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

[Scott Nordlund]

I think this could use a little more elaboration...

(a) (coax) would mean that the wire is carrying the signal and the shield is grounded.  The shield doesn't have to be connected on both ends, but it may help (assuming it doesn't introduce ground loop issues...).  For (b) (twisted pair) to work as described, the wires must carry complementary currents, the signal and its return path, as this is the "loop" (which couples to some other "loop" like a transformer, the amount of coupling being proportional to the loop's enclosed area).  If you twist a wire together with a ground wire that isn't carrying a complementary current (which I think has been discussed here before), it's just a weaker form of (a), but this may be good enough in most cases.

Of course you could also use optical fibers....

> Date: Mon, 6 Jul 2009 17:27:51 +0300
> From: ajhuovil at cc.hut.fi
> To: lanterma at ece.gatech.edu
> Subject: Re: [sdiy] On shielded cable (or not) for audio
> CC: synth-diy at dropmix.xs4all.nl
> 
> 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
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

_________________________________________________________________
Windows Live™ SkyDrive™: Get 25 GB of free online storage.
http://windowslive.com/online/skydrive?ocid=TXT_TAGLM_WL_SD_25GB_062009