Until Friday our sound board has worked without a problem. Since then we have started getting a loud hum every time i bring up one of the hanging mics. Even if all the other mics are down, we still get the humming. All the other hanging mics are fine, and nothing has been moved or changed. If i cut out the low frequencies, the hum goes away, but doing that messes up everything that mic picks up. Any one have any suggestions on what i could try to get rid of the hum?
Thats probably 60Hz from the AC line, probably there's either an AC cable crossing a mic, or a shield has come loose at some mic cable / connector. I'm assuming it's not feedback since you say nothing has been changed, check the house EQ anyways.
Chances are, someone yanked on the mic cable somewhere, probably where it plugs into the board, and tugged the shield wire loose. I doubt it's the board itself.

Simple troubleshooting steps. First, if you have anything plugged into the insert jack on the board for that channel, unplug it. If the hum goes away, it's either the insert cable or the gadget you're inserting.

If the hum stayed, swap that mic's XLR connection with the one next to it. If the hum stayed on that channel of the board, then the board is at fault - possibly a cold-solder joint on pin 1 of the XLR. If the hum followed the mic cable to a new channel, the board's okay and it's either the cable or the mic. itself.

If this is a permanent install, chances are the cable plugged into the board is relatively short and goes to a wall-plate with a bunch of XLRs that are, in turn, wired to jacks on and around the stage. Go to the wall plate and swap the same two cables you just swapped at the board. If the hum stayed on its new channel, it's the cable between the board and the wall plate. If it went back to its old channel, it's either the installed wiring to the stage jack or the cable from the mic to the stage jack or the mic itself.

Do the same swap at the stage jacks. If the hum stayed on the same channel of the board, call an electrician - your installed wiring has a problem. If the hum moved to the other channel, the installed wiring is good, it's either the mic or the cable between the mic and the stage jack. Swap mics. If the hum follows the microphone, you've got a bad mic. If it stays on the same channel, it's a bad cable.

You may want to try gently tightening the mic element after all the above checks; depending on the style of the mic if the elemt comes loose it may have the same effect as breaking the shield. Similarly, if it has some kind of power unit check that out too.
What DMXtools has stated is a very thorough and comprehensive check and follows a very simple and very effective principle that will work in most situations.

That is - change the constant variables that you have to eliminate possible causes. Eliminating possible causes is probably the quickest way to fault find.

Nephilim also has a valid point, which you should check if there is still a problem.

The only thing that I will add is just check if the amp (or any other equipment) has an Earth or Ground "lift" button. Sometimes they can switch if the equipment gets a bit of a knock, or some one wonders "Hey - I wonder what this does".

What this does is isolate the signal earth from the power earth. As Inaki2 has suggested power can introduce a lovely bottom end hum and when all else fails, isolating the two earths can stop this. You should power down your amps when pushing this button.

Hope this helps,
wow, thanks for the fast reply! i will beable to run those check later tonight and see if i can narrow down the problem. thanks again! ill let you know how it goes
well i've narrowed it down to either a low frequency feedback, or problems with the cable. we are goning to play with it some more tomorrow and see what we can do to get rid of it!
thanks for all your help!

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