekend" Warrior isn't going to buy such a coverplate because they have no idea why they need it. And besides, it won't be available in the style that his wife wants.

Hey, I just report the news. Here's how an ungrounded, GFCI protected receptacle is supposed to be marked, according to the NEC (diagram courtesy Mike Holt Enterprises).

(http://howtosound.com/images/gfcibig.jpg)

I think that verbiage should be sufficient to make a stinger cap floated ground GFCI legal according to the NEC. But something like "Current Limited Safety Ground" or "Instrument Noise Ground" would be more descriptive. I'm going to draw up a schematic and send it to my code monkeys to see what they think. Then it's build/test time.

Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on September 30, 2013, 05:08:26 PM
This is a very nice video from Taylor guitar describing the problem at hand.

http://www.taylorguitars.com/media/audio-noise-and-grounding-david-hosler

Yes, a great video. This is exactly what I demonstrate in my NoShockZone seminar, except I use a B&K Variable AC supply to bias the guitar amp's ground from zero to 120 volts AC.

Way cool...  8)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on October 01, 2013, 08:58:24 AM
Here's the basic diagram of my SafeGround for guitars.

(http://howtosound.com/images/SafeGround.jpg)

Now, don't go building this yet as I still have to double-check all the fault paths, build a demo, gather empirical data as to peak and sustained fault currents, check the code legality, and see if it operates safety without making a guitar amp buzz. But it seems like a possibility.   
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Stan Sakamoto on October 08, 2013, 02:56:32 AM
Why does a guitar amp buzz when you're not touching the strings, and why does it stop when you touch the strings or anything metal on the guitar? I'll provide the answer and explanation next week.  8)

Thanks to Al Keltz for telling me about this.  ;)

I've come across this many times working with musicians. The hum and buzz problems can come from several issues such as a grounding problem from the amp to the guitar. First make sure that your amp has a good ground. When you connect your cable from the guitar to the amp that has no good grounding the cable becomes like an antenna picking up the noise transients from the guitar to the amp which amplifies the noise. Sometimes just by changing the cable from the guitar to the amp with a balanced shielded cable may fix the problem. Another issue is that the pick ups on the guitar is not grounded correctly and your strings and cable are looking for a good ground and stops humming when you touch the strings since your body is grounded. A hum in low frequency at 60Hz usually means that you have a ground loop.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bob Leonard on October 08, 2013, 12:02:04 PM
Here's the basic diagram of my SafeGround for guitars.

(http://howtosound.com/images/SafeGround.jpg)

Now, don't go building this yet as I still have to double-check all the fault paths, build a demo, gather empirical data as to peak and sustained fault currents, check the code legality, and see if it operates safety without making a guitar amp buzz. But it seems like a possibility.

Are you showing this as a separate device between the wall outlet and amplifier plug?
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on October 08, 2013, 12:51:28 PM
I won't speak for Mike, but my understanding is that this could be built into a GFCI outlet strip.

This could actually protect against reverse bootleg ground, while I don't like lifting the equipment safety ground from properly wired outlets.

This is like the stinger cap inside a two wire line cord guitar amp but chassis is cap connected to ground instead of neutral.

I might suggest a slide switch for each outlet that could selectively lift the ground.

I am still working on a late night beer-soaked design project to combine a non contact voltage sensor to detect for even reverse bootleg grounds and not even connect if it senses the hot ground present. it could even sense for and correct reversed hot-neutral, if I drink enough I may be able to tell ground from neutral and identify a floated ground. 

JR

Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jerome Malsack on October 08, 2013, 01:20:11 PM

Are you showing this as a separate device between the wall outlet and amplifier plug?

What the question has been mostly is if the guitar player is not touching the strings or a metal part on the guitar, why does it buzz. 
If you approach a guitar it will buzz also as you get close enough until you touch the strings. 

Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on October 08, 2013, 08:47:12 PM
I won't speak for Mike, but my understanding is that this could be built into a GFCI outlet strip.

This could actually protect against reverse bootleg ground, while I don't like lifting the equipment safety ground from properly wired outlets.

This is like the stinger cap inside a two wire line cord guitar amp but chassis is cap connected to ground instead of neutral.

I might suggest a slide switch for each outlet that could selectively lift the ground.

JR

Yup, the idea is to build a GFCI outlet strip with a few of the receptacles set up with a floated "SafeGround" for guitar players. I don't think the NEC or UL will allow any kind of switches on the individual safety ground paths, even if there's a GFCI involved. But I think they might allow a few receptacles on the end of the strip (or the entire strip) to be permanently ground lifted with a stinger cap and bleeder resistor, but only as long as there's a GFCI in that circuit. The idea is to eliminate hard-grounding the meat puppets in the first place, but protecting them from a hot-chassis to earth-ground hand-to-hand fault. And yes, this circuit should protect the musician (puppet) from an RPBG outlet induced shock between the mic and the guitar, even if the mic OR the guitar was the source of the hot-ground voltage.

More to experiment with, but I think this simple grounding system would eliminate electrocution hazards for the guitar player from his amp, while stopping ground loop hum, high-freq buzz, and even providing shock/electrocution protection from a "hot" microphone. And there would even be protection from shock due to Reverse Polarity Bootleg Grounds. I might even have a better filter design than the stinger cap and bleed resistor, but if that works and envisioned I'm keeping it proprietary for a potential patent.

I'm heading out to Texas for 12 days to teach a bunch of seminars, but I'll build one of these when I get back and test it for guitar-to-mic fault currents and buzz abatement. In the meantime, unless you're qualified to work on live voltages and measure them safely, then PLEASE don't go building one of these and wade into the kiddie pool holding your guitar. This is all still highly experimental.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Christian Güssmer on January 02, 2014, 01:38:53 PM
1st: sorry for bringing up this old topic
2nd: thanks for that discussion, really interesting stuff in here
3rd: is there any recommendation about what to do to stop that buzzing? From what I understood, the discussion about fuses and caps is about human protection... I mean standing on a stage in the middle of a soundcheck and there is exactly that buzz that shouldn't be there.

I have a diagram to post later, but according to my logic the answer is The Strings Are Grounding You. When you're holding the guitar but not touching the strings (or anything else grounded) then your own body is ungrounded and picking up all sorts of noise from the environment. This is the same buzzing noise you'll hear if you touch the tip of a 1/4" phone plug that's connected to the input of your guitar amp. Since your belly is close to the back of the guitar, this noise voltage is coupled into the unshielded, high-impedance circuity of the guitar (including the pickups). The reason it's a high-frequency "buzz" and not a low frequency "hum" is that there's high-pass filter circuit formed by the capacitor formed by distance of your "belly" to the internal guitar wiring.

When you touch the guitar strings with your hand, that provides a low-impedance earth-ground path from your body, through the strings/bridge/tailpiece of your guitar, via the shield of your phone plug, to the chassis of the stage amp, then finally through the power cord to the (hopefully) grounded wall outlet. So any of the ambient electrical noise your body was picking up via a capacitive coupling to the wiring in the walls (and more) has now been shorted to ground, and the noise stops.

If you ask guitar players and sound engineers how this works, I would guess that 99% of them think the opposite, that your body is grounding the strings. But being that the strings are always grounded (should be), then that can't be what's happening.

However, I do like the discussion on decoupling caps and 10 mA fuses which could probably stop musicians from being electrocuted. But very few guitars have these features, with the vast majority being wired with strings and all metal pieces being "grounded" to the sleeve of the 1/4" phone jack. I'm sure it's 99.999% that way in the guitar world unless someone knows something I don't know.
(...)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on January 03, 2014, 07:43:47 AM
1st: sorry for bringing up this old topic
2nd: thanks for that discussion, really interesting stuff in here
3rd: is there any recommendation about what to do to stop that buzzing? From what I understood, the discussion about fuses and caps is about human protection... I mean standing on a stage in the middle of a soundcheck and there is exactly that buzz that shouldn't be there.

There's quite a few different types of buzzes you can hear during a sound check. The one referenced in this thread is caused internally by electric guitars with high-impedance electronics. It occurs when a human is standing very close to the back of the guitar (within a few inches) but not touching the guitar strings. The body acts like an antenna booster, collecting harmonics of the 60 Hz hum all around us, and coupling it into the guitar electronics as buzz. If you turn the volume knob(s) down on the guitar or move away from the back of the guitar, this type of buzz should stop. If it doesn't stop, then there's a different reason for the buzz. For instance, if the guitar player is using an unshielded speaker cable instead of a shielded instrument cable to plug into his amp, then the amp will probably be nearly quiet with the guitar volume turned down, but buzz like a swarm of angry bees with the guitar volume turned up, even if your body is far away from the guitar body. If the amp buzzes or hums even with it's volume control turned down, then the amp itself likely has an open capacitor in its power supply. Also, I've been on stages with a lot of big electrical conduits and wiring underneath, and the guitar or bass would buzz/hum depending on where the musician stood or oriented the neck of the guitar.

During a sound check you can only do quick fixes, so if the guitar/amp is buzzing, get the musician to move around a bit and see if it changes. Have them try a different instrument cable and see if it changes. If the backline amp buzzes just sitting there with the volumes turned down, then tell them to get a backup amp.

Also, I found a Yamaha bass guitar a few months back during a sound check that had double pickups and a balance knob to set the relative volume. It hummed/buzzed when the bass player turned this balance knob all the way to the left or right, but was hum/buzz free in the middle. I'm guessing it was some sort of split humbucker pickup arrangement, but once we got his bass hum/buzz free during sound check, I was good to go for the gig and didn't do more research.

Oh yes, remember that many XLR outputs on GK bass amps are neither balanced nor does their ground lift switch actually lift the ground. You'll want to add your own external DI box to fix this buzz/hum.

