Reducing Hum from Mics/mixer

I've got a mixer with 5 choir mics (SM81) input to it that produces a slight hum when the mics pick up any noise. The hum will go away once there is no noise for about 10 seconds OR when someone speaks into the podium microphone. It can also continue for some time and isn't always predictable.

Anyone know why the hum goes away when someone speaks into the podium mic? How about getting rid of the hum altogether? I've tried new cables, different microphones, different inputs, a hum reducer inline from the mixer to the mic input, and just about every dial I can find on the mixer to no avail.

What kind of mixer? What are you listening though? Are you feeding the house system? If so, then there's a hum in your choir mic and mixer setup. The sound goes away because the built-in sound system will switch to the podium (and mute the chior mics) by default - either because someone speaks into it or after a time out of no sound on the other inputs.

All good questions, thanks for the quick response, I'm working on it this morning/afternoon. The mixer is an Alesis S-16. We are listening through the house system, that is what we are feeding it back into.

The mixer accepts the 5 choir mics and the podium mic (from an output that is on stage right) and then feeds it both through the left balanced output to the house system as well as through an aux out through a balun (which eventually makes its way to the other building). I'm not sure if the hum translates into the feed in the other building or not, haven't got that far. I have tried a single mic, any mic (both choir or reg), as well as different house inputs and a variety of cables but can't get the hum to go away.

I'm afraid that during our meeting the hum will be a bit annoying and don't know a good way to prevent it, I may have to rely on the fact that the podium mic stops the hum and much of the meeting will be talking.

I can't find a manual for that mixer. I'd try with just a simple hand mic into the sound system and see what happens. It's possible that's just how the sound system works.

I'm still unclear on just how you have the things wired up. The fact you have a line running to another building makes things ripe for problems.

If possible, you may want to try feeding through the EJ-8 "crab" box and use the hum eliminator switch, or a similar piece of equipment. Ground loop hum is one possible source - it comes by having two different points connected to group with differing potential.