Installs Picking up radio station on installed system

[Morte615]

I have an installation that over the last few years were picking up an AM radio station through the system anytime they plugged something into the floor pockets on the stage. This year they redid all the jacks and wires under the stage and included shielded XLR wire for the connections. Just last night though I heard the return of the radio station, it only happened when I had a 50' XLR cable plugged into a direct box on one end and the center stage floor pocket on the other (there are 3 floor pockets on the stage and 2 power pockets, in either of the other pockets nothing happened.)

If I unplugged the direct box the station went away, I was told that this used to happen all the time when they plugged a keyboard in to the stage pockets also.

So any suggestions on ways to get rid of this? An idea floating around here is to connect the shield on the cable to earth ground but I am worried it may introduce a ground loop then (and we already have enough problems with that as it is.)

 

[FMEng]

You are apparently in a high RF environment, with a nearby AM transmitter. The gremlins may be seasonal, depending upon how the water table affects earth conductivity. Your instinct about grounding is right on. Adding grounds will introduce loops and won't remove the RF, because most grounds aren't really grounds at RF frequencies.

If you can plug a dynamic mic into the same cabling (layed out the same way) and it is clean, then we can focus in on the DI. First, experiment with the ground lift switch. Typically, if the DI is used with an instrument amp, lift the ground. If plugged into an instrument with no amp, ground it. But, it never to hurts to try both. RF or not, this applies.

If it is an active DI: It has two or three antennas connected to it, the input cable(s) and the output cable. They introduce RF into amplifier circuitry inside, which can demodulate the RF into audio. You could replace the DI with a more robust one, or you can keep the RF from reaching it. You can put a toroid core donut on each cable connected to the DI, right where they plug into it. The toroid core material has to be tuned for the right frequency range. For AM stations, I would use the FT-193-J from Amidon. The more wraps of cable around the donut, the better.
http://www.amidoncorp.com/ft-193a-jc/

If it is a passive DI: Then the DI is simply passing RF along to the mixer input, and the mixer is doing the demodulating. Again, a better DI could solve the issue, or we can block the RF from reaching the mixer. In this case, I would put the toroid on the mic cable near where it plugs into the mixer. If the cables aren't long enough, use some short jumpers to wrap around the donut.

This is the same approach I would use if you have AM issues with mics. Torroids on all the mixer inputs could be good preventative medicine. It can also help to put one on the AC cord, near where it enters the mixer. Occasionally, a condenser mic can have RF issues, where a toroid will need to go on the cable within a few feet of the mic. I would try winding the cable onto the torroid about 6 ft from the mic so that it isn't obnoxious looking.

Even if another solution solves your issue, I would keep a few torroids handy. When the environment has a strong RF field, it isn't a question of if it will cause issues but when. A few donuts can save a show.

 

[TimmyP1955]

Check all XLRs to see if pin 1 is connected to the shell. Usually it should not be.

 

[BobHealey]

What's the gain level when you start getting AM radio? There are some places around here I can get reliably get AM 810 in, either by running long distance unbalanced (did not realize the TRS outputs on console were unbalanced until I downloaded the manual after getting home) or by cranking the head amp gain up to its limits (30+year old wiring, multiple analog and digital consoles) with nothing connected to the XLR.

Actually, on that tangent, are all 3 pins on the problem snake channel continuous back to the console?