Eliminating RF pick-up

[rbalewski]

Hey all!

I'm using a few condenser mics (Behringer C-2's and a small Shure) for our choir. I sometimes have to turn them up quite a bit, and when I do the local college radio station comes thru slightly. I'm wondering if maybe there's some kind of preamp I can put right at the mics so I'll have a hotter signal going thru the lines? Or maybe some RF eliminator? Or any other suggestions you may have would also be appreciated!

Thanks!

Ron

 

[FMEng]

The following info applies if the interfering radio station is an FM. The RF energy from the station gets picked up by the cables and it can be demodulated into audio by the preamp in the console, or by the active circuitry in a condenser mic. Cable shielding isn't effective at preventing this.

An effective block of the RF signal is a toroid. The toroid has to be made of a certain material to be effective at the correct frequency band. The following will work and can be applied easily.
http://www.amidoncorp.com/2x44-3551p2/

I suggest buying two for every affected microphone. Work with one mic at a time. Start by wrapping a few turns of the mic cable through the toroid, within a few inches of the connector that plugs into the console. The more turns through it, the better. That will fix it if the console is the culprit. If not, do the same thing near the microphone. Try near the connector first. If it cleans up, try moving the toroid a few feet away, so that it rests on the ground or someplace less obnoxious, since they are heavy. Sometimes it takes a toroid at both the console and mic, and sometimes just one end will fix it.

For AM station interference, the solution is similar, but type J material is used. They aren't available as snaps-ons. Instead, a donut is used.
http://www.amidoncorp.com/ft-193a-jc/

 

[rbalewski]

Thanks FMEng! Will give those a try! It's a college FM station that's causing the interference, so we'll order the appropriate toroids.

Ron

 

[rbalewski]

Just a quick follow-up. Finally got a couple snap-on toroids to try out. What a difference! It seems the sound board was demodulating the signal. When I put them right at the inputs, the radio was gone! Now to get more for the other lines... Thanks FMEng for the great tip!

 

[howlingwolf487]

You can also try Neutrik's EMC series of connectors for adapters/rewiring.

 

[FMEng]

I've heard that the Neutrik EMC is effective for GSM cell phone interference, which well up in the UHF band. That tends to be a big problem in conference rooms and council meetings with phones laying next to mics, so that's what they were designed for. I doubt they are effective at low VHF frequencies, where broadcast FM lives, because the ferrite bead is tuned too high.

 

[howlingwolf487]

I've heard that the Neutrik EMC is effective for GSM cell phone interference, which well up in the UHF band. That tends to be a big problem in conference rooms and council meetings with phones laying next to mics, so that's what they were designed for. I doubt they are effective at low VHF frequencies, where broadcast FM lives, because the ferrite bead is tuned too high.

The cutoff starts at 10MHz, I believe. Their product literature isn't terribly forthcoming on the numbers...up front, anyway.