I have my Yamaha RX-V773WA L/R pre-outs connected to a Crown XLS 1500 PA. When L/R speakers are directly connected to the AVR, I get no buzz. The AVR pre-outs are connected to the PA inputs over RCA. So here is what I did so far to identify/isolate source of the problem:
1. From the PA, disconnected all inputs and turned on the PA - No buzz
2. Connect pre-outs from the AVR to the PA inputs - buzz re-appears
3. No buzz when speakers are directly connected to the AVR
4. Leave the RCA cable plugged into the PA but disconnected on the AVR end - no buzz
My conclusion so far is that:
1. Buzz isn't getting induced into the pre-amp otherwise it would appear even when AVR's amp is being used.
2. Buzz isn't getting induced at the PA RCA inputs or by the cable - not sure about this because if I use a thinner RCA cable, the buzz gets louder.
So somehow the RCA pre-out connectors on the AVR are picking up AC noise and inducing them into the signal?
Don't have the appropriate cables yet but I was going to try:
1. Using RCA-to-XLR cable. But RCA is unbalanced and XLR is balanced. Not sure if that would kill the buzz if it is getting induced at the AVR RCA pre-outs and not by the cable.
2. Run the pre-outs through a hum killer (have a Behringer noise killer at hand) - But I don't like too many interconnects. IMHO, connectors are the worst culprits when it comes to picking up stray interference. And, not sure if chokes inside the hum killer will actually destroy it. IMO, humm/buzz is best destroyed at source.
I read a few threads about ground loops but not sure what could be done here since the AVR power plug is two prong and the AVR chassis has no ground lug on the back. The PA has a three-prong power plug. Make matters a little complicated, the power outlets don't seem to be grounded (and getting them grounded is going to be another project, unfortunately).