[sdiy] On shielded cable (or not) for audio

[Jerry Gray-Eskue]

How much care you use in shielding depends on the environments you are
anticipating. If you are going to setup for studio use only you can
sometimes readjust equipment locations to minimize unwanted signals in you
system, but if you are on the road or doing gigs you want the max you can
build in. You could occasionally end up near high power transmitters or very
close to florescent lights or dimmer panels. Having "ten four good buddy"
pop out in the middle of a performance, recording or demo can really disrupt
the mood....

The more amplification the signal may see (as in preamp stages) and the
longer the wire run the more critical the shielding is.

- Jerry

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Antti
Huovilainen
Sent: Monday, July 06, 2009 9:28 AM
To: Aaron Lanterman
Cc: synth diy
Subject: Re: [sdiy] On shielded cable (or not) for audio


On Fri, 3 Jul 2009, Aaron Lanterman wrote:

> I've been going through google images looking at the insides of people's
> modulars to get mounting ideas.
>
> I noticed that when running to audio jacks, some folks (like Paul S.) (a)
use
> shielded coax-like cable. Others (b) just use a couple of wires and twist
> them.

A is electrostatic shielding, B electromagnetic (you minimize the loop
area).

The rule in tube amps is to use B for heater wires (to prevent high
current causing inductive hum. A is sometimes used for the first stages in
high gain amps, but there's a catch: shielded wire seems to have
approximately 33-50pF / meter capacitance which can cause loss of highs in
very high impedance circuits.

> Thoughts on when (a) is essential (or equivalently, when you can get away
> with (b))?

As hinted in previous paragraph, you don't need to use B unless you have
reason to suspect inductive interference (sending or receiving). PSU leads
might be one case.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova
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