Shielded wiring

StephenG

hi all,

I want to do a complete rewiring job on a tele, and want to use some decent quality shielded wiring.

any suggestions where i can get some decent wire in capetown for the task at hand?

thanks
s

Kirfew

Yebo Electronics? I rewired my strat with plain old hookup wire and it sounds fine, I think the bit that you need to worry most about with regards to shielding from noise is to use copper tape to shield the body cavities.
Check out this article:
http://www.guitarnuts.com/wiring/shielding/tele.php

Brastep

Albeit my experience is limited to one build, I think most of the wiring is too short to be affected by hum.... I shielded only my pickup cavities and one longer set of wires (from pickup switch on upper part of the body down to the pots). I stripped the insulation off a section of TV antennae wire to get to the copper foil and mesh shielding which i wrapped around the wires. A tricky messy job but it seems to work fine.

AlanRatcliffe

If your cavities are shielded, screened cable doesn't make much difference, but still adds an extra layer of shielding that will do no harm. I like overengineered stuff. ?

I won't usually bother upgrading to a screened cable if there is cavity shielding, but I will then twist the longer lengths of unshielded wire into twisted pairs, which also helps (and helps make the wiring neater too).

A guitar with shielded cable can still benefit from cavity shielding because of the high impedances and the exposed wire ends (which all add up to a couple of inches of unscreened wire).

Also note that screening comes in different types (lap, woven, foil) which affects performance and flexibility. Foil offers the best coverage, but is the least flexible, lap screened is the most flexible, but has the worst coverage. Woven screen sits nicely in between the two.

External, uninsulated wire shields can work if they are done properly with rigid wiring (like the braided shield cloth wiring used in an older Les Paul), but careful with them otherwise, as they do introduce the possibility of shorts.

Riaan

Is it necessary to solder the strips of copper cavity shielding tape together as demonstrated in lots of books? If so, why?

Tonedef

Alan Ratcliffe wrote: I will then twist the longer lengths of unshielded wire into twisted pairs, which also helps.

This is a common misconception regarding EMI. Twisted pairing only works for signalling circuits that are designed for it; that is, the signalling design must be using differential signalling, and both wires in the pair must be impedance matched.

Twisting them together in of itself does nothing. It definitely neater, but you may even be inducing capacitance between the wires by twisting them together. Nevertheless, I don't think it's really that big of a deal - it's not like we're dealing with digital circuits operating at the megahertz level.

AlanRatcliffe

Riaan wrote: Is it necessary to solder the strips of copper cavity shielding tape together as demonstrated in lots of books? If so, why?

Depends on the tape. Many tapes you need the solder to make sure there is an electrical connection between the strips. The stuff from Stewmac has electrically conductive glue so you don't have to solder it.

AlanRatcliffe

Tonedef wrote: This is a common misconception regarding EMI. Twisted pairing only works for signalling circuits that are designed for it; that is, the signalling design must be using differential signalling, and both wires in the pair must be impedance matched.

So it's only effective in a balanced (differential) circuit? I was taught that it simply needed to be forward and return conductors of any circuit.

Tonedef

Not quite. Simply because you have a wire going 'up' and a wire going 'down' in the pair, does not imply that the phase is shifted through 180 degrees between the two. Indeed, you'll most likely have no shift at all, which turns your wire into an antenna!

Also, if you have something like a pot at the end of the twist (to use a simple example), you have a voltage drop past the pot into the one wire, and so your line is, ipso facto, not balanced anymore (if it ever was in the first place).

The phrase that is commonly bandied about is "common mode rejection", but CMR only works when you have a differential amplifier between the two lines that can detect voltage differentials and conduct them to ground.

The simple fact that the wires aren't twisted together in the first place, means that no differential amplifier is present in the axe.

If EMI was keeping me awake at night, I'd simply install good quality shielded wiring, ground it carefully (this can also be tricky) and be done with it.

Tonedef

Riaan wrote: Is it necessary to solder the strips of copper cavity shielding tape together as demonstrated in lots of books? If so, why?

Any conductor that moves within an electromagnetic field (or in the our case, a static conductor within a changing EM field, same thing), will attempt to generate an electric current by absorbing much of the EM radiation. However, if the electrical potential in the conductor cannot go anywhere, the conductor cannot absorb anything and is thus useless as an EM shield, as the radiation will simply pass through it. Indeed, it's this very priniciple that makes electric pickups possible.

Seeing as your signal wiring are perfectly good conductors too, you need to put them into what amounts to a protective conductive cage that gets hit by the em first, thereby creating two electrical circuits - one for clean signal and one for em noise current.

Thats why its necessary to ground your shielding material - cable shrouds, copper cladding, the lot. That way the noise current generated can flow to ground and not become part of your tone signal.