The key to this sort of troubleshooting is to have a quick bag of tricks you can quickly try. If it's really the instrument doing it, then all you can really do is put a gate on that channel. But I like being able to help a musician figure out what's really wrong with their rig which helps the next sound guy to work with him or her.   Pay it forward... 8)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jerome Malsack on January 03, 2014, 09:36:36 AM
Here's the basic diagram of my SafeGround for guitars.

(http://howtosound.com/images/SafeGround.jpg)

Now, don't go building this yet as I still have to double-check all the fault paths, build a demo, gather empirical data as to peak and sustained fault currents, check the code legality, and see if it operates safety without making a guitar amp buzz. But it seems like a possibility.

So has this been approved for use? 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on January 03, 2014, 10:35:35 AM
So has this been approved for use?

Nope, it's just an experiment on my part. If I get some time I'll build one in the next few weeks and measure all the worse case fault currents. I'm pretty sure it's safe, but getting it code approved could be an issue, but who knows...
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Christian Güssmer on January 03, 2014, 05:15:23 PM
There's quite a few different types of buzzes you can hear during a sound check.

Hi Mike,

I've been in situations like this during the last years quite often and do have some "tricks" at hand, but unfortunately exactly this kind of buzz is hard to eliminate. Also I confess that I would have ticked the wrong answer in the poll, so thanks again for discussing it. At the moment I just wanted to know if I maybe missed a point in this quite large text.

Moreover there are often bands touring Europe with their backline on US power and those transformers, powerstrips and different wiring schemas add another layer of complexity - up to dangerous issues.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on January 03, 2014, 06:38:58 PM
I've been in situations like this during the last years quite often and do have some "tricks" at hand, but unfortunately exactly this kind of buzz is hard to eliminate.

The trick is to learn what causes these various unwanted noises so we can come up with quick fixes. And sadly, a standard electric guitar with high-impedance single-coil passive pickups is an antenna waiting to pick up interference. I would always build a Faraday cage from copper foil for my own guitar players which helped a lot in the crappy clubs full of neon beer signs buzzing. But that's not something you can do during a sound check.

At a gig last week I found than when the guitar player leaned against the concrete block wall behind him, his strat copy hummed (not buzzed) like crazy. When he stepped a foot or two away the hum went away. I looked on the other side of the wall and saw a big circuit breaker panel. I'm sure there was enough magnetic coupling to the pickups that no amount of shielding would have stopped it. I told him to stay away from the wall and his guitar wouldn't hum, which he hadn't figured out for himself.

Quote
Moreover there are often bands touring Europe with their backline on US power and those transformers, powerstrips and different wiring schemas add another layer of complexity - up to dangerous issues.

I had the reverse happen when Chumbawumba (Tub Thumppin') did their first show in the US on the roof of a garage in Washington DC. I have the dubious distinction of running sound for that gig. The funny thing is that I was using the MHA Audio M6 British rig (the original AC/DC Hells Bells system) which used Cameleon power amps and processing on 230-volts. We had a huge transformer that stepped up a 100-amp 120-volt feeder to 230-volts with all UK wiring and distro. We also had a second American distro split for the standard Edison 120-volt back line.

When Chumbawumba's manager called me from the hotel to advance the show, he insisted I needed to provide 120-to-230 volt transformers for the band's UK gear. But I told him I was running the only sound system in the USA with British plugboard 230-volt power running around stage, and the lads would feel right at home plugging in without any transformers or adapters. 

The rest is history with helicopters circling the building and nearly everybody in the band getting arrested except for my sound crew (their plan from the beginning).
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mac Kerr on January 03, 2014, 07:43:31 PM
The trick is to learn what causes these various unwanted noises so we can come up with quick fixes. And sadly, a standard electric guitar with high-impedance single-coil passive pickups is an antenna waiting to pick up interference. I would always build a Faraday cage from copper foil for my own guitar players which helped a lot in the crappy clubs full of neon beer signs buzzing. But that's not something you can do during a sound check.

At a gig last week I found than when the guitar player leaned against the concrete block wall behind him, his strat copy hummed (not buzzed) like crazy. When he stepped a foot or two away the hum went away. I looked on the other side of the wall and saw a big circuit breaker panel. I'm sure there was enough magnetic coupling to the pickups that no amount of shielding would have stopped it. I told him to stay away from the wall and his guitar wouldn't hum, which he hadn't figured out for himself.

Bill Whitlock tells a story of a stage with the same problem, and it turned out to be caused by a current loop in the building plumbing that was connected between 2 buildings with different electrical service. Putting in an isolating section in the pipe fixed the problem.

Mac
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jonathan Johnson on January 03, 2014, 10:16:28 PM
Bill Whitlock tells a story of a stage with the same problem, and it turned out to be caused by a current loop in the building plumbing that was connected between 2 buildings with different electrical service. Putting in an isolating section in the pipe fixed the problem.

That would be interesting to see on a rider: "Ground lift adapter for 1" FIP threaded water pipe"
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Stephen Swaffer on January 04, 2014, 10:32:36 PM
Heading off topic a bit, but for a current loop to exist there must be potential (voltage) difference leading to a possible hazardous situation at the isolating section, besides the NEC requiring any metal parts-plumbing, etc to be bonded to ground. Obviously fixed one problem (so does clipping a ground pin off a 3 prong plug sometimes) but maybe creating another??  Or am I thinking wrong?
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on January 04, 2014, 10:40:15 PM
Heading off topic a bit, but for a current loop to exist there must be potential (voltage) difference leading to a possible hazardous situation at the isolating section, besides the NEC requiring any metal parts-plumbing, etc to be bonded to ground. Obviously fixed one problem (so does clipping a ground pin off a 3 prong plug sometimes) but maybe creating another??  Or am I thinking wrong?

Sometimes there will be a failed neutral in a building bus and a water pipe becomes the actual neutral return path through the neutral/ground in another panel. So while there might only be a volt or two difference between the remote ends of a water pipe, isolating it in the center could cause a 120-240 split-voltage problem in one of the buildings. I would be extremely careful to measure everything BEFORE cutting into any pipes you know are carrying current. However, it would be cool to put a clamp-on ammeter around a water pipe and read the current, which could considerable with up to hundreds of amperes. I would tread lightly in this instance.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mac Kerr on January 04, 2014, 11:26:47 PM
Heading off topic a bit, but for a current loop to exist there must be potential (voltage) difference leading to a possible hazardous situation at the isolating section, besides the NEC requiring any metal parts-plumbing, etc to be bonded to ground. Obviously fixed one problem (so does clipping a ground pin off a 3 prong plug sometimes) but maybe creating another??  Or am I thinking wrong?
Sometimes there will be a failed neutral in a building bus and a water pipe becomes the actual neutral return path through the neutral/ground in another panel. So while there might only be a volt or two difference between the remote ends of a water pipe, isolating it in the center could cause a 120-240 split-voltage problem in one of the buildings. I would be extremely careful to measure everything BEFORE cutting into any pipes you know are carrying current. However, it would be cool to put a clamp-on ammeter around a water pipe and read the current, which could considerable with up to hundreds of amperes. I would read lightly in this instance.

IIRC in this case there was a difference in ground potential in each of the attached buildings due to different location of the cold water pipe's connection to actual dirt, and the separate electrical service to each building. Isolating the 2 buildings so they each were grounded to dirt only at the CW pipe got rid of the current in the pipes. It was not a case of miswiring, but of multiple ground potentials because of location. Ground potential varies across distance because physical dirt is a high impedance path.

Mac
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on January 05, 2014, 10:32:54 AM
Sometimes there will be a failed neutral in a building bus and a water pipe becomes the actual neutral return path through the neutral/ground in another panel. So while there might only be a volt or two difference between the remote ends of a water pipe, isolating it in the center could cause a 120-240 split-voltage problem in one of the buildings. I would be extremely careful to measure everything BEFORE cutting into any pipes you know are carrying current. However, it would be cool to put a clamp-on ammeter around a water pipe and read the current, which could considerable with up to hundreds of amperes. I would tread lightly in this instance.
Good point but if a water pipe is actually serving as a safety ground and carrying fault current, what better time to discover that then while sorting out the wiring.  :o

While the term ground loop is widely over used, plumbing could form a classic loop were major magnetic fields in the area generate loop currents and voltage.

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mac Kerr on January 05, 2014, 10:40:38 AM
Good point but if a water pipe is actually serving as a safety ground and carrying fault current, what better time to discover that then while sorting out the wiring.  :o

While the term ground loop is widely over used, plumbing could form a classic loop were major magnetic fields in the area generate loop currents and voltage.

JR

In the example I referred to there was no fault current. This was a case of differing ground potential in different locations (because that is what "earth" does) creating current in the plumbing of 2 attached buildings. If you want the whole explanation attend one of Bill's classes on grounding.

Mac
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on January 05, 2014, 12:32:54 PM
In the example I referred to there was no fault current. This was a case of differing ground potential in different locations (because that is what "earth" does) creating current in the plumbing of 2 attached buildings. If you want the whole explanation attend one of Bill's classes on grounding.

Mac
Understood.. I was commenting on Mike's hypothetical about potential life threatening voltages, not Bill's actual venue example.

