PA system gone nuts

[PineGrove99]

Thanks for all the help. Is curiosity a good or bad thing? Nevermind, don't answer that. just keep thinking about this so you can help us out...

I like the idea of an unbalanced high-impedance line going into a balanced mixer input. If something is improperly grounded, could it really get into the mixer's main out channels when the output sliders are down? Stranger thing have happened, I guess.

[PineGrove99]

I reread some of the posts. The noise seems to be on only one of the OUTPUT channels. So does that mean that the power-supply-noise-source theory is more likely?

[Ben Milne]

I'd now say it now seems more like it... you said you swapped out the amplifiers?
due to the regular nature of your issue I'd start looking at things like hot water system etc anything that could be on a timer.
Is the desk metering your noise when it happens?
It might be worth getting a licenced electrician to meter your power at all points you're are using at the times your issue surfaces. a Furman power conditioner might help, though since the issue continues with your master faders down, it might not be something it can deal with either..

Certainly sounds like a frustrating issue.
good luck and keep us posted.

[Charlieshafer]

Another monkey wrench! Are the speakers hard-wired, and hung or permanently installed? If not, and you've traded speakers to make sure that it's the one channel, then I'm back to the power supply clean-up.

[...and Master of None]

Not necessarily "more likely" - more like "still likely"

If you're sure it's only on one of the output channels (and not just showing up only on that channel because one of the pre-fader aux controls is feeding a bad input to just that channel) then you're pretty much almost done. Swap your connections between the output of the board and the eq, amp, or whatever they feed. If the problem moves, it's the board. If it doesn't, start from whatever the board is connected to and work towards the speakers.

[david blair]

I Always take care of my cords. Coil them neatly, and store them in a zip lock bag. I plug the cords in so that nothing pulls the plugs or bends the wire.
Unbalanced speaker wires should not run parallel to power lines, and all equipment plugged to the soundboard should share the same outlet.
Have you tried swapping your cables? Mark them with tape. Is it always the same speaker? It smells like a loose XLR jack or a bad cord to me. These things just bug...

[PineGrove99]

OK, I think the noise stops if both wired mics are disconnected. (The noise stops on its own so so disconnecting a mic and hearing the noise stop does not prove anything 100%).

Listen to the last report... The PA system is off. Someone is using a digital recorder plugged into AC power with I think a dynamic mic on a 10 foot cable. Nobody hears anything acoustically during the recording. On playback the noise is overlaid onto the recording.

Sounds like the noise source is the AC power to me. Now, I will escalate attempts to clean up the power.

[PineGrove99]

I talked to the person who set up the recorder. It had two mics connected of unknown provenance each with a 40 ft. cable. Back to the RFI theory.

[PineGrove99]

The ultimate cause seemed to be one or two older, permanently installed 150 ft. mic cables picking up EMI. Rane explains how mixers and other equipment have transitioned (mostly) from grounding to the signal ground to the chassis ground. Some day I may get curious enough to troubleshoot the cables. Quite likely one of the cable shields is shorted to one of the balanced signal lines, or something like that.

[mandroid]

probably just run parallel external wiring , temporarily, till your church funds a proper electrician and replaces those cables
and perhaps installing a conduit of metal to run them thru , it can add to the shielding..

[mandroid]

Perhaps getting a high quality microphone snake installed , will be a good step ..you will have a remote plug in
for say 8 channels of microphones, then .. and get the wiring re done from the amplifier to the installed speaker set too..

[Charlieshafer]

Yeah, many custom hard-wired installations simply used weak cable/wires for the runs, and not necessarily shielded, especially when done by volunteers (as in our church) Or, the insulation may have been compromised, or even a wire at a terminal has loosened up enough for interference to set in. I'm with mandroid, just run a high-quality cable around the perimeter for the time being. Snakes are great, as they're made to handle long runs, but then you have the big ugly box at the end. There are smaller snakes, though, meaning fewer channels. Oh, and I use snakes for the shows, as the original hard-wired runs are weaker than a (fill in the blank with whatever you want)

[mandroid]

a snake fuctionally can be built in, input jacks near to the normal microphone locations.

there are flush floor mount audio connectors to hide the audio input, until needed .


http://www.parts-express.com/pe/show...TOKEN=38543718

http://www.parts-express.com/pe/show...TOKEN=38543718
as well as those that can be put in the wall
such as these http://www.parts-express.com/pe/show...number=248-304