Is String Grounding Necessary?

[fieldwrangler]

Folks -

Even in high impedance situations I've often thought that getting as close to 100% shielding as possible (which includes things like forming a tube for traditional Tele jack tunnels) seemed to make string grounding unnecessary.

Now I've found a situation in which this doesn't seem to be the case at all.

Thoughts? Have I just been getting lucky all these years?

Non-functional 3rd prongs in the wall socket (or deliberately lifting that ground) make for a good torture test, by the way.

Bob Palmieri

[Mike Sulzer]

Folks -

Even in high impedance situations I've often thought that getting as close to 100% shielding as possible (which includes things like forming a tube for traditional Tele jack tunnels) seemed to make string grounding unnecessary.

Now I've found a situation in which this doesn't seem to be the case at all.

Thoughts? Have I just been getting lucky all these years?

Non-functional 3rd prongs in the wall socket (or deliberately lifting that ground) make for a good torture test, by the way.

Bob Palmieri

Well, an EE or physicist will tell you that a perfect shield keeps out all electric fields, but that is not necessarily the whole story for preventing hum resulting from electric fields. For example, one potential problem is that the shield of the guitar cable is used to carry signal current. The problem is that there could be hum currents flowing on this shield as well, currents that are a result of the shield doing its job to neutralize electric fields. The resulting voltage in series with the input (because the shield has some resistance) is not very big, but on the other hand, guitarists sometimes play with a lot of gain.

There is a way around this, in principle, although I have not tried it. It involves using a stereo jack for the guitar and amp and two conductor shielded cable. You then have two signal leads, call them high and low. The high signal is what it is, and it goes to the tip. The low signal is what people usually call ground, but you have to make sure that no actual shield is connected to it, and it goes to the ring. So all the copper foil, pot cases, etc. connect to the shield of the cable and at the other end it goes to the chassis of the amp. The low side of the signal runs directly to the bottom of the cathode biasing resistor of the first stage. (Yes, that point is at "ground" potential, but it is signal ground rather than chassis ground, and they can be a bit different.)

Both guitar and amp should remain compatible with the normal world since with the use of mono cable and plugs, the ring contact on the jacks gets connected to the shield. As I said, I have never tried it, but a separate conductor for the shield is standard in some applications. (The amp might have more hum that it should when using a mono cable because the chassis and signal ground are connected together at a non-optimum point.)

[fieldwrangler]

Well, an EE or physicist will tell you that a perfect shield keeps out all electric fields, but that is not necessarily the whole story for preventing hum resulting from electric fields. For example, one potential problem is that the shield of the guitar cable is used to carry signal current. The problem is that there could be hum currents flowing on this shield as well, currents that are a result of the shield doing its job to neutralize electric fields. The resulting voltage in series with the input (because the shield has some resistance) is not very big, but on the other hand, guitarists sometimes play with a lot of gain. There is a way around this, in principle, although I have not tried it. It involves using a stereo jack for the guitar and amp and two conductor shielded cable. You then have two signal leads, call them high and low. The high signal is what it is, and it goes to the tip. The low signal what people usually call ground, but you have to make sure that no actual shield is connected to it, and it goes to the ring. So all the copper foil, pot cases, etc. connect to the shield of the cable and at the other end it goes to the chassis of the amp. The low side of the signal runs directly to the bottom of the cathode biasing resistor of the first stage. (Yes, that point is at "ground" potential, but it is signal ground rather than chassis ground, and they can be a bit different.)

Both guitar and amp should remain compatible with the normal world since with the use of mono cable and plugs, the ring contact on the jacks gets connected to the shield. As I said, I have never tried it, but a separate conductor for the shield is standard in some applications.

Mike -

Thanks, as always, for specifying key aspects of the problem & solution. In this case I'll be "telescoping" the cable shield, using 2 conductor cable to carry the signal and attaching shield to ground at the guitar end only (unless someone thinks there's a good reason to attach it at the amp end.) However, I won't be able to take control of what will constitute the amp's input conditions; customers, after all, will be customers.

While we're here, though, it might be a good time to clear up some odd info that often gets circulated regarding string grounds. I've read that the reason it's used is that the player's "body capacitance" is somehow sinking the high-z electrostatic buzz field. I always thought that it basically grounded the player's body, which seems to be a fine "source" for some of this noise.

[Mike Sulzer]

Mike -

Thanks, as always, for specifying key aspects of the problem & solution. In this case I'll be "telescoping" the cable shield, using 2 conductor cable to carry the signal and attaching shield to ground at the guitar end only (unless someone thinks there's a good reason to attach it at the amp end.) However, I won't be able to take control of what will constitute the amp's input conditions; customers, after all, will be customers.