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on January 05, 2014, 02:45:50 PM
Good point but if a water pipe is actually serving as a safety ground and carrying fault current, what better time to discover that then while sorting out the wiring.  :o

That's true, but the caveat is to watch all water pipes and ground wires for currents and/or voltages when being disconnected. One of the plumbers working in my apartment some 40 years ago nearly died while touching a "ground wire" that was disconnected from the panel. He swore that a green ground wire could never have voltage on it, but I showed him with a meter that it indeed did have 100+ volts. Same for metallic water pipes and electrical conduits which can indeed be carrying all sorts of "ground loop"or "neutral return" currents. Disconnecting one without being careful to monitor for voltage could get deadly in a hurry.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Stephen Swaffer on January 05, 2014, 03:47:40 PM
I would be careful even if you are dealing with nonmetallics.  I used to work on electric vacuum furnaces that used water cooled transformers (always wondered about the wisdom of piping water and 480 VAC to a device but....) and the cooling lines were rubber hoses.  We learned that we needed to use non-conductive rubber hoses after a mechanic tried to do a repair by clamping off  a 3/8" hose with vise grips to cut and install a splicer to repair a pin hole leak.  When he cut the hose there was 120 VAC across the 2 pairs of vice grips-at least he warned the rest of us so we didn't have to learn the hard way.  Really an eye opener to put a meter on a rubber hose and get a solid voltage reading!
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on January 05, 2014, 04:43:34 PM
In the example I referred to there was no fault current. This was a case of differing ground potential in different locations (because that is what "earth" does) creating current in the plumbing of 2 attached buildings.

And you can have different "ground" voltages from one side of a building to the other of the SAME building. I've measured 5 volts AC between sub-panels bonded to the far separated walls of an industrial building. That's why I get worried when a see an electrical sub-panel that's bonded to building steel. Connect any kind of chassis bonded/shielded signal cable between gear powered from those two different sub-panels, and you'll almost certainly create ground loop currents. For me it was coax cables carrying video signals from cameras on the dock to the guard shack. That's when I learned about video hum, baluns and checking for ground currents in an industrial building with a clamp-ammeter. That was 35 years ago, but the problem still exists, especially in large audio systems plugged into multiple power outlets.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 04, 2014, 07:17:22 AM
Just a heads up. I have a phone meeting tomorrow with Taylor Guitar  to discuss their guitar string ground fuse technology. The basic concept is a 10 mA fuse wired between the cable shield and strings to protect the player from dangerous shocks from a stage amp gone bad.  https://www.taylorguitars.com/taylorware/universal-string-ground (https://www.taylorguitars.com/taylorware/universal-string-ground)

Let me know if you have any specific questions you would like me to ask the engineer who designed it. I'm proposing an experiment with my NoShockZone setup to determine just how well this actually works. I'll keep you all posted.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on March 04, 2014, 10:34:29 AM
Well the obvious first thing is thank him (them) for being proactive and addressing customer safety.

It's been a while since I read the literature on this but from my recollection 10mA will not stop you from getting a shock, but should not cause death to a healthy heart. I believe I targeted 6 mA or less when I last looked into this.

Q#1. What does fuse opening characteristic look like? IIRC mechanical (thermal) fuses generally pass more current for short period of time then fail open. They are likewise not very precise.

Q#2. Could you source a smaller fuse? (My recollection is, not easily, from the last time I looked). 

Q#3. Have you evaluated using a small capacitor (perhaps in parallel with a large resistor) to provide the shield ground. This could be cheaper, and not prone to requiring repair/replacement after a shock event.

Q#4..  As a design engineer are you tempted to design a solid state current limiter? Hypothetically this could be set to protect at a much smaller current. Probably would not be as clean of a ground as the fuse in the simplest implementation.

The fuse is the ideal KISS solution, if only they made smaller current versions, but the technology is not available AFAIK. Since the 10mA fuse should save the vast majority from lethal shocks it's all good.

JR
 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Stephen Swaffer on March 04, 2014, 10:49:41 AM
GFI technology should be easy to implement-designs already exist.  My first thought is if the fuse blows, there is no shield which means now there is noise.  First thing someone is gonna do is check the amp-which has a hot chassis since the shield was hot.  Instead of being shocked by the guitar, they would be shocked by the amp.  Why not just GFI  the amp and shut it down if there is a fault?
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Tim McCulloch on March 04, 2014, 11:00:28 AM
GFI technology should be easy to implement-designs already exist.  My first thought is if the fuse blows, there is no shield which means now there is noise.  First thing someone is gonna do is check the amp-which has a hot chassis since the shield was hot.  Instead of being shocked by the guitar, they would be shocked by the amp.  Why not just GFI  the amp and shut it down if there is a fault?

Why? Because amp owners are not sufficiently concerned about their role in safety - they what their mystical, unicorn-inspired "tone" and will risk a great deal because once the unicorn delivers, the owners will NOT change a single part of the signal chain nor will they allow any of those parts to be modified in any form or fashion.  If the amp leaks current & voltage and trips a GFI, they will run an extension cord to a conventional OPD outlet.

I love the guitar but find most guitarists to be problem children in this respect.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on March 04, 2014, 11:07:35 AM
GFI technology should be easy to implement-designs already exist.  My first thought is if the fuse blows, there is no shield which means now there is noise.  First thing someone is gonna do is check the amp-which has a hot chassis since the shield was hot.  Instead of being shocked by the guitar, they would be shocked by the amp.  Why not just GFI  the amp and shut it down if there is a fault?

I have been proselytizing for use of GFI outlets strips for back line use.

You do not have access to enough information inside a guitar to implement a proper GFCI, while it could be added inside the guitar amp. In my experience guitar amps are generally sharp pencil exercises so GFCIs are unlikely unless mandated by law. Which is probably also unlikely.

Perhaps a variant on the GFCI that trips for any path current (back to my solid state fuse suggestion). 

Yup, GFCI trips in the 4-6mA range so if used on the back line gear should trip well before the 10mA Taylor in-guitar fuse.

JR

@ Mike:  when an outlet GFCI trips does it open ground, or just the hot?  If the hot shock hazard is not coming from the back line the GFCI on the back line may just kill the amps but not the shock hazard. Dealing with the ground path inside the guitar should protect against shocks from either direction.

JR

Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 04, 2014, 03:41:05 PM
@ Mike:  when an outlet GFCI trips does it open ground, or just the hot?  If the hot shock hazard is not coming from the back line the GFCI on the back line may just kill the amps but not the shock hazard. Dealing with the ground path inside the guitar should protect against shocks from either direction.

JR

A GFCI only opens the line/hot wire, not the neutral and certainly not the ground. And there's no current sensing the ground wire at all, so a hot microphone will NOT trip a backline GFCI on a properly grounded guitar amp, and it certainly won't trip or disconnect your guitar amp from a RPBG (Reverse Polarity Bootleg Ground) outlet. But the Taylor fuse WOULD protect the guitarist under both conditions. Yup, they would feel a 10 mA shock, but that shouldn't be life threatening to a healthy heart. I've had one of my readers on another forum ask about shock dangers if they're wearing a pacemaker, so that's another area to research. 

I'm going to ask Taylor for a bunch of their 10 mA fuses to try, and do a few experiments to see how long they take to open under standard shock conditions.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jonathan Johnson on March 04, 2014, 04:27:37 PM
Just a heads up. I have a phone meeting tomorrow with Taylor Guitar  to discuss their guitar string ground fuse technology. The basic concept is a 10 mA fuse wired between the cable shield and strings to protect the player from dangerous shocks from a stage amp gone bad.

My first thought is that any protective device should not cause another problem when actuated -- don't trade one problem for another. The typical mantra is that Ground Shall Never Be Broken -- that is, a broken ground is no different than any other malfunctioning safety device. Although since this is on the output of a guitar, it's probably not considered a safety ground.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on March 04, 2014, 04:52:03 PM
A GFCI only opens the line/hot wire, not the neutral and certainly not the ground. And there's no current sensing the ground wire at all, so a hot microphone will NOT trip a backline GFCI on a properly grounded guitar amp, and it certainly won't trip or disconnect your guitar amp from a RPBG (Reverse Polarity Bootleg Ground) outlet. But the Taylor fuse WOULD protect the guitarist under both conditions. Yup, they would feel a 10 mA shock, but that shouldn't be life threatening to a healthy heart. I've had one of my readers on another forum ask about shock dangers if they're wearing a pacemaker, so that's another area to research. 

I'm going to ask Taylor for a bunch of their 10 mA fuses to try, and do a few experiments to see how long they take to open under standard shock conditions.

While these current numbers are not precise as human skin conduction is pretty variable, 10 mA is generally considered a threshold for sticking to the current or being able to release.

People with weak hearts and pace makers probably need to avoid even 10 mA shocks.

-----

Yup, even a GFCI outlet strip will not protect against RPBG, but it could if specially designed to.  If an outlet tester can ID RPBG, a protection device could be executed using similar technology. I scratched up some circuit ideas but expect this is too small a market, to justify paying to protect against a threat that wouldn't happen if people were competent.

I would still want to protect at lower current than a nominal 10 mA fuse (but I think I understand the design decision).

For back line if the more common threat is still rouge guitar amps, a standard GFCI outlet will protect against that. If the threat these days is more from RPBG wiring and not shady guitar amps, get a non contact meter and use it. In fact get and use a non contact tester either way.

JR

PS: OK I still think a simple capacitor is the practical solution for this, here is my exotic off-the-wall idea for the day. Connect the ground through the primary winding of very high impedance step down transformer, that would saturate at modest mA currents. Then short the secondary with the 10mA fuse. At modest currents where the transformer is not saturated, and the fuse is still shorting the secondary, the short would reflect through the transformer to the primary and look like a relatively low impedance. A 10:1 step down transformer could cause 1 mA of primary current to open the 10 mA secondary fuse. For this to work the primary winding impedance needs to be tens of K ohm... Probably not your common off the shelf transformer or cheap... Just a little mental masturbation.  I still would use the cap solution first, and solid state current limit second. I wonder if you could make a PTC fuse that small? I suspect for tiny currents, mechanical approaches suffer from environmental concerns (like ambient temperature) that could be a confounding variable.
 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on March 04, 2014, 05:08:48 PM
My first thought is that any protective device should not cause another problem when actuated -- don't trade one problem for another. The typical mantra is that Ground Shall Never Be Broken -- that is, a broken ground is no different than any other malfunctioning safety device. Although since this is on the output of a guitar, it's probably not considered a safety ground.

