[CQ-Contest] Hum and stubs

[Barry]

Recently installed a new computer and having an odd problem. 
When transmitting with the amp on, I get hum in my computer speakers 
(Harman Kardon with a subwoofer and 2 tweeters), but only when my 
stubs are switched in, and only on 15m. It's either coming from the 
speakers' AC line or the connection between speakers, as the hum is 
still there when not connected to the computer. I've tried ferrites on the 
audio and AC lines, but that didn't help. Ideas?
Tnx,
Barry--
Barry Kutner, W2UP              Internet: w2up at mindspring.com
Newtown, PA                     Frankford Radio Club