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

[Antti Huovilainen]

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