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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 16074 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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Tune it or don't play it... please

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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Bill Whitlock
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Jensen Transformers, Inc.
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"Sadly, marketing has become the art of deception by omission."
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