While we're here, though, it might be a good time to clear up some odd info that often gets circulated regarding string grounds. I've read that the reason it's used is that the player's "body capacitance" is somehow sinking the high-z electrostatic buzz field. I always thought that it basically grounded the player's body, which seems to be a fine "source" for some of this noise.

Yes, grounding the guitar shield separately from the amp ought to work just as well as to the amp chassis (or just as badly if the problem is something else).

As for the string ground:

The player is probably not normally grounded, but must be at some potential resulting from fields in the area or possibly contact with something else that is not at ground potential. So when the player touches ungrounded strings, they are raised to the same potential above ground. Thus a field develops across an imperfectly shielded pickup or other guitar component, causing hum. The voltage developed in the circuit is referred to the guitar ground: so put the strings at this ground potential, and the player touching them goes to the same potential, or almost so. Thus in summary, if the shielding is imperfect, get rid of nearby fields by grounding the player. Too bad if the player is also connected to another source at a different potential with some force behind it!

[salvarsan]

To cut electrical noise from the strings, many guitars have a wire connecting the bridge or string ends to the guitar electrical ground.

I was apprehensive about using a straight wire.

My luthier connected the bridge to the guitar ground by a parallel RC widget, 270k/0.02uF. Seemed plausible, worked, cost little. The 270k resistor was chosen as a generic value to limit current flow should the amp's 400VDC mysteriously short to the guitar ground while I stood in salt water playing my guitar. Yeah, it could happen. No, really.

[John Kolbeck]

To cut electrical noise from the strings,

So if your strings are not grounded, are they somehow acting as an antenna, even though they are not connected to the circuit? Is your body also acting as an antenna, even though it too is not connected to the circuit (considering the noise level drops when you touch grounded strings)?

[salvarsan]

So if your strings are not grounded, are they somehow acting as an antenna, even though they are not connected to the circuit? Is your body also acting as an antenna, even though it too is not connected to the circuit (considering the noise level drops when you touch grounded strings)?

Personally, I hold to the theory that a subspecie of noise demons were attracted to that particular luthier's instrument strings, and that we provided them with an escape route through a pair of talismans that had been pre-incantated and graven in runes (red-purple-yellow, and 20nf-400V) by their creator. It's said to be a minor example of conservation of thaumaturgic aether, or so they tell me. I have no real understanding of this because I'm not a thaumaturgist. That said, I prefer Magic that works irrespective of my belief in it.

There was string noise. We stopped it with 50 cents in parts used as described.

[rjb]

There was string noise. We stopped it with 50 cents in parts used as described.

We'll have none of that pragmatism here, sir.
We must know why, why, WHY?

[salvarsan]

We'll have none of that pragmatism here, sir.
We must know why, why, WHY?

Sir,

I refuse to inject reproducible facts and rational thought into an on-topic discussion, for with those, it can't degenerate into personal invective and ignorant hand-waving arguments.

[potatofarmer]

The thing about balanced cable though is that both conductors need to have equal impedance to ground. Plus then you'd need a bal/un transformer at the amp (or the first effects pedal) unless you're making a wholly balanced rig. Which would be impressive and commendable!

[Jason Rodgers]

Personally, I hold to the theory that a subspecie of noise demons were attracted to that particular luthier's instrument strings, and that we provided them with an escape route through a pair of talismans that had been pre-incantated and graven in runes (red-purple-yellow, and 20nf-400V) by their creator. It's said to be a minor example of conservation of thaumaturgic aether, or so they tell me. I have no real understanding of this because I'm not a thaumaturgist. That said, I prefer Magic that works irrespective of my belief in it.

There was string noise. We stopped it with 50 cents in parts used as described.

You pick up those parts at the shop on Perdido Street?

[rjb]

The thing about balanced cable though is that both conductors need to have equal impedance to ground.

Spoilsport.

[John Kolbeck]

You'd think a company like Fender, that produces at least one product somewhere along the entire signal chain, could introduce something like a balanced guitar rig more readily than anyone else, and exploit the selling point.

[salvarsan]

You pick up those parts at the shop on Perdido Street?

I wish, but no, it was across the bay from Tokyo in the Port of Chiba where ...

the sky above the port was the color of television, tuned to a dead channel.

[salvarsan]

You'd think a company like Fender, that produces at least one product somewhere along the entire signal chain, could introduce something like a balanced guitar rig more readily than anyone else, and exploit the selling point.

It'd be good engineering and a good product but you'd still have guitarists, seldom the sharpest tools in the shed, using them.