Behringer c2 condenser mics - How to fix RF Interference - Please HELP!
We have two mics which provide the sound for a web cam on a nesting pair of sea-eagles. The interference from nearby AM radio towers is really bad. Any suggestions much appreciated.
Environment as follows.
2 x shotgun mic Behringer c2 condenser mics
48 volt phantom power
connected with double shielded twisted pair cable
total cable to preamp, distance is 250 metres
mics located 20 metres of vertical in different trees (preamp -> 250 metres to tree -> 20 metres up to nest) 2nd mic (preamp -> 250 metres to tree -> 20 metres up to camera)
AM radio transmitter towers x 2 are around 500 metres away.
We have ground the shield at base of tree, (earth stake) and at 180 metres from preamp - also earth stake
ferrite on both cables at 20 metres (base of tree) prior to vertical
ferrites at pre-amp
If you want to hear how bad it is here is our ustream channel.
This is quite a long cable run here which might be the culprit. First I would try to get the pre-amp closer to the mics if possible as it's better to have amplified signal closer to the source. The twisted pair cables themselves might also be a contributor. Try using quad-cables as they have a higher resistance to interference. Obviously that length of cable would be expensive so first conduct a test with one mic using a shorter cable to see how much if any interference you have. But maybe the signal is too low which is causing you crank up the volume on the other end which up course cranks up the AM radio.
Also, check the power situation where the pre-amps are connected. Are they connected directly into the wall or are you using any kind of power conditioner with RF filtering? That would also be something else to try.
I would agree with Ben. That is quite a distance to run on mic level signal. One of the things that the preamp does is change the signal from mic level (typically 100mv pp) to line level (1v pp), which is 10 times as strong. But preamps require AC and I doubt the sea-eagles will provide you 110VAC at the nest. I would suggest you get the preamp as close as practical. But if the signal is a balanced signal from the mic to the camera, in theory the distance should not matter (to a point) since when the combined signal and inverted signal is processed, it cancels out any interference. That's the theory anyways.
But in listening to your ustream channel, I don't think you are getting RF interference. You are getting the actual AM broadcast. That is a result of something that is acting as an antenna. Past experience tells me it is usually due to a poor signal ground, frequently caused by poor (or bad) solder joint(s) somewhere. Signal ground is not necessarily the same as the physical earth or chassis ground.
Many moons ago, when I was high school age, our family music group was setting up to provide music at a local (conservative) church, when all of a sudden we were getting the local rock station blaring out of the PA. It turned out to be a bad solder joint on the ground plane of the circuit of our keyboard. Years later at another church where I was doing sound, we started getting the truckers on their CB's during the Sunday service - again bad solder joints.