Are the power cords plugged into the same circuit? To test, use an extension cord and plug them into the same outlet.
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It doesn't sound like the speakers because both started at the
same time. Try the interconnects into a preamp,or even a walk man type cd player,any source with a volume control. Try it with the new source unplugged from the outlet too.That might show a ground loop problem. Besides the above mentioned,I don't have any other ideas.
It sounds like you have a ground loop because it happens when you connect the RCA cables.
One of the speakers may not be grounded properly at your power bar - when you connect the RCA you are connecting the grounds between the two speakers with your Y connector that goes to the Airport Express - a microamp current flow along the RCA ground wire (negative) can easily generate auidble hum.
Seriously you should look into getting XLR cables - I know the Airport does not support them but you could easily add a DAC/PRE with optical in and XLR out.
My 12L's never hummed as long as I used them with my stereo which was all located at the same end of the room. When I tried them in home theater plugged
in to the opposite end of the room from my receiver, they hummed loudly. I assumed it was because I had created a ground loop.
Hifitime offers a good suggestion. Try your inputs one at a time to try and find the loop.
Ok. This all makes sense. I have wanted to go back to XLR cables and I might have to look into a DAC/ or Preamp at some point.
I wanted to also bring up the fact that I have a turntable in the mix and when I use go from the turntable to my phono preamp to the speakers, there is still a little bit of a hum, but not as much as when I go from the computer to airport express to the speakers. That make sense?
My thought is that it is most certainly the ground loop problem connected to the RCA cables since it happens with the turntable and with the computer. Does everyone agree with that?
Also, I don't have any dimmers in the office.