That commandment gets a little scary in the context of a RPBG...  At Peavey we had a customer killed by the guitar amp's safety ground when two amps were plugged into two different outlets, one of which was RPBG. That benign safety ground killed him (actually the mis-wired outlet killed him).

I suspect I am getting repetitious but I like the small cap on place of hard ground solution for inside the guitar which is NOT a safety ground. KISS
----
For my smart outlet strip, it should be possible to detect which is neutral vs hot and connect the right leads to the right destination. Discriminating safety ground from neutral may not be as simple, especially if ground is already corrupt and noisy. While this seems a less serous fault. 

JR

PS: No I am not seriously thinking about making smart outlet strips... but I have thought about safety grounds a lot over the years after losing a customer playing his UL approved, and perfectly assembled guitar amp.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 04, 2014, 06:04:40 PM
That commandment gets a little scary in the context of a RPBG...  At Peavey we had a customer killed by the guitar amp's safety ground when two amps were plugged into two different outlets, one of which was RPBG. That benign safety ground killed him (actually the mis-wired outlet killed him).

Even though at first blush I was feeling a little uncomfortable about putting a fuse in series with the string ground, in reality it would have limited the shock current to your customer to 10 mA before the fuse blew. Now of course, there will be larger peak shock currents involved and I don't know how many line cycles it will take for the fuse to open up. That's what I'm going to experiment with and confer with one of my shock/electrocution buddies who's been studying marine shocks for years.

Yes, your mileage will vary - depending on the condition of your heart, pacemaker, recreational drugs, etc... And I sure wish there was a way to reset the "fuse" and an indicator to let the musicians know it was "blown". But as a KISS fix, it's certainly an interesting idea.

I do love talking to other engineers, so I'm really looking forward to my Taylor talk tomorrow.     
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Stephen Swaffer on March 04, 2014, 08:00:00 PM
Why? Because amp owners are not sufficiently concerned about their role in safety - they what their mystical, unicorn-inspired "tone" and will risk a great deal because once the unicorn delivers, the owners will NOT change a single part of the signal chain nor will they allow any of those parts to be modified in any form or fashion.  If the amp leaks current & voltage and trips a GFI, they will run an extension cord to a conventional OPD outlet.

And I respectfully argue that if a 10 mA fuse blows, they will find a fuse sufficiently large enough not to blow-until it makes them say ouch.  (95% or better of the edison fuses I remove from service have 30 A fuses on #12 wire-and 20 A edison fuses are as easy to buy as 30 amp fuses-I am betting it will be far easier to find a 1 amp replacement than a 10 mA replacement) 

That  is why I would argue for an internal GFCI-or perhaps a current sensing relay that would disconnect the amp from mains anytime current above a threshold of say 5 mA was sensed on the shield wire-since there should never be current there, there would be no need to compare to anything.  Essentially this would work like a remote trip circuit breaker, where an outside fault condition trips the breaker forcing a manual reset.  We used a similar concept to protect electric furnaces from an overtemp situation-if overtemp was sensed we killed mains-same idea here-current going where it should not disconnect everything-hot and neutral can be disconnected simultaneously meeting current standards.  Not sure you can do that with a ground-and to an extent you have to protect against  true faults and not human stupidity.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Tim McCulloch on March 04, 2014, 09:33:54 PM
And I respectfully argue that if a 10 mA fuse blows, they will find a fuse sufficiently large enough not to blow-until it makes them say ouch.  (95% or better of the edison fuses I remove from service have 30 A fuses on #12 wire-and 20 A edison fuses are as easy to buy as 30 amp fuses-I am betting it will be far easier to find a 1 amp replacement than a 10 mA replacement) 

That  is why I would argue for an internal GFCI-or perhaps a current sensing relay that would disconnect the amp from mains anytime current above a threshold of say 5 mA was sensed on the shield wire-since there should never be current there, there would be no need to compare to anything.  Essentially this would work like a remote trip circuit breaker, where an outside fault condition trips the breaker forcing a manual reset.  We used a similar concept to protect electric furnaces from an overtemp situation-if overtemp was sensed we killed mains-same idea here-current going where it should not disconnect everything-hot and neutral can be disconnected simultaneously meeting current standards.  Not sure you can do that with a ground-and to an extent you have to protect against  true faults and not human stupidity.

Steve, I don't think we're disagreeing. :)
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 04, 2014, 10:10:58 PM
And I respectfully argue that if a 10 mA fuse blows, they will find a fuse sufficiently large enough not to blow-until it makes them say ouch.  (95% or better of the edison fuses I remove from service have 30 A fuses on #12 wire-and 20 A edison fuses are as easy to buy as 30 amp fuses-I am betting it will be far easier to find a 1 amp replacement than a 10 mA replacement) 

I think these fuses are odd enough that you're not going to put in a 1 amp replacement. However, as I've alluded to many times, no matter how much you try to idiot proof something, they keep building better idiots. It may not be perfect, but at least it appears to be a step in the right direction. But I'll wait until I do some actual testing to venture a real opinion.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 05, 2014, 10:08:37 AM
Once I do some experimenting with the Taylor String Fuse Technology, I'm going to build my SafeGround power plug to try out. As a review, it's a combination of the old "stinger cap" idea, along with a ground bias resistor hung on a GFCI to make it code compliant (or should be with proper lobbying).

Don't worry as I'm not going to grab this with my own hand to see if I'll get shocked or electrocuted. I'll build a shock dummy with appropriate internal body resistance for the testing.

Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on March 05, 2014, 11:07:40 AM
Even though at first blush I was feeling a little uncomfortable about putting a fuse in series with the string ground, in reality it would have limited the shock current to your customer to 10 mA before the fuse blew.
A minor point the way a fuse works is that it fails open circuit from over current. How fast the fuse opens depends on how much over current it experiences. So it does not limit current until it blows, in fact it passes plenty of current until it heats up, then zero after it opens up.

I didn't want to have to pull out my pocket protector and do some actual engineering but here goes.

From the littlefuse data sheet
a 10mA fuse will pass 100% or rated current for 4 hours (min)
a 10mA fuse will pass 135% of rated current for 1 hour (max)
a 10 mA fuse will pass 200% of rated current for 5 sec (min) 30 sec (max)

Looking at the cold resistance of the 10mA fuse of 3300 ohms, means a full 120V fault. Would draw 36 mA for less than 5 seconds (72 mA for a 240V fault). I suspect as the fuse heats up it's cold resistance will drop causing it to draw even more current so trip even faster. At 350% overload we are off the data sheet, so I'll leave this to Mike to determine empirically. 

Of course this is data sheet perfect world, but it looks like a safe ASSumption that the 10 mA fuse will limit the time that it passes a few tens of mA to only seconds or less.
=====
Cooper Bussman makes a fast acting series that perhaps oddly has similar time vs overload characteristic but offers a significantly lower cold resistance of 155 ohms. So .77A instantaneous current for 120V fault, 1.5A for 240V. So perhaps we do not want to mess with fast blow fuses in our axe. Bussman lists a 1/500A (2 mA) fuse with a cold resistance of 1750, passing 68/134 mA short term.

I haven't seen the 2mA fuses for sale so who knows how expensive that would be, these are not common fuse values.

It looks like the slow/normal acting 10m fuse will keep the peak short term fault current in the low tens of mA (for 120V) until it blows..     

Quote
Now of course, there will be larger peak shock currents involved and I don't know how many line cycles it will take for the fuse to open up. That's what I'm going to experiment with and confer with one of my shock/electrocution buddies who's been studying marine shocks for years.

Yes, your mileage will vary - depending on the condition of your heart, pacemaker, recreational drugs, etc... And I sure wish there was a way to reset the "fuse" and an indicator to let the musicians know it was "blown". But as a KISS fix, it's certainly an interesting idea.

I do love talking to other engineers, so I'm really looking forward to my Taylor talk tomorrow.   

You could literally design a bilateral (AC) semiconductor current limiter set for a couple mA that would just limit the current no matter what. This would require high voltage parts and seems overly complex compared to the simple C or C with large R in parallel that would likewise limit the current due to mains frequency. I would be tempted to use a C in parallel with any solid state current limiter anyhow to clean up the ground, so why not just KISS.

JR

Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 05, 2014, 12:25:44 PM
You could literally design a bilateral (AC) semiconductor current limiter set for a couple mA that would just limit the current no matter what. This would require high voltage parts and seems overly complex compared to the simple C or C with large R in parallel that would likewise limit the current due to mains frequency. I would be tempted to use a C in parallel with any solid state current limiter anyhow to clean up the ground, so why not just KISS.

JR

I think that my above diagram with an R-C circuit and GFCI receptacle has the best potential for using off-the-shelf products that are already UL listed and Code documented. If something goes terribly wrong it would trip the GFCI, and even RPBG situations should be shock safe. Of course, you'll want to use a "portable" GFCI which needs a physical reset every time it's plugged in. That's so a neutral failure will allow it to trip, unlike a standard install-GFCI which needs power across the H-N input to trip itself. Again, I've only drawn this out on paper and haven't bread-boarded one for experiments yet, so nobody go building one and jumping in the pool with your guitar strapped on. We don't want to see you on the Darwin Awards...
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on March 05, 2014, 12:47:43 PM
I think that my above diagram with an R-C circuit and GFCI receptacle has the best potential for using off-the-shelf products that are already UL listed and Code documented. If something goes terribly wrong it would trip the GFCI, and even RPBG situations should be shock safe. Of course, you'll want to use a "portable" GFCI which needs a physical reset every time it's plugged in. That's so a neutral failure will allow it to trip, unlike a standard install-GFCI which needs power across the H-N input to trip itself. Again, I've only drawn this out on paper and haven't bread-boarded one for experiments yet, so nobody go building one and jumping in the pool with your guitar strapped on. We don't want to see you on the Darwin Awards...

