OK...I used to be a DJ and a soundman....not really good at either but here is a idea...
how many amps are you running?..i mean sound amplifiers...is it possible to disconnect all inputs going to the amplifiers ?...if so, try that and see if you can eliminate half the equipment...if the hum does not stop, try a different circuit for the amplifiers...next, check the sheild on the cables going to the speakers...you can use an ohm meter for this and since the problem is intermitent, move and shake the cord while securly attached with the ohm meter as moving the test leads around during ohm checks can cause fluctuations...is cables are good and amplifiers are good then it must be a power problem must likely a ground-loop of some kind....i had an amplifier one time that did this and to see if it was the amp or the power , I pluged a two prong adapter in the wall then plugged in the amplifier...in a sence, disconnecting the ground prong on the amps plug...the noise was gone....but I am not convinced this is your problem...but try it.
also....if you can time the hum, it will give you a good idea as to what might be causing it....sometimes I have seen the cooling fans inside sound equipment kick on and cause a hum...and I have seen a refridgerator do the same as the compressor kicks on...air conditioners, lights and other things can cause these problems so don't limit your self to your power supply and sound equipment....
also keep us updated as I want to know what the bugger is...
Thanx , Mike
