Nasty, nasty buzzing in our PA

[bassman314]

I have a funny buzzing issue.

In our Youth department's sound set-up, we run all of our A.V stuff into a switch box, then into a Mackie Board on one of the stereo channels.

In the A/V Switch, we have a VCR, DVD, CD and a desktop computer.

When the Switch box is on the computer, and the fader for the channels, mains, and sub-group are all on, we get NASTY buzzing. It's the only time we get that sort of buzzing going on. Here's the set-up:

PC -(1/8" - RCA)-> Switch Box -(RCA - 1/4" TRS)-> Ebtek Hum Eliminator -(1/4" TRS - 1/4" TRS)-> Channel input.

We're using a Mackie CFX 16, a Dell Dimension 4700, and a Radio Shack 4-channel A/V switch.

Any thoughts?

[bassman314]

Through some investigation work (cable chasing), I found the problem.

The computer is plugged in separately than the rest of the system. They are on the same power circuit, but plugged into different wall receptacles.

I checked the cord at the wall, and it was out about 1/8". I adjusted it and it solved MOST of the buzzing. It's MUCH quiter now. We only hear it when the faders for the channel, subgroup, and master are set at +10 db. We leave the faders at/around -25db most of the time.

The buzz is only at lower frequencies, as well. Think it might be time to replace some wall receptacles and check on our wiring.