Ground Topology inside a Guitar

[Kesh]

Ground loop means at least two different things, so here I'm asking about the pattern all the ground wiring makes inside an electric guitar, the ground topology.

I understand a star topology is desirable, that is the ground circuit forms no loops with itself. Is this correct? And if so why?

If it is correct then it seems difficult to achieve with copper shielding within the guitar. All the pots' cases will ground to this, but it isn't the most reliable thing to use as a common ground. Most wiring diagrams for my guitar (Fender P-J set up) run wires between the pot cases to form the ground's backbone. Will this be a problem if there is also a ground along the copper tape?

Thanks.

[pdf64]

I'm having trouble thinking how a ground loop could occur inside a guitar?
Do you think your guitar has a ground loop at the moment, or are you just trying to find out what best practice is?
As I understand it, there's no opportunity for a loop to form, so just ground everything using the criteria of convenience and reliability / robustness.
Pete.

[Kesh]

I'm having trouble thinking how a ground loop could occur inside a guitar?
Do you think your guitar has a ground loop at the moment, or are you just trying to find out what best practice is?
As I understand it, there's no opportunity for a loop to form, so just ground everything using the criteria of convenience and reliability / robustness.
Pete.

Here's a different example, not my guitar just a pic that lead me to wonder about this. You see how with the red wire, the ground bus forms a loop, as where the three screened cables to the right are joined, their screens are also joined. In my guitar the copper tape takes the place of the wire screening.

It's for a renovation project, I want to get it right.

[pdf64]

Don't worry about this, I really can't see how it will make any difference whichever way you do it.

[Steve Conner]

Me neither. If you do get a ground loop in a guitar it doesn't matter, because even the best humbucking pickups will pick up vastly more hum than the worst ground loop you could possibly create.

Having both a ground bus and copper tape is (IMO) the right thing because it allows for a backup ground path if the copper tape doesn't make good contact everywhere.

[enkindler]

People confuse two diffrent grounding issues with guitars,

First it is common for guitars to have noise caused by non-existant shielding, this will cause issues with louder noise from near buy AC sources or natural background noise. This issue will never be fixed by star grounding.

The second can be caused by odd interactions of diffrent metals and grounding paths that can create complex inductive and capacitive interactions between particular componants like a bridge which is only grounded by the copper shielding tape. This can cause noise that most people call "grounding loops" but in a guitar this is a misnomer as your only ground is the input jack. This problem can be AVOIDED by making sure you do a star ground of all major componants and when you are building a guitar it is simple to make sure everything connects back to a common point via a low impedance path. This is also solvable in a dozen other ways including a bus ground.

When I am working on guitars, I tend to use what is already there but when I am building a guitar I consider the shield and the ground two systems to keep things straight in my mind. I would never go back and add a star ground to "fix" a non-existant problem or with the idea that it will give you some magic tone.

But for me it is an easy way to prevent odd problems and to prevent missing a ground.

[Kesh]

Thanks for the reply, but how do you create a star, when the whole control circuitry is encased in, and touching at many points, a copper shield?

The wiring is easy to make in a star topology, but all kinds of loops then appear because of the shielding/conductive paint forming part of the ground circuit. Do people really insulate the shielding from the pots, switches, jack and everything else and just have it connecting to ground at only one point?

[Steve Conner]

As far as I know, no they don't, so there is no true star grounding in a guitar with a shielded cavity. But as I already said, it makes no difference in practice.

[Kesh]

Sorry, missed your post. I guess a real obsessive could just about insulate the shielding from everything that would make a loop, but I don't think I'll bother.

[enkindler]

Getting a good electrical contact between the peices of copper tape is hard and to be honest most people don't realize or don't electrically connect the parts of the sheild anyway. The adheasive is not conductive and will thus act kind of like a capacitor. Actually thinking about it I have yet to open a DIY copper foil install where they even electrically connect the various pieces of tape at all.

The Mechanical electrical connection from screwing in the pots is good for shielding but if you have a loose pot or corrosion etc... It can cause issues over the long run. Not to mention the random drop out of the volume or tone circuit if the retaining nut gets a little loose.

The above LP circuit the physical "loop" is not a ground loop, a ground loop requires two systems with a ground at a diffrent potential. At the voltages we are working at almost any good reliable physical conneciton will be good enough to prevent that from happening.

if it is a restoration go with stock if it is for a renovation just run a wire as a ground IMHO.

[Kesh]

My copper tape was advertised as having conductive adhesive, and a quick poke with a multimeter confirmed this.

[Steve A.]

Gibson wires up their Les Pauls with 3 bus wires going from pickup to pickup- if there was a complete loop it could conceivably act as an antenna. At least that was Gibson's explanation.

As for star grounding there are no high currents going around in a guitar as there can be in an amp so in that sense star grounding is not necessary. However there is a similar concept which I first read about at the old GuitarNuts site back in 1998: keep the signal returns and ground wires separate and join them at a single point which goes directly to the ground terminal of the output jack.

Before I read that I would consider the signal returns and the ground wires to be interchangeable; in my more complicated wiring harnesses I thought that the shorter length of wire used would keep the noise level down. Nope. When I started isolating the signal returns (the "cold" lead from a pickup and the cold terminal of the pots) from the ground and shield wires there was a noticeable reduction in noise. The purpose of copper shielding is to intercept some of the RF/EMI noise in the atmosphere and dissipate it to ground. However if that noise is routed through the audio circuit I think that some of it will end up in the signal from the guitar.

While guitars and amps are wired for two conductor jacks I think that you could reduce the noise level by using a cable with 2 center conductors and TRS plugs and jacks. That way the signal return would isolated from the ground and shield for the length of the cable. The amp could be wired to keep the two lines isolated until the initial tube stage- I would connect the signal return to the ground point of the cathode resistor.

A simpler version of that would be to use a TRS output jack on the guitar using a shielded cable with two center conductors with a TRS plug on the guitar end and a mono plug on the amp end with the signal return connected to the ground terminal.

While I don't think any of those ideas would lower the noise level very much especially in an environment with a lot of RF/EMI noise***, I think that you would get a clearer signal from your guitar in less noisy environments. Or not. (It is just a hypothesis at this time.)

Steve Ahola

*** Although technically not correct I use "RF/EMI noise" to refer to all of the crap that might cause noise in a guitar rig, whether from the electrical wiring or transformers or dimmers or TV's. Some of which is reduced by shielding while other noise is reduced by hum-canceling coils.

[enkindler]

My copper tape was advertised as having conductive adhesive, and a quick poke with a multimeter confirmed this.

Most likely you have 3M tape, it's adhesive has lots of small metal balls and is conductive and would appears so with a Multimeter as it only has a .005 ohm resistance. This is OK for EMI shielding which it was designed for but I am willing to bet if you put an LCR meter on there you would see some oddities.

It is likely it would be OK but that tape is also very expensive and still has the same problems of most foils tapes or even metal boxes. When the nuts that hold the pots in place becomes loose you will get noise or intermittent operation if you are using the mechanical connection of the pot as the signal path..

[Kesh]

i just ran a wire from the copper shielding to the ground bus, which is soldered to pot cases

[pdf64]

The copper tape that I used from Stewmac had conductive adhesive, but I made solder connections between them all for added reliability.
However, my experience of trying to implement best practice has left me skeptical that shielding guitar cavities has any significant benefit, with regard to rejecting buzzy type interference.