[LAU] Ground lifts, TRS, etc. to PA? USB/other signal crossover to audio?

Will Godfrey willgodfrey at musically.me.uk
Thu May 8 20:50:00 UTC 2014

On Thu, 8 May 2014 19:30:48 +0000
Jonathan E Brickman <jeb at ponderworthy.com> wrote:

> Last night I came to at least a working resolution of a really weird problem.  I'm hoping others can tell me what is really going on here, and what best to do about it.  I am not an electrical engineer or electrician or anything like that, I just learned a lot from my dad and friends and trial and error.
> This has to do with my MIDI synth, which is a headless Linux box, often used with three Yoshimis running simultaneously.  Input is just MIDI, either through USB direct from keyboards or through USB MIDI adapters.  The problem is other really odd high-pitched non-static signals which are contaminating the audio, when plugged into any of three different PAs.

To me, a high pitched sound like this suggests that something (or a combination
of somethings) is marginally stable and different signal chains and earth loops
can set it off unpredictably. If there is a lot of gain in the system it can even 
be something like speaker wires close to something not completely screened.

Usually, TRS is used for balanced signals but there can sometimes be benefit in
using it for unbalanced lines, in that the screening is separate from the return
path - mind you, in some cases that can actually cause RF interference problems!

All in all, I'd suggest going right back to first principles and connecting just
the absolute minimum system (even if that means listening on headphones),
ensuring you are free of the problem, then carefully adding bits. Do keep in
mind that if the problem re-appears it is not necessarily the last bit you
added that is at fault. It might just be completing a loop that triggers the
real culprit.

Will J Godfrey
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.

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