Scenario: In my classroom at work I have a computer connected to a Yamaha portable PA system (StagePas 400i) to run sound for various purposes. I have a Roland keyboard, my Taylor guitar, and the PC all hooked up to the built in mixer. Instruments using standard instrument cable (no issues here) and PC using a 50' 3.5mm to RCA from Monoprice. Whenever I use the RCA connected to the PC I get a VERY loud buzz/hum from the speakers that is not present at all with other sources.
Things I've tried:
-A different source connected through same cable: no hum
-Different cable from the same source: hum still present
-Unplugging the computer from the wall: Hum still present!
-Switching power strips/surge protectors: Hum still present
-Using both the Audio Out and Headphone jacks: Hum still present
It boggled my mind when the buzz didn't stop when I unplugged the computer from the wall.
Any other ideas or probable causes? I'm going positively mad here. The stagepas is new and works/sounds great with everything except this PC.