I wonder if any RF experts can help me with this one.
I work in an area where there are a lot of microphones in a room connected to a control unit. If a GSM cell phone is present in the room, I get a nasty hum obliterating my audio tracks every time the phone communicates with the tower. It's important that I record the audio without missing a second. We have no cell phone rules, but it's impossible to know who's got a phone on vibrate in their pocket and people just won't turn them off.
I thought putting a ferrite bead on each Microphone cable near where it plugs into the control box would block most of the audio hum at least enough for the audio to be made out. I just tried it and there's no difference in the hum with or without the beads.
Check to see if it does it with the microphones unplugged. If not then is is defiantly pick-up on the leads and not direct into the amplifiers.
You probably need a bit more than a bead, something like an in line inductor. You could get this by raping the lead several times through a ferrite ring but you don't get too much inductance like this. You might have to resort to a filter box that actually puts a series inductor or two in the signal line along with small capacitors to ground.
With the microphone unplugged there is a lot less hum. There's about 6 microphones in the room and I can't unplug them all, so I think the quieter hum is coming from a more distant microphone.
It's very limited what I can try because I don't actually do technical work there, I just use the equipment. I can wrap the lead a few times if I buy a bigger ring. I don't have the large 3-prong microphone connectors though to put something inline.