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Why does a guitar amp buzz stop when you're touching the strings?

Your body is grounding the guitar
- 14 (43.8%)
The guitar is grounding your body
- 5 (15.6%)
Touching the strings creates a ground loop
- 0 (0%)
The strings are acting like an antenna
- 9 (28.1%)
You've got an electric personality
- 4 (12.5%)

Total Members Voted: 32

Voting closed: September 26, 2013, 09:44:26 AM


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Author Topic: Guitar stops buzzing when I touch the strings  (Read 102443 times)

Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #100 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.

Jonathan Johnson

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Re: Guitar stops buzzing when I touch the strings
« Reply #101 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.
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John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #102 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.
 
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John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #103 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.
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Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #104 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.     

Stephen Swaffer

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Re: Guitar stops buzzing when I touch the strings
« Reply #105 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.
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Tim McCulloch

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Re: Guitar stops buzzing when I touch the strings
« Reply #106 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. :)
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Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #107 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.

Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #108 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.

John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #109 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

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Re: Guitar stops buzzing when I touch the strings
« Reply #109 on: March 05, 2014, 11:07:40 AM »


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