N_Cook wrote:
Someone wanting a large mixer amp repairing, not seen yet , as left the
owners do do a bit more exploration, as on phone prompting, the problem only
occurs with a feed from a MAC based system, not when instrument feeds only.

Reported as a whirring , booming noise, but not mains hum, at bass
frequencies but noticeable at all levels
Told them to borrow someone elses pc, or transfer to CD/tape etc to rule in
or out , firstly. Assuming a perfectly good pc system and a properly working
mixer/pa, does this sort of intrusion sound familiar to those familiar with
PC feeds, perhaps beats between SMPS oscillator and mains, badly mismatched
impedances on feed hookup, ground loop/lift problem, just a few thoughts.

Yes, it`s power supply noise from the pc, feeding the signal via a
decent transformered di box sometimes works - it`s often a case of suck
it and see.

Ron(UK)