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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 27294 times)

John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #120 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
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Tune it or don't play it... please

Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #121 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.

John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #122 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
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Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #123 on: March 06, 2014, 02:52:18 pm »

and they want ice water in hell....

JR

Here you go...  ;D
« Last Edit: March 06, 2014, 02:55:18 pm by Mike Sokol »
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Bill Whitlock

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Re: Guitar stops buzzing when I touch the strings
« Reply #124 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.
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Bill Whitlock
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Bill Whitlock

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Re: Guitar stops buzzing when I touch the strings
« Reply #125 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!
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Helmut Gragger

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Re: Guitar stops buzzing when I touch the strings
« Reply #126 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 specīs 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, 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



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Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #127 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.

Mac Kerr

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Posting Rules
« Reply #128 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.
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Helmut Gragger

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Re: Posting Rules
« Reply #129 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 donīt 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
« Last Edit: February 02, 2015, 04:03:55 am by Helmut Gragger »
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