"Mystery Radio or Ground Loops from Hell"
© Mike Konopka

While working in Chicago's "The Second City" theatres, I came across a mic input that was picking up hefty amounts of "WBBM News Radio 78 AM". From my experience, radio interference is usually caused by poor grounding or a ground loop. For those unfamiliar with the term "ground loop", this refers to a system or device's multiple paths to ground.

So, I knew there had to be a ground loop due to the blaring radio, but where? Inspection of the system showed that the mixer was properly grounded and all outboard and power amp AC cords were lifted. Next I checked the video recorder and camera grounds. This equipment was used for taping and dressing room cues. Nine times out of ten, ground loops in theatres can be traced here. I've even encountered ground loops caused by cable TV installations. Cable ground loops can be remedied by  installation of a lifting transformer on the audio section, available at Radio Shack (in the car audio area).
But none of the above was the cause of my radio problem. After climbing thru the suspended ceiling of the theatre, I finally found the cause of my headaches. Years ago during the original mic snake installation, the installer grounded the case of the XLR connectors to pin 1 (a practice that I don't recommend). It turned out that the shell of one of these connectors was touching against the metalwork of the suspended ceiling, which in turn was touching the stage lighting grid... turning the entire ceiling and lighting system into a giant radio antenna. One little snip of the shell to ground connections solved all my problems with "News Radio 78". The weird thing was that as soon as I fixed this radio interference problem, in walked a reporter from News Radio 78 to interview the cast!!

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