As a ham radio operator, tracking down these sorts of signal problems is very common. My approach is to disconnect cables one at a time until it goes away. Another approach is to disconnect everything and plug things back in one at a tiem until it starts. I recently had to apply these techniques to hum that eventually was caused by a ground loop with my subwoofer.

There are just too many devices in our setups not to take a systematic, one-step-at-a-time approach to identifying the culprit.

-Brian


2xM60ti, 2xM3ti, 2xQS4, VP100, EP350, N2's