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 the problem.

The key here is to “divide and conquer.” It is 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.

Tools to aid in Radio Frequency Interference (RFI) Troubleshooting:

Here are some basic things that you will need for trouble shooting RFI problems. There are MUCH more esoteric gad gets 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.

Headphone amplifier
