I really don't think power supplies are the most likely candidate, are they? I think the fact that the hum happens *only* wheh the USB or serial cables are connected means something is coming from my PC, right?

Actually, the fact that the hum happens with the data cables connected practically guarantees that you're looking at a ground loop problem- and the way the various components are powered/grounded is likely to be most of the issue.

Check out the FAQ thread titled "Common installation problems". Although this is for cars, one of the sections is about ground loops and links some documents on the subject. Perhaps it would help you to look at this information.

It's interesting- you're the second person on this BBS to complain about noise problems when sending the Empeg through a mixer. The first happened to be Doug316, the original owner of my Mark1. I wonder what it is about mixers that brings out the worst in computer equipment?

I should ask my audio-producer friend about this. His job is to work with computer-based audio mixing products and he's got a serious garage studio with multiple mixers.

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Tony Fabris