Jump to content

Signal Ground Vs. Chasis Ground In Guitar Body


Recommended Posts

I suppose this has been done to death, but I don't really know how to phrase the question, so my attempts to search the archive have been fruitless.

Building a fretless bass to use nylon-clad strings. Want to shield it. Guitarelectronics has one build a faraday cage shield around the control area, and separate the signal ground entirely from this shielding. The shielding is connected to you via the bridge wire and a 33pf 400v cap. (I think this is how it goes. See


for before and after schematics.

The problem is that with the nylon string cladding, I won't be providing the contact when I touch the strings.

I guess my question is "would it be somewhat effective to build the shielding shown, but connect it to the signal ground?

If so, what purpose would connecting the bridge to the signal ground serve?

Or if this has been done a lot, how can I look up the relevant archived material?

Edited by Wonko
Link to comment
Share on other sites

I'm not sure what that 0.33uF (which, btw, is 330nF or 330,000pF) cap is for exactly. My first reaction was that it was for shock protection, but the only time I've seen those is when they are parallel with a resistor. Either way, the ground and shielding are not completely separated, they just have that extra component between them. Connecting the ground directly to the shielding is really the only way shielding even works. Connecting the bridge to ground reduces hum when you are touching the strings. However, as you have pointed out, with nylon coated strings, it may not work (I'm sure it depends on how thick the coating is; for instance, I've never witnessed Elixir strings affecting noise reduction due to their coating).

You could always consider piezo pickups for a fretless if you have yet to buy pickups.

EDIT: Maybe the capacitor effects the frequency of the noise being introduced into the circuit, sort of like a tone pot for the hum? Just a though, and probably completely wrong.

Link to comment
Share on other sites

just a thought I had : will the balls of the string still not be in contact with the bridge? therefore grounding the strings? I know you wont get the same effect as you would with bare strings (Where when you touch the strings you become grounded too) but it might help a little still. You might have to keep one hand touching the bridge though which could be a problem depending on your playing style.

Link to comment
Share on other sites

This describes star grounding, the prefered method of wiring to eliminate ground loops by bring all the grounds to one point, I think the cap is to disallow DC voltages (only AC signals can pass through a cap)...

This is particularly needed on strat type single coil pickups which are suseptable to interferance noise and hum...

Your problem is that on a bass you usually wont have contact with the bridge and the strings will be insulated due to the coating...

I would suggest that some form of humbucking pickup, P-bass, stacked or other humbucking design would be the way to go to at least minimise the effect of hum and noise...

Anyone got any other ideas?


Link to comment
Share on other sites

The string ground is only to use your body as part of the shielding for the circuit. If you shield the circuit, your body isn't going to help much so the string ground can be left off.

In a guitar with passive electronics, while it might seem "neet" to claim star grounding and point towards the elimination of ground loops as the justification - without an active component to detect any difference in ground potential, no ground loop will exist. A ground loop is much more than multiple paths to ground.

And when it all comes down to wiring, the shield and the signal ground will share the same grounding point on a correctly wired instrument - that being the output jack.

The use of a cap or even a cap/resistor to make the string ground is highly touted to eliminate shocks - but it hasn't been proven in practice, and I wouldn't stake MY life on it regardless of which website says it will.

Proper grounding of equipment will always have my priority above the use of a cap or cap/resistor in the guitar.

Your choice and your life duration may vary.

Link to comment
Share on other sites

you guys are mostly on the right track here.

John's intention for the cap was to protect against an extremely rare failure in a tube amp, that could cause high voltage DC to be present at the shield of the guitar cable.

the cap lessens, but won't eliminate the current through your body, for AC voltages.

it is in no way a substitute for checking for proper grounding on the AC outlet you plug your amp into.

and IMHO GFCIs are a guitarists best friend.

Joe, as strange as it might seem, the string ground isn't to use your body to help in the sheilding, but the other way round. it's to ground your body so it's not a 'big ol' bucket o' noise'.

it is true that if you have better shielding on your guitar, your body won't be able to couple the hum and noise as effectively to the guitar, but both good shielding and grounding yourself through the strings is better yet.

Robert is right about the ball end of the strings contacting the bridge, and there still being the issue of contact through the strings to the fingers. my guess is the capacitive coupling through the insulated strings will definitely be better than nothing, but not nearly as effective as direct coupling to unclad strings. so HB pickups and attention to detail in cavity shielding will be very valuable here.



Edited by unklmickey
Link to comment
Share on other sites

ok, im freaked out now.. so besides grounding the bridge and the rest of the electronics, what should I do to avoid being shocked?

A - take a deep breath and relax, it's not that difficult to be safe.

B - use an outlet tester (cheap and available at most home centers or hardware stores). lights indicate proper wiring or wiring errors, instructions right on the tester.

C - if your amp has a power cable with a 2-prong plug , replace it (or have it replaced by a pro) with a 3-prong cable.

D - get a short extension cord for your amp with a built-in GFCI. plug it into the outlet, plug your amp into the cord. do the same for your PA system. ditto for your bass player, etc.

E - if you use a tube amp, and are worried about the EXTREMELY RARE possibility of DC shock, read the appropriate sections on the GuitarNuts site.

F - FINAL TEST. have your ex-wife hold your guitar, while you hand her the microphone. if she doesn't light up like a Christmas tree, you're safe.

if she DOES, just blow off the gig, and wait to play somewhere that doesn't have stuffed wiring.

..........you won't need the money from this gig anyway, you just saved a ton in alimony!

bitter, who, me, bitter? :D

Link to comment
Share on other sites

You forgot one important safeguard: wear clean dry sneakers. But that other stuff shore helps. <g>

I did figure something out to get around the nylon strings. I can take a single auto jumper cable, remove the insulation a 3 feet from one end, and wire that to the grounded bridge. The long end, when setting up, is connected to a water pipe, like that fire sprinkler over your head. The short end is clamped on to any suitable protrusion on the bass player's body. An ear, the tongue, whatever. After all, safety first! This should go over particularly well with the piercers....

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...