Story: My pre-pro (Anthem AVM60) only has one 12v trigger output, so I use an Emotiva ET-3 to trigger several devices. Since the blue LEDs on the front are set to laser-blind-the-room brightness, I hide the ET-3 behind my equipment rack. It creates a blue glow on the wall.
Yesterday I turned on my system and noticed a horrible 60Hz hum coming from my left speaker and left subwoofer. Oh no, I thought, what's wrong? I pulled the system out from the wall and looked for anything that might have become unplugged or otherwise dislodged. I wondered if my Belkin power conditioner had broken. I wondered if maybe there was a new device or dimmer someplace in my house that was creating such a DC offset that the Emotiva CM-2 could not handle it. I dreaded the thought that my pre/pro or an amp or two had been fried in the night by some huge power spike I was unaware of. I assumed the worst.
Then I noticed the ET-3. It had fallen down behind the rack somehow and was resting against the speaker wire bundle with the super bright LEDs lighting up the insulation. I put it back where it belonged. The hum stopped. I put it back on the wire bundle. The hum returned. Aha.
Warning: Based on this, if you use an ET-3, keep it away from unshielded signal wires. The light-up-the-night LEDs can cause a nasty and scary hum. But it's nothing to worry about. Just position the thing where it can do no harm.
That was your first mistake. You took your lucky break and broke it in two. - Paul McCartney