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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

[Bob Leonard]

Where?  Between the bridge/strings and ground?  That won't affect the tone.


Steve.

And it won't do anything either.

[John Roberts {JR}]

And it won't do anything either.

I am not sure which is the right page in the ARRL handbook (my 1961 copy is long gone), but this is about relative impedances, voltage dividers and simple one pole filters..

A few 10k pF can be a relatively low impedance to the hum coming from a nearby meat antenna, and low relative to the megohm  input impedance of a guitar amp.
------
Cap coupling just the strings and external metal parts, while hard grounding the pick-up seems OK if the pickup does not present exposed metal (like a grounded can), but that still seems like a lesser risk than having all the metal parts hard connected to ground.   

I still wouldn't mess around inside any axe that may have some legacy value. A cap wired into a guitar cable connector shell seems less invasive. and if your audience complains about the tone, you can blame it on the cable, and swap it out. 

One of you guitar boys could experiment with how small of a cap value you can get away with before you hear the low frequency fall off (this can depend somewhat on the amp input Z), and shielding effectiveness diminish. I was suggesting a 0.1uF (for 120VAC). One experienced guy on that other forum says that he typically uses 0.02uF. If a .02uF works, my .1uf is 5x over-kill.

Ok this is not apples to apples, but i just did another little experiment. With me sitting here ungrounded touching the center pin of an RCA cable connected to my computer speaker amp. As expected this makes a bunch of hum... Then I tried grounding myself through my trusty old capacitor diddle box while touching the pin. Grounding myself through 100 pF made almost no different in the hum level. 1000pF made a very audible reduction in hum. .01uF knocked it way down, with only slight (but audible) differences between .02uF and .22 uF. I also noticed a difference from how tightly I gripped the diddle box lead, so my skin resistance was a factor too. While a guitar amp has more voltage gain than my computer speakers, you are not literally touching the input lead of your guitar amp, only standing near the pickup coil so my little experiment may be representative.   

I'm sure somebody will claim to hear a 10 uF cap in series.. 

JR
 

[Bob Leonard]

JR,
Send me the schematic for the gadget you were talking about in a PM. I have an SG we use for a test bed.

I have a couple of other methods for killing hum in guitars that are unique. I'll post them when I get time.

[John Roberts {JR}]

JR,
Send me the schematic for the gadget you were talking about in a PM. I have an SG we use for a test bed.

I have a couple of other methods for killing hum in guitars that are unique. I'll post them when I get time.

I do not have a schematic... just open the ground in the cord going to the guitar and solder a cap in series with that ground. My calculations suggest a 0.1uF will limit current from exposure to 120V 60Hz to around 5 mA, that should be sub-lethal. Other people who have actually performed this mod say 0.02uF is adequate...

My crude testing suggests that .02uF may work OK and will be only 1 mA of fault current.

Ideally this cap should be 200V or more but for noise testing any old cap will do.

JR

[Steve M Smith]

I find it strange when people are suggesting that having exposed metal parts connected to ground could be dangerous.

In the UK, all exposed metal parts are supposed to be connected to ground/earth for safety.

Surely it's only a problem if your ground connection isn't really grounded.

This seems to be quite common in the US from what I have read on this forum but almost unheard of here.


Steve.

[Bob Leonard]

I do not have a schematic... just open the ground in the cord going to the guitar and solder a cap in series with that ground. My calculations suggest a 0.1uF will limit current from exposure to 120V 60Hz to around 5 mA, that should be sub-lethal. Other people who have actually performed this mod say 0.02uF is adequate...

My crude testing suggests that .02uF may work OK and will be only 1 mA of fault current.

Ideally this cap should be 200V or more but for noise testing any old cap will do.

JR

OK, I've tried that in the past with no success, but I'll try it again. I have another SG with P-90 single coils that makes all kinds of noise, especially in my basement under fluorescent lights. I'll try with that first.

[Mike Sokol]

I find it strange when people are suggesting that having exposed metal parts connected to ground could be dangerous.

In the UK, all exposed metal parts are supposed to be connected to ground/earth for safety. Surely it's only a problem if your ground connection isn't really grounded. This seems to be quite common in the US from what I have read on this forum but almost unheard of here.

Steve.

Steve, you are correct. I work with UK sound crews at times and they're typically aghast at US power and grounding. We do tend to have loosey-goosey grounding here.

[Jerome Malsack]

I have also seen someone on the internet who setup a guitar as balanced to the amp.  Would the CMRR help with this??

[John Roberts {JR}]

I find it strange when people are suggesting that having exposed metal parts connected to ground could be dangerous.

In the UK, all exposed metal parts are supposed to be connected to ground/earth for safety.

Surely it's only a problem if your ground connection isn't really grounded.

This seems to be quite common in the US from what I have read on this forum but almost unheard of here.


Steve.

To be logical about this there only needs to be a solidly grounded chassis, when there is dangerous power circuitry inside that chassis that could fail and energize the metal chassis. Grounding the chassis to the mains system trips the over-current breaker in the case of such a fault.  For the guitar there is no mains power inside that guitar, so little risk of wiring or insulation failure energizing those metal parts. Safety agency rules stipulate that exposed metal parts do not present elevated temperature or voltage to consumers. 

Ironically perhaps, most musician shocks related to guitars come from the guitar amps being dicey. Many old ungrounded line cord guitar amps used stinger caps to cap couple the chassis to the neutral pin of the two wire line cord for less hum. Musicians often get a shock from touching the real ground connected microphone while holding the guitar now energized by the stinger cap (if stinger cap connected to hot side of mains).   

The metal parts of guitars are routinely grounded to the amp to reduce hum noise pickup, not for safety. Many old guitar amps were not grounded themselves, so the guitars are just connected to the amp chassis, whatever potential that may be.   

Note: The stinger caps are mainly delivering a painful shock not stopping hearts, but musicians have fallen off stage or suffered incidental injuries after getting shocked. This may seem like much ado about nothing since deaths from this is rare, but they aren't zero. The sub-lethal shocks are unpleasant so less of that is better too.

I consider the amps with stinger caps a self inflicted wound that is mostly harmless, but venues with dangerous power distribution should not be tolerated.

JR

PS: Yes, the US is a little flaky about mains and outlet wiring. We need to be proactive about our home and work place wiring. 

[John Roberts {JR}]

I have also seen someone on the internet who setup a guitar as balanced to the amp.  Would the CMRR help with this??

Not likely. By balanced I ASSume you mean 3 wire, +/-/ground. Floating the pickup and reading it differentially will not diminish the  hum that looks like valid signal to the pickup.

JR

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