Shielded Wire And Ground Wire

[Jude]

As I understand it, if you use shielded wire you only ground one end, hence it cannot be used to ground say, a pickup selector switch or the ground to the output jack, and a separate ground wire is required. Is this correct?

Thanks

[dpm99]

Here's the basic principle. Everything wants to go to ground. If you touch a ground wire to any line, the signal will immediately go to ground. (That's how most kill switches work, by the way.) A shielded wire is really two wires. The wire on the inside carries a signal. The shielding acts as a second wire, and it connects to the ground. That's how shielding works. If any stray interference is in the vicinity of the wire, it will connect with the shielding and get sent immediately to ground, before it ever has a chance to interfere with the signal line.

A pickup selector switch deals only with the signal line. It's not typically shielded or grounded, though some might argue that shielding would be helpful.

All grounds must connect to the ground on the output jack. Otherwise, they're not grounded. The ground is carried through the output jack, through the cable, through the amp/pedals/whatever, then sent to the electrical ground in the plug.

I'm not sure I entirely understand the question, but I hope that helps.

[Sporky McGuffin]

Shielded wire should be grounded at both ends, otherwise it acts less as a shield and more as an antenna. It's not difficult to demonstrate that connecting the shield at both ends results in better noise rejection than connecting it at only one end. The practice comes from the lazy way to solve a ground loop; this was picked up on by some of the less scrupulous cable manufacturers and sold as a virtue (pretty much anything used to sell cables is nonsense ).

Connect it at both ends. Ideally there should be a central point for all those grounds to come back to - either the output jack or a star point.

[Jude]

Thank you. I'm not electronically illiterate and always wondered why it was recommended that shielding be only grounded at one end. The ground loop explains it perfectly.

[kmensik]

It can be done both ways.

1. The more common asymetrical way: the shield is also the ground, carries the return of the signal. A coaxial cable shield connected to a volume pot on one side and to an output jack on the other is a typical exymple.

2. The other way is the symetrical one (more common in pro audio than in guitars). That way you would have two wires in a common shield leading to the output jack - one signal hot, one signal ground, one shield. All shields should connect at one end only, at one point, and in a guitar to the signal ground too. More on guitarnuts.com

[curtisa]

In many cases you don't get a choice. A humbucker with a 4-core shielded cable for example (4 signal wires and a 5th separate shield conductor) is technically only grounded at one end by deliberate action of the guy installing the pickup, ie by soldering the shield wire to the ground system in the guitar. The other end of that shield wire goes to the backplate of the pickup which is usually floating in mid air behind a scratchplate or screwed to a big lump 'o wood. By definition the pickup shielding is grounded at one end only.

There's a lot of voodoo floating around regarding grounding and shielding in guitars. Probably the most important factors in grounding in guitars is not that you explicitly use star grounding or bus grounding or Eric Johnson's super-mysterious Alpha Centauri grounding system 5659-a rev 38, but that your work is clean and any grounds you use or make are low impedance. Chances are you'll probably settle on a wiring scheme for your builds that follows no rule in particular, but works every time and is easy for you to execute.

[dpm99]

Shielded wire should be grounded at both ends, otherwise it acts less as a shield and more as an antenna. It's not difficult to demonstrate that connecting the shield at both ends results in better noise rejection than connecting it at only one end. The practice comes from the lazy way to solve a ground loop; this was picked up on by some of the less scrupulous cable manufacturers and sold as a virtue (pretty much anything used to sell cables is nonsense ).

Connect it at both ends. Ideally there should be a central point for all those grounds to come back to - either the output jack or a star point.

Sporky, could you clarify that a little? According to my understanding, it's supposed to function like an antenna. That's how it shields. It gathers the ambient 60 cycle hoobledeegooblee in the air and sends it to the ground. I'm not an electrician, but that's always worked for me. It certainly wouldn't hurt anything to connect it at both ends, of course.

[KeithHowell]

Ground loops inside guitars are somewhat overrated. Hum and noise are directly proportional to the current in a circuit NOT the voltage. In a guitar the currents are to low to really be a problem for ground loops. Amplifiers where you have several hundred volts (tube/valve) or lower voltages and lots of amps in the solid state case ground loops become much more important. A few volts potential difference between ground points will result in current flow and the resultant hum.

Shielding inside guitars is more important than worrying to much about loops

[curtisa]

Yep, most definitely.

Most people probably haven't heard what an actual ground loop sounds like. Hums and buzzes in a guitar are often mis-labelled as a ground loop when it's more likely to be induced noise, either through poor shielding or something inside the guitar cavity being grounded poorly. A true ground loop will have a fundamental tone of 60Hz (or if you live in the UK, Aus, NZ etc 50Hz). Anything that sounds buzzy, raspy, hissy, squealy, static-y etc is not due to a ground loop and is far more likely to be caused by a wiring or shielding issue in the guitar. If you can change the buzz by moving around the room, or turning something off, or playing in a different house it's not a ground loop.

The oft-touted star grounding method in a guitar does not guarantee noise-free operation of the instrument. Diagrams that show all grounds returning to the case of the volume pot as "star grounded" are not done because it IS star grounding, but because it's a handy central place to attach the many points in a guitar circuit to ground. The fact that it is star grounding is just a happy coincidence and probably only adds to the confusion.