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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 16069 times)

Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #110 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...

John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #111 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
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Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #112 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

Stephen Swaffer

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Re: Guitar stops buzzing when I touch the strings
« Reply #113 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.   
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Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #114 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???

Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #115 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

John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #116 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 
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frank kayser

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Re: Guitar stops buzzing when I touch the strings
« Reply #117 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.
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Jonathan Johnson

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Re: Guitar stops buzzing when I touch the strings
« Reply #118 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."
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Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #119 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.
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