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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 102456 times)

Mac Kerr

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Re: Guitar stops buzzing when I touch the strings
« Reply #90 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
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John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #91 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
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Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #92 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.

Stephen Swaffer

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Re: Guitar stops buzzing when I touch the strings
« Reply #93 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!
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Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #94 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.

Mike Sokol

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Re: Guitar stops buzzing when I touch the strings
« Reply #95 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

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.
« Last Edit: March 04, 2014, 07:20:12 AM by Mike Sokol »
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John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #96 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
 
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Stephen Swaffer

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Re: Guitar stops buzzing when I touch the strings
« Reply #97 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?
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Tim McCulloch

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Re: Guitar stops buzzing when I touch the strings
« Reply #98 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.
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John Roberts {JR}

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Re: Guitar stops buzzing when I touch the strings
« Reply #99 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

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Re: Guitar stops buzzing when I touch the strings
« Reply #99 on: March 04, 2014, 11:07:35 AM »


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