Yup... the more I look at the fuse the more I like the cap.  8)

As I think I posted during an earlier inspection of this we had a long and winding discussion of this on another forum a few years ago and participants there determined empirically that a cap could be used inside the guitar and sized such that it both provided a low enough shield impedance to reduce hum/buzz, while still a high enough impedance at 50/60Hz to provide human safety current limiting.  Of course the cap should be high enough voltage to handle mains voltage. There are caps specified to work across mains voltage, so they should be more than adequate IMO.   

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 05, 2014, 03:56:58 PM
Yup... the more I look at the fuse the more I like the cap.  8)


Taylor is sending me a bunch of their 10 mA fuse kits for evaluation, and I'm going to build a cap/resistor string grounder and give that a shot as well. Since I can try this using a bunch of different hot-chassis voltage conditions, we should get a hint as to how effective any of these technologies would be in preventing shock and/or electrocution.

Let the testing begin...  ;D
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Stephen Swaffer on March 05, 2014, 07:43:09 PM
I think Mike is really a pyro at heart-he just uses electricity instead of fire to burn things up-but that's OK someone's gotta do it.  I hope he can come up with a workable solution.

Unfortunately, those fuses look identical to the 10 A fuses that are used internally on Honeywell PLCs.  Granted, you can't buy them at Radio Shack or the auto parts store.   
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 05, 2014, 07:48:34 PM
I think Mike is really a pyro at heart-he just uses electricity instead of fire to burn things up-but that's OK someone's gotta do it.  I hope he can come up with a workable solution.

Ummmmm.... I really was a pyro. REALLY!!!!

I did wait until my boys were 16 years old before I would share my favorite napalm recipes with them. That's not too young, is it???
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 05, 2014, 07:55:50 PM
Hey JR,

I should probably use a Solid State Relay with a zero-crossover start in order to have all peak current experiments start on zero part of the line cycle. If not, then there's a big variable as to what the instantaneous voltage of the 60-Hz wave is when I hit the shorting switch for the peak current test. Anything else I should think about when building the test rig? I'll use a 1K Ohm resistor as a substitute for the meat puppet, as I don't think a human has any significant capacitance or inductance to worry about. What about dreadlocks though?  :o
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on March 05, 2014, 09:24:27 PM
Hey JR,

I should probably use a Solid State Relay with a zero-crossover start in order to have all peak current experiments start on zero part of the line cycle. If not, then there's a big variable as to what the instantaneous voltage of the 60-Hz wave is when I hit the shorting switch for the peak current test. Anything else I should think about when building the test rig? I'll use a 1K Ohm resistor as a substitute for the meat puppet, as I don't think a human has any significant capacitance or inductance to worry about. What about dreadlocks though?  :o

The path R for the meat puppet is very important and hugely variable. If you asked me earlier I could have measured my skin resistance right after I finished my 5 mile run today, and was coated in a layer of salty sweat. Right now my skin is clean and dry and I have a hard time measuring less than 1M ohm.

The musician who was killed by the RPBG, happened on a hot summer night, without air conditioning, so he was likewise sweaty and lower resistance than my present 800K ohm.

My suspicion right now is if I grabbed a hot mains lead that 10 mA fuse would never blow.

So 1k Ohm sounds low, but clearly the people who do get electrocuted conduct better than my dry skin.

OK according to wiki IEC uses something between 1200 and 3200 ohm with 50% measuring 1875 ohms (at 100V). This sounds authoritative and very precise, I suspect reality is much more variable.

Note: Dry skin is pretty much an insulator, while we are a sack of salt water inside so any break in our skin can cause lower resistance and more current. Sweaty skin also matters.

This is pretty arbitrary but I'd use closer to 2k for your test, and this will still over current the fuse and let it open. I wouldn't worry too much about starting at a zero crossing... There are other much larger variables involved (like actual skin resistance) making that much precision un-important.

JR 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: frank kayser on March 05, 2014, 09:27:54 PM
Ummmmm.... I really was a pyro. REALLY!!!!

I did wait until my boys were 16 years old before I would share my favorite napalm recipes with them. That's not too young, is it???
Naw.  I think that age is just about right.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jonathan Johnson on March 05, 2014, 10:50:51 PM
Ummmmm.... I really was a pyro. REALLY!!!!

I did wait until my boys were 16 years old before I would share my favorite napalm recipes with them. That's not too young, is it???

Overheard at a church campout: a father, telling his preteen sons, "if you're going to play in the fire, play in it responsibly."
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 06, 2014, 01:35:56 AM

This is pretty arbitrary but I'd use closer to 2k for your test, and this will still over current the fuse and let it open.

JR

My WAG of 1K was due to the guestimate of damp hand-to-lip resistance, since that's the most likely the situation for a guitar player. And you are correct that this resistance changes a lot depending on skin moisture. For example, with dry skin from finger-to-finger you can't feel a 9-volt battery, but touch that same battery to your tongue and it really hurts.  :'( Anything that breaks this dry skin barrier really lowers our basic resistance, and hence increases our fault current and potential electrocution. So I'll do some experimenting using both 1K and 2K meat puppet loads just for grins.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on March 06, 2014, 10:40:11 AM
My WAG of 1K was due to the guestimate of damp hand-to-lip resistance, since that's the most likely the situation for a guitar player. And you are correct that this resistance changes a lot depending on skin moisture. For example, with dry skin from finger-to-finger you can't feel a 9-volt battery, but touch that same battery to your tongue and it really hurts.  :'( Anything that breaks this dry skin barrier really lowers our basic resistance, and hence increases our fault current and potential electrocution. So I'll do some experimenting using both 1K and 2K meat puppet loads just for grins.

I tried sticking one lead of my ohmmeter in my mouth and the DCR still remains pretty high...

Wetting both hands with salt water gave me the lowest reading around 40k from hand  to hand, so still lots of dry skin involved. Path is probably wet hand to core, and core to other wet hand.  I suspect the dangerous combination is a full body sweat where there is a damp/wet path covering lots of surface area reducing the resistance into and out-of the lower resistance core.

Note: Core resistance varies too with how hydrated we are... while that is probably a secondary effect relative to surface salt water.

Have fun, maybe measure current with your RC shock blocker too.

Good advice, try not sweating or having open wounds while performing, or working around electricity.  8)

Note: While perhaps not obvious this skin resistance varies somewhat with voltage, so higher voltage will measure lower resistance, and when skin gets damaged by high voltage, the resistance drops even lower. There is also a body capacitance effect affecting AC current, but the LF DCR should characterize it adequately for our inquiry (120v-60Hz).   

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 06, 2014, 01:05:02 PM
I tried sticking one lead of my ohmmeter in my mouth and the DCR still remains pretty high...

JR

I'm sure I speak for everybody on this forum that we want to see a picture of that.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on March 06, 2014, 01:13:21 PM
I'm sure I speak for everybody on this forum that we want to see a picture of that.

and they want ice water in hell....

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on March 06, 2014, 02:52:18 PM
and they want ice water in hell....

JR

Here you go...  ;D
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bill Whitlock on June 13, 2014, 12:43:38 AM
Heading off topic a bit, but for a current loop to exist there must be potential (voltage) difference leading to a possible hazardous situation at the isolating section, besides the NEC requiring any metal parts-plumbing, etc to be bonded to ground. Obviously fixed one problem (so does clipping a ground pin off a 3 prong plug sometimes) but maybe creating another??  Or am I thinking wrong?

Sorry to be so very late to this party (long story) but let me clarify the situation I described:  The small voltage difference (required in every ground loop to cause current flow ... basic physics) was generated by the voltage drop in the neutral wire (50-100 amps can commonly flow in this) between two power poles in the alley behind these two adjacent buildings. Since each had its own service drop, there was now a small voltage difference between the neutral/ground bond points in the two breaker panels. Code requires (and rightfully so) that there be a connection between the neutral/ground bond and plumbing in each building. The purpose of this connection is not to "ground" the electrical service (that must be done outside the building walls with a rod - and provides a path for lightning) but to prevent the possibility of electrical safety ground being at a different voltage than the building's plumbing system. Both buildings properly had this connection to plumbing ... and the plumbing was fed from the same water main in the street. Now there's a loop ... and I found some 13 amps flowing in the incoming water line ... which ran about 12" under the floor at the "bad" end of the stage. The cure was the aforementioned insulated plumbing connector. There is no Code violation here since Code simply requires that a building's plumbing be tied to safety ground. It does not require that you ground the plumbing outside the building. In Code language, the connection is not serving as a "ground electrode".  Such situations can be easily found using an inexpensive magnetic field meter (I use the $150 Tri-Field meter). Normal "ambient" magnetic fields in buildings are under 2 milli-gauss, usually less than 1 milli-gauss. I found that at the "good" end of the stage in this rehearsal studio, but as I walked to the other end, the meter climbed to 30 milli-gauss at waist level and rose to over 100 milli-gauss when on the floor ... hence my question to the owner "What's under there?".  The other clue to guitar hum being of magnetic field origin is that it will change with the orientation of the guitar. If you stand up with it and rotate your body, the hum will change if a magnetic field is causing it. Anyway, I'm starting to get into lecture mode here ... and I don't have the time tonight. My working situation has changed and I expect to be monitoring this forum much more in the near future. Hope this little dissertation was helpful to guitar players ... hum can be caused 3 ways:  bad shielding (well placed aluminum foil or ground wires to guitar's hardware will fix); ground loops between pedal stuff and amplifier ... or using a DI box that can't isolate ground; or ambient magnetic fields as described here.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Bill Whitlock on June 13, 2014, 01:35:49 AM
There's one primary reason for this effect, and certainly a few smaller ones. So please, post whatever mix of this and that you like. Nobody here's going to judge you for a crazy answer. In fact, a lot of my fundamental ideas on power and sound systems seemed a little crazy at first read (RPBG's for instance) but have since been validated by peer review.

No little green Martians, though.... as I'm pretty sure they have nothing to do with guitar buzz.

I'd say "none of the above". This behavior is clearly a shielding (against electric fields, not magnetic) issue. A guitar that has every metal part connected to the sleeve of its output jack ("grounded"), and similarly grounded metal shielding enclosing all the switches, pots, and wiring will not behave this way. Only if you have a "magnetic" personality ... and AC at that ... could that be a cause. In most of the cases I've dealt with, it's the unshielded internal wiring that's the culprit. If you don't touch any of the guitar's metal parts (presumably grounded), the ambient electric field, which your body assumes unless you purposely ground yourself, is coupled through the backside of the guitar into the wiring, causing buzz. When you touch the grounded metal parts of the guitar, your body is now at the same (grounded) potential and the buzz will stop.  That's my theory ... and I'm sticking with it!
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Helmut Gragger on February 01, 2015, 04:27:38 PM
Hi folks,
I am new here, just stumbled over this discussion through a web search.

May I add a few remarks.

I have a Strat guitar equipped with single coils. All cavities and metal parts are shielded and tied to a common point. This used to be connected to signal ground with a 600V cap as described above (470 nF if I recall correctly). I bridged the cap later since it made problems I cannot remember. So it is stock so to speak as far as ground wiring goes.

I have installed a big air-coil á la Suhr which makes it vastly insensitive to external hum and the like.
This is just to describe the situation I am talking about.

This guitar is totally insensitive to proximity. It is quiet, whether I touch the strings or not.

I also own a LP look-a-like equipped with L-90 pickups (humbuckers), treated as above with the cap intact.
I noticed some funny hum problems when I turned the volume pots which I attributed to the cap.
It was clearly a ground loop problem, not mains hum. The pickups are dead quiet, which can be proven if they are switched to single coil.
For the moment, the cap is disabled and that hum is gone.
However this guitar is very susceptible to the described phenomenon of string noise when untouched despite proper shielding, grounding etc.

If I find anything I will post it here.

One remark on the 100kOhm resistor: although 1/4 W may suffice for matters of power disappation, the resistor may not be rated for such big voltages. For this reason, I would recommend using a 2W type metal oxide or the like. A look at the spec´s will make you wiser.

I will see if this "mod" does make the earlier described hum problem go away.

Also, I agree that an amplifier should be grounded properly, but this does not nedessarily mean that the signal ground is on earth potential. In fact it may not even have any relation to earth. AFAIK there is no global rule for doing this. Up till now the best version I saw was here (http://sound.westhost.com/earthing.htm), but this does not help us for given equipment.

I recently had a lunchbox amp that produced funny crackling noises dependent on the guitar when you wiped your finger over the plastic - static electricity.
It turned out the thing was not earthed at all, it was fully encapsulated, which is technically correct.

The phenomenon was cured by connecting a cable from its recording output to, say, a Fender amp input (turned off), which is grounded. This was ridiculous and the amp was returned despite any other virtues it may have had. The company was not aware of such problems, but it is a shame that they are not aware of persistent noise problems coming from antenna pick-up.

have fun,

Helmut, from the Tyrol, Austria



Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on February 01, 2015, 05:11:14 PM
Helmut,

Per the rules of this forum you need to use your full first and last name to post messages. Please go to the control panel and change your screen name to your actual name.

Thanks very much. And welcome to the forum.
Title: Posting Rules
Post by: Mac Kerr on February 01, 2015, 05:12:20 PM
Hi folks,
I am new here

Please go to your profile and change your display name to your real name as required to post in these forums.
Title: Re: Posting Rules
Post by: Helmut Gragger on February 02, 2015, 03:17:46 AM
Please go to your profile and change your display name to your real name as required to post in these forums.

Some forae require that. Done.

May I dwell on the theme of this thread.
I tried the 100k mod, and it changes nothing for me.

I noticed the hum that appears with the cap comes from my body.

As soon as I touch the earth lug on my power socket it becomes markedly lower. So, although I like the idea of a cap, it has to go.

I also noticed that the crackling and hissing noise that appears, when I don´t touch the strings, is worst in humbucking mode. In single mode the noise level rises anyway, so it is hard to say, but in parallel mode the guitar is almost silent. If you know what to listen for, it is there, but much, much diminished.

This too goes away with earthing, so this supports the theory that the human body is the cause of the problem, not the guitar.
The body seems to act as an antenna for all sorts of electromagnetic junk. Huh, were you aware of that?

One logic explanation for the humbuckers to fail cancellation is the fact, that their principle is to combine an out-of-phase signal with the original signal for cancellation. The catch is that the distance of the coils must be small compared to the wavelength of the to-be-cancelled signal, otherwise no 180 deg phasing would occur, a paradigma that is certainly not given for all frequencies. In fact, if the phasing is suitable, the noise might increase.

So it looks like the only cure would be the earlier mentioned wrist strap for earthing. BTW, such things are readily available for workplaces for assembling surface mount devices. They are called "ESD wrist strap", or, indeed, "ground bracelet".

-helmut
Title: Re: Posting Rules
Post by: Helmut Gragger on February 02, 2015, 04:55:23 AM
Please note. Important.

The aforementioned ground bracelet will be an elegant solution at least for recording situations, when the optics does not hurt.

Note that this device has a 1MOhm resistor to ground. It is meant to discharge electrostatic voltages to ground (earth), where no noteworthy currents are involved. I have verified the function thereof for quietening the human body as an antenna in the proximity of a guitar.

Although touching real ground does the same, one should resist to ground the body by a direct hard connection (without a resistor), because the vicinity of an amplifier with potentially malfunctioning earth connection would create a beautiful, potentially lethal current path through the body as displayed on the video earlier up this thread.

The solution is to maintain contact to the guitar ground with such a bracelet, which most conveniently is accomplished by attaching the alligator clip to one of the strings. Any which way the bracelet is applied, the 1MOhm resistor will limit all currents to a safe level.

-helmut

Title: Re: Posting Rules
Post by: Kevin Graf on February 02, 2015, 10:49:42 AM
This too goes away with earthing, so this supports the theory that the human body is the cause of the problem, not the guitar.
The body seems to act as an antenna for all sorts of electromagnetic junk. Huh, were you aware of that?
-helmut
The human body acts as a capacitor connected to Mother Earth.  The capacitor value can range from about 100 pF to 700 pF depending on a number of factors.  A Non Contact Voltage Probe needs this capacitance to work as do portable radios (at some frequencies). 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on February 02, 2015, 11:28:42 AM
Welcome to this very old thread.

You mention that cap coupled ground was somehow problematic at 470 nF but a 1 Meg ohm via wrist strap connection was adequate.

First 470 nF seems large, IIRC 47 nF is more reasonable to keep current from a mains connection down to human safe levels.

I can imagine any cap being problematic in series with the guitar pick-up but that is not the intended connection, it is only between the exposed metal parts and the guitar amp input ground.

I had to google Suhr air coil and that appears to patented guitar pickup technology? This may be familiar to some of the guitar players here (not me, but google is my friend).

I am interested in human safety aspects of guitar players exposed to dangerous voltages from faulty guitar amps or improper stage wiring that presents a dangerous voltage potential between guitar and microphone. A performer was just killed down in Argentina from such an accident.

Your 1M wrist strap is surely not a shock path hazard, but I wouldn't hold my breath for getting musicians to wear one, or even one of the less invasive factory static management methods (like anti-static carpets and grounding foot straps) etc..

JR 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Helmut Gragger on February 02, 2015, 03:15:28 PM
Welcome to this very old thread.

I was warned that the last post was more than three months ago. Who cares, nobody found the solution yet.

You mention that cap coupled ground was somehow problematic at 470 nF but a 1 Meg ohm via wrist strap connection was adequate.
First 470 nF seems large, IIRC 47 nF is more reasonable to keep current from a mains connection down to human safe levels.

The 470nF was recommended by the guitarnuts site (http://www.guitarnuts.com/wiring/shielding/shield3.php).
And yes, a 1MOhm between me and the earth contact did the job.

I can imagine any cap being problematic in series with the guitar pick-up but that is not the intended connection, it is only between the exposed metal parts and the guitar amp input ground.

That´s the way it is used.

I had to google Suhr air coil and that appears to patented guitar pickup technology? This may be familiar to some of the guitar players here (not me, but google is my friend).

Just for the records: it is explained here (http://www.fralinpickups.com/bpncs.asp).
I just mentioned this to make clear that it is not mains hum I am talking about. The Suhr coil is a big, core-less air-coil that functions akin to the second coil of a humbucker.

Your 1M wrist strap is surely not a shock path hazard, but I wouldn't hold my breath for getting musicians to wear one,

Yes, it would be detrimental to somebody´s macho image of a long-haired axe-wielder, but nobody would see it if they were desperate in a recording session ;D, sssh!

or even one of the less invasive factory static management methods (like anti-static carpets and grounding foot straps) etc..

This works?

Well, the outcome of this experiment was, there is currently no non-invasive solution. The cap made funny problems, maybe I should try a smaller one as you here suggested.

-helmut
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on February 02, 2015, 03:44:37 PM
I was warned that the last post was more than three months ago. Who cares, nobody found the solution yet.
I guess that depends on what problem you are trying to solve. My focus (and this forum) is generally on trying to prevent electrocution deaths, not single coil pickup hum.
Quote

The 470nF was recommended by the guitarnuts site (http://www.guitarnuts.com/wiring/shielding/shield3.php).
And they were trying to block DC voltage from a rogue guitar amp, not AC current from miswired mains.

470 nF is 6.7k @ 50Hz. 230VAC is 34mA and enough current to disturb heart rhythm if traveling through the musician's core.

Quote

And yes, a 1MOhm between me and the earth contact did the job.

That´s the way it is used.

Just for the records: it is explained here (http://www.fralinpickups.com/bpncs.asp).
I just mentioned this to make clear that it is not mains hum I am talking about. The Suhr coil is a big, core-less air-coil that functions akin to the second coil of a humbucker.

Yes, it would be detrimental to somebody´s macho image of a long-haired axe-wielder, but nobody would see it if they were desperate in a recording session ;D, sssh!

This works?
In a recording studio the threat from bad mains wiring is much less, and extraordinary shielding has been used inside some studios (up to and including a faraday cage). I wonder if a wide open single coil pickup would sound natural with no hum at all, not unlike the fake feedback they add to movie soundtracks whenever an actor uses a microphone in front of a PA.  8)
Quote

Well, the outcome of this experiment was, there is currently no non-invasive solution. The cap made funny problems, maybe I should try a smaller one as you here suggested.

-helmut

I can not guess what you mean by "funny" problems. A smaller cap is likely to be even funnier if wired in series with the guitar pickup. Like i tied to explain the cap is supposed to be wired only between the ground coming from the guitar amp, and the exposed metal parts on the guitar (not in series with the audio circuity).  The premise is that the reactance of a 47nF cap,  which is lower impedance than your 1M wrist strap, should be adequate to quiet down the player's self noise when he touches that exposed metal on the guitar. The rationale for the smaller value cap is to keep current from a mains wiring (or faulty amp dumping AC like from a bad stinger cap) below safe levels. (single digit mA).   

JR

PS: Reportedly the cap in series with exposed guitar metal does not make it quieter or noisier, just safer. 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Helmut Gragger on February 03, 2015, 01:40:20 PM
I guess that depends on what problem you are trying to solve. My focus (and this forum) is generally on trying to prevent electrocution deaths, not single coil pickup hum.

I understand that. Yet the initial subject of this thread was the question where the noise comes from. That´s how I stumbled over it.

And they were trying to block DC voltage from a rogue guitar amp, not AC current from miswired mains.

Thats a point. I agree on a smaller cap.

I can not guess what you mean by "funny" problems.

I cannot remember why I finally removed it again on the Strat, this was months ago. There was some problem. However, on the LP it made the problems described earlier, like some hum in certain positions of the volume pot, which came and went with the proximity of the body and disappared totally when I made direct contact. I cannot explain that.

A smaller cap is likely to be even funnier if wired in series with the guitar pickup.

Looks like my two attempts to make that clear were a failure. The cap was never in the tone path, always the way you describe.


PS: Reportedly the cap in series with exposed guitar metal does not make it quieter or noisier, just safer.

I´d be happy if it would not make the guitar louder, but it does (BTW, I am not talking about single coils). However the discussion here turned my focus to the practical tests and insight that (and here we are back to the original subject of this thread...)  there is nothing on the guitar that can be done to remedy that buzz.

-helmut
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on February 03, 2015, 03:22:07 PM
FWIW I understand the guitar players are picky about their sound so any safety approach must not interfere with best sound.

One guitar maker uses something like a 7mA fuse inside their guitars but those are pretty expensive and 7mA is on the higher limit of what I would like to see (clearly better than nothing). Simple fuses down at low single digit mA do not appear to be practical and/or cost effective.

In another thread I am looking at two different hypothetical solutions for leaving the guitar fully grounded and still protecting the human.

The more expensive approach involves a 3 pole relay that disconnects power and ground when it detects a shock hazard. My less expensive approach that is barely at the schematic stage now, is to fabricate an electronic fuse with a pair of power mosfets in series, that when turned off provide 400V of isolation. I propose using a 100 ohms resistor in series with this solid state fuse to sense for current, but a smaller R with more precise detection could be used if really needed. I suspect 100 ohms is adequate for shielding.

Good luck with your mission and thanks for the clarification.. I wonder if guitar controls (like pots) might benefit from having their metal case hard grounded, but floated from the guitar's external metal shielding that is cap coupled?

Ideally providing my safety precautions external to the guitar is probably more acceptable to the guitar players than messing around inside their axe. 

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Helmut Gragger on February 04, 2015, 03:55:51 PM
FWIW I understand the guitar players are picky about their sound so any safety approach must not interfere with best sound.

 ;D You nailed it my friend. View it that way: good tone saves their day, so rather die if the tone is bad.

In another thread I am looking at two different hypothetical solutions for leaving the guitar fully grounded and still protecting the human.

That is very interesting. Could you point me to this thread? Please understand, I find this forum is not one of the usual guitar modding places I am hanging around, just the title of the thread was suggesting so. I cannot imagine where this would be buried under. Thanks.
I yesterday tried to find this thread starting from the top of the forum - no chance. I had no idea where this would be dug under. Had to resort to the search function.

I wonder if guitar controls (like pots) might benefit from having their metal case hard grounded, but floated from the guitar's external metal shielding that is cap coupled?

Hmm. That´s easily tried. That would also make the shielding more solid (which is not accessible from outside). I use a smaller cap as you suggested (a Y-cap perhaps? I have tons of them lying around.) and let you know.

-helmut
Title: Re: Guitar stops buzzing when I touch the strings
Post by: John Roberts {JR} on February 04, 2015, 04:19:59 PM
;D You nailed it my friend. View it that way: good tone saves their day, so rather die if the tone is bad.
Yup, they may use some unsafe legacy guitar amps for the tone, but science is our friend, and we may be able to protect them from themselves without stepping on the tone.
Quote
That is very interesting. Could you point me to this thread? Please understand, I find this forum is not one of the usual guitar modding places I am hanging around, just the title of the thread was suggesting so. I cannot imagine where this would be buried under. Thanks.
The parent forum is about live sound reinforcement, but the sub-forum where this discussion is located specializes in "AC power and grounding."  During live performances guitar players who also sing can find themselves bridging between two different power drops. As long as both power drop grounds are 0V no problem, but stuff happens, so a mic and guitar ground may have mains voltage between them.  :o   
http://forums.prosoundweb.com/index.php/topic,152668.msg1400186.html#msg1400186
Quote

I yesterday tried to find this thread starting from the top of the forum - no chance. I had no idea where this would be dug under. Had to resort to the search function.

Hmm. That´s easily tried. That would also make the shielding more solid (which is not accessible from outside). I use a smaller cap as you suggested (a Y-cap perhaps? I have tons of them lying around.) and let you know.

-helmut
A Y cap (self healing) is best for worst case scenario.

JR
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on February 04, 2015, 04:59:23 PM
;D You nailed it my friend. View it that way: good tone saves their day, so rather die if the tone is bad.

Yes, an oldie but goodie thread comes back to life. I just had an interesting install gig that's on topic for this thread. I'm installing a new digital console with a digital snake in a church with a rather large platform/stage. It's some 60 ft wide and 3 ft tall. They're putting a portable stage directly in front of the main stage for their new Saturday night youth service. And this B-stage has a drum kit, electric guitar, electric bass, multiple acoustic guitars, and a bunch of singers. Just what you would expect for this sort of service. Last week I was there to assist with the first load-in and sound check for this new stage, and the e-guitar player was complaining about her guitar buzzing "again" and that it must be ungrounded because if she touches the metal pickup guard or holds the strings the buzzing will stop. This was a single-coil Telecaster which as we all know is subject to RF induced buzz. A quick demonstration of me moving the guitar close to her body (buzz) and moving it away (no buzz) showed that she was the source of the noise. Then I grounded myself on another back line amplifier I had previously tested for grounding, and had her touch my hand while her guitar was hanging on the strap in front of her and buzzing like crazy. As we could predict from this thread, the buzzing stopped.

OK, that's old news that we all understand, but here's the next experiment. I had her spin in a circle slowly in buzz mode while standing on the stage and the buzz didn't change. But when she stepped away from the stage and stood in front of it, the buzzing was reduced significantly. It was like the stage itself was radiating the same kind of buzz we get from neon signs in a bar.

A week later I was climbing around under the stage to hook up a few speaker drops and saw that it's all metal studding. And there's many J-boxes connected to the studs for strength. And lots of the receptacles are the orange isolated ground types. So I started to wonder if the metal studs under the wooden stage were grounded/bonded to the electrical system. Then today the tech director drops the big bombshell, that they regularly cross connect some of the dimmers to power stage boxes and non-dimming fluorescent fixtures, and just set the lighting console so that the "dimmer" channel can be only 0% or 100%. I explained to her that Triac dimmers never really get to 100%, it's more like 99% on, and that plugging audio gear into a Triac patched receptacle was a VERY bad idea that could be causing her buzz. Especially since she now tells me that ANY electric instruments she puts up on the stage, including digital drums, buzz like crazy.

My hypothesis is that the metal stage structure may never have been bonded to the electrical service ground, and there could be a bunch of DIY installed electrical receptacles that are improperly bonded to stage metal. That's might causing this stage to re-radiate the dimmer hash from the 99% full-on Triacs. Or something like that.

I'm supposed to be doing some stage box rewiring at this church in the next week or so, and should have time to set up an experiment with buzzing guitars and drums. I'll begin by bonding the metal stage structure to the ground in the service panel. I'll also bring an o-scope for the AC power and look for the tell-tale spike caused by a Triac dimmer at "full-on". If I'm correct, then bonding the stage superstructure to the service panel ground and un-patching the dimmer racks from the stage receptacles should eliminate the buzz in the e-guitars and d-drums. But we shall see.

Sounds like another really good reason that metal stages should be properly bonded to ground. In this case an ungrounded stage really COULD mess up the sound of the guitar with a bunch of extra buzzing.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Scott Holtzman on February 05, 2015, 03:37:41 AM
Yes, an oldie but goodie thread comes back to life. I just had an interesting install gig that's on topic for this thread. I'm installing a new digital console with a digital snake in a church with a rather large platform/stage. It's some 60 ft wide and 3 ft tall. They're putting a portable stage directly in front of the main stage for their new Saturday night youth service. And this B-stage has a drum kit, electric guitar, electric bass, multiple acoustic guitars, and a bunch of singers. Just what you would expect for this sort of service. Last week I was there to assist with the first load-in and sound check for this new stage, and the e-guitar player was complaining about her guitar buzzing "again" and that it must be ungrounded because if she touches the metal pickup guard or holds the strings the buzzing will stop. This was a single-coil Telecaster which as we all know is subject to RF induced buzz. A quick demonstration of me moving the guitar close to her body (buzz) and moving it away (no buzz) showed that she was the source of the noise. Then I grounded myself on another back line amplifier I had previously tested for grounding, and had her touch my hand while her guitar was hanging on the strap in front of her and buzzing like crazy. As we could predict from this thread, the buzzing stopped.

OK, that's old news that we all understand, but here's the next experiment. I had her spin in a circle slowly in buzz mode while standing on the stage and the buzz didn't change. But when she stepped away from the stage and stood in front of it, the buzzing was reduced significantly. It was like the stage itself was radiating the same kind of buzz we get from neon signs in a bar.

A week later I was climbing around under the stage to hook up a few speaker drops and saw that it's all metal studding. And there's many J-boxes connected to the studs for strength. And lots of the receptacles are the orange isolated ground types. So I started to wonder if the metal studs under the wooden stage were grounded/bonded to the electrical system. Then today the tech director drops the big bombshell, that they regularly cross connect some of the dimmers to power stage boxes and non-dimming fluorescent fixtures, and just set the lighting console so that the "dimmer" channel can be only 0% or 100%. I explained to her that Triac dimmers never really get to 100%, it's more like 99% on, and that plugging audio gear into a Triac patched receptacle was a VERY bad idea that could be causing her buzz. Especially since she now tells me that ANY electric instruments she puts up on the stage, including digital drums, buzz like crazy.

My hypothesis is that the metal stage structure may never have been bonded to the electrical service ground, and there could be a bunch of DIY installed electrical receptacles that are improperly bonded to stage metal. That's might causing this stage to re-radiate the dimmer hash from the 99% full-on Triacs. Or something like that.

I'm supposed to be doing some stage box rewiring at this church in the next week or so, and should have time to set up an experiment with buzzing guitars and drums. I'll begin by bonding the metal stage structure to the ground in the service panel. I'll also bring an o-scope for the AC power and look for the tell-tale spike caused by a Triac dimmer at "full-on". If I'm correct, then bonding the stage superstructure to the service panel ground and un-patching the dimmer racks from the stage receptacles should eliminate the buzz in the e-guitars and d-drums. But we shall see.

Sounds like another really good reason that metal stages should be properly bonded to ground. In this case an ungrounded stage really COULD mess up the sound of the guitar with a bunch of extra buzzing.

Mike, in data centers and in transmitter shacks at radio stations we bond everything.  The fence, the racks, the conduit.  Usually a halo is installed for easy connection.  A giant ungrounded portable platform is an antenna, plain and simple.

For safety and RF reasons we should be as diligent in our grounding practices.  For portable it doesn't even have to be awesome, I would just get some welding cable (nice and soft, easy to role) and a selection of clamps.  For permanent structures Cadweld is your friend.

Title: Re: Guitar stops buzzing when I touch the strings
Post by: Jonathan Johnson on February 05, 2015, 03:32:35 PM
Sounds like another really good reason that metal stages should be properly bonded to ground. In this case an ungrounded stage really COULD mess up the sound of the guitar with a bunch of extra buzzing.

Previous discussions of stage grounding have centered around the human-safety aspect. This points out that there is also an electronics aspect. Where a metal-framed stage with a wood deck might be given a "pass" and not be grounded for the human-safety aspect since there is a nonconductive surface between the talent and the structure, your example shows that a wood-decked stage with a metal substructure should still be grounded for technical reasons if not for human safety.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on February 05, 2015, 04:32:36 PM
Previous discussions of stage grounding have centered around the human-safety aspect. This points out that there is also an electronics aspect. Where a metal-framed stage with a wood deck might be given a "pass" and not be grounded for the human-safety aspect since there is a nonconductive surface between the talent and the structure, your example shows that a wood-decked stage with a metal substructure should still be grounded for technical reasons if not for human safety.

That's exactly my point. Currently whatever is happening around this stage or its AC power distro is making it nearly unusable for electrically amplified instruments with high-impedance signals. Hard-wired low-z mics seem to be fine. And RF mics seem to be fine. But anything hooked up with unbalanced connections is a buzz factory. So any single-coil pickup guitar is nasty, and a digital drum kit with unbalanced outputs run into multiple DI boxes is buzzy. At first blush it appears the tech crew is doing everything right. But the buzzing is making them do dangerous things, like cutting off ground pins on extension cords. Again, that's a recipe for disaster. I'm going to look at this stage very closely over the next few weeks and try a few grounding/bonding experiments while instruments are buzzing on stage. This could become the poster child for properly grounding/bonding metallic stage structures, even if they're NOT a shock hazard for the musicians.

Again, this is all speculation, but I could be on to something.... 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on February 05, 2015, 06:11:02 PM
Previous discussions of stage grounding have centered around the human-safety aspect. This points out that there is also an electronics aspect.

This also suggests that something like an non-grounded metal railing around a stage could cause guitars to buzz a lot more than usual. What about drop-ceiling grid? I know that the drop ceiling grid in my kitchen isn't bonded to ground. Doesn't that become a big antenna for radiating buzz? Of course, I don't play a lot of guitar in my kitchen, but I've been in a lot of performance spaces with a metal grid drop ceiling.

Seems like a really interesting area of study, especially since it could help make stages safer in addition to being electrically quieter.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Stephen Swaffer on February 05, 2015, 08:41:18 PM
Most drop ceilings have lights installed in them. Can lights would effectively ground the grid, flourescent troffers might be iffy because of paint.  However, I have never met a drop ceiling grid that wasn't happy to provide an effective ground for the electrician on top of a fiberglass ladder.

The metal framing is a safety issue in that if a hot wire in one of the boxes touches the box it will energize the framing.  If this were properly wired (at least as inspected around here) the boxes would all be grounded regardless of the receptacle configuration.

Another typically ungrounded metal screen is the wire mesh used for stucco, etc.  Out of sight out of mind!  In older buildings, tin ceilings are usually ungrounded.  The one in our church auditorium was actually energized for some time due to some volunteer wiring.  Surprisingly, it never created a hum issue in the sound system.
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Mike Sokol on February 05, 2015, 09:14:25 PM
Another typically ungrounded metal screen is the wire mesh used for stucco, etc.  Out of sight out of mind!  In older buildings, tin ceilings are usually ungrounded.  The one in our church auditorium was actually energized for some time due to some volunteer wiring.  Surprisingly, it never created a hum issue in the sound system.

Just a clarification. I'm not suggesting that an ungrounded structure like a metal ceiling will cause a HUM. My point is that it can cause a BUZZ. Those are two totally different sounds caused by different interferences. While HUM is typically caused by ground loop currents circulating in low-z signal shields connected between different pieces of audio gear, BUZZ can occur when high frequency energy caused by dimmers, switching power supplies and neon signs radiates through the air and couples into high impedance audio circuitry such as single coil guitar pickups.

The first thing I do when troubleshooting these noises is to determine if we're hearing Hum, Buzz, Hiss or Hash (which I've covered in another thread) then try to define just what the noise source is. For instance, I had an electric keyboard with a BIG 60-Hz HUM a few months ago which was NOT a ground loop. They had used a non shielded speaker cable to go from the keyboard output to the DI box input. Again, this didn't BUZZ, but it really HUMMED. I guess it Hummed rather than Buzzed because the keyboard had a low enough output impedance that it shunted out the high "buzz" harmonics, but allowed a magnetic coupling to the 60 Hz sine waves in the electrical wiring all around them. 
Title: Re: Guitar stops buzzing when I touch the strings
Post by: Stephen Swaffer on February 05, 2015, 10:40:31 PM
I'm not suggesting that an ungrounded structure like a metal ceiling will cause a HUM. 

Understood-and since it did not create an issue maybe it is not surprising-in this case I wondered why 2000  + sq feet of energized tin, in close proximity too speaker wiring and parallel to choir mic (condensers) snake wiring for 75 feet did not induce some hum.  Perhaps, since no current was flowing there would not have been any magnetic fields.

What I find interesting is the various wiring schemes over the years.  When our building was converted from gas lighting to electric in the 30's all of that wiring was done with rigid conduit.  Even though it is rusty in many places, it still provides a good ground electrically-not sure how good it is for technical gear as all of that is on newer wiring.  We did have a bad CFL create a lot of noise in the system-and they are run on the rigid conduit wiring.