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.
"I have a tremendous will to live and a tremendous 'joie de vivre,' alternating with irritability." - Joni Mitchell