You turn on the sound system and you hear a radio station. Now what?
Let’s lay aside the ‘magic fixes” and “voodoo methods” and set forth a methodical procedure to deal with this problem, which is called Radio Frequency Interference (RFI).
The key is to “divide and conquer.” It’s essential that the problem be localized to one part of the sound system. If more than one problem exists, these tests will help disclose that also.
Start with simple tests and proceed to more rigorous ones. Problems range from simple to complex, but more fall into the simple category.
You’ll need some basic tools to troubleshoot RFI problems. Note that there are MANY more esoteric gadgets out there which can prove invaluable under many circumstances.
But before we pull out the “big guns” let’s look at some inexpensive tools that will locate most of the problems:
—Mic to line preamp
—Battery-operated phantom power supply
Divide and Conquer
Start by dropping the level of the main fader on the console/mixer. If the RFI drops in level, you’ve localized it to the front end of the mixer (at least ahead of the main potentiometer).
If it doesn’t go away, unplug the output of the mixer from the rest of the system. If there is still RFI at the output of the system, there is help later in this document.
If unplugging the mixer output stopped the RFI, the problem lies in the mixer and/or devices connected to it.
Check Mic Lines
Drop the channel faders on the console one by one. If the problem goes away when a specific channel is dropped, you have isolated the problem.
If the RFI drops in level a little with each channel, you may have found many problems! It is important to determine whether the RFI is getting into the mic lines (very common) or somewhere else.
Listen to each mic line individually through the headphone amplifier. You may need an external phantom power supply for condenser microphones.
If the mic lines are clean through the headphone amp, but have RFI through the mixer, the mixer input may not be grounded properly or may not be RFI immune. See the section on mixer inputs for some other things to try.
Pick up the mixer (if it’s not too large/heavey), and turn it. The RFI will either get better, get worse, or stay the same. If this changes the level of the RFI, consult the manufacturer of the mixer.
Too Many Grounds?
Using an appropriate outlet tester, check the AC socket that the mixer is plugged into to make sure that it is properly grounded. Disconnect each mic line from the mixer.
Using an ohm meter, check for shorts between any of the three conductors and a building ground. The building ground should be accessible on the 3-prong of the AC outlet that the mixer is plugged into.
Mic lines often get “unbalanced” by a conductor getting shorted to conduit, etc. somewhere up the path. They will still work, but will be noisy since there is no common-mode rejection at the balanced input of the device.
Electricians often ground audio shields to conduits, jack plates, etc. The only ground on a mic line should be at the mixer! Find the improper grounds and disconnect them.
If you’re in doubt at all about the condition of the building electrical ground, have a qualified electrician check it out.
It’s usually a bad idea to drive a dedicated ground rod for the audio system, as it establishes different ground potentials for different electrical devices in the same building.
Using an ohmmeter, check for shorts between shields of pairs of multi-conductor cables. Each pair of a snake cable should be individually shielded, and each shield should be isolated from the other shields.
When the outer jacket is removed from a snake cable for wiring purposes, it is important to heat shrink the individual pairs to maintain their isolation.
This is a time consuming process, and many installers overlook it. If there are shorts between shields that can’t be located at either end, you may need to pull new wire.
Ron Steinberg of Rentcom Communications told of an installation that was an RFI/EMI nightmare. Upon testing the newly installed mic lines, it was discovered that there was no continuity between the drain wire and the foil shield (an extra layer of Mylar was isolating them).
Very unusual, but not impossible. The manufacturer replaced the wire without question.
The moral of the story? Assume nothing.
Choir Mic Problems?
Are the choir mics causing RFI problems? Most choir mics have a “module” that goes on the end of the line from the mixer.
A small diameter cable proceeds from that point to the microphone. On most choir mics, this is an unbalanced line.
Such lines should be cut to length and never coiled up. Unplug the mic line from the module, leaving the module plugged into the mixer.
Does the problem go away? No? Unhook the module and plug in a regular dynamic microphone into the mixer through the same mic line.
Does the problem go away? You may need to install filters on the mic input to the module. Better yet, consult the manufacturer of the microphone for some remedies.
Chances are you aren’t the first one that has had this problem.