I've been a tinkerer and hobbyist for 20 years now, but mostly in software. I've done my fair share of simple soldering (replaced controller ports on an original Xbox with actual USB ports, for example) but only have the vaguest idea of how electricity actually works.
All that being said, I've got a virtualization setup that I love and for which I'd like to sort out an audio playback solution. Basically, I just want to be able to funnel two analog stereo sources into one amplifier and not damage any of my hardware.
The first thing I did was buy one of these and ran it backwards, effectively. The amp was connected "Amp Input" and my two sources were connected to the A and B channel outputs. That worked fine, but before long I wanted to eliminate one of the (now three) boxes on my desk.
After some googling, I wound up with this (likely often cited) article: http://www.rane.com/note109.html and set to building myself what I understood as the "right" circuit. Enter this fella:
PC 1 is onboard audio on a fairly high-end motherboard, originating as a TRRS 1/8" connector. PC 2 is the 1/4" TS output from a Native Instruments Komplete Audio 2 USB audio interface. The amp takes two RCA inputs.
My solder job really sucked, but the connections were sturdy. I just used some high end(?) RCA cable I had lying around and a few other connectors. Once complete, I wrapped the whole shebang in electrical tape (no heat shrink at hand) and connected everything. And wouldn't ya know it - everything worked!
I have since discovered an issue, however, and I'm nearly certain it's my cable.
If I have the amp cranked, there is a very audible, pulsing hum. It sounds like cicadas, actually. It didn't take long for me to realize the pulses matched exactly with the "breathing" effect from my motherboard's LED controller. I've got two strips of white LED's pulsing, and the buzzing corresponds with the fade in either direction. When they're either fully off or fully on, there's no noise.
Predictably, when I remove the 1/8" TRRS connector from the motherboard, there's no interference. A little more interestingly, the signal noise also vanishes if I disconnect the outputs of the USB sound card.
So... what have I done? What might solutions be other than changing the lighting? ;) What did that little box do that I could replicate, perhaps?
Thank you all in advance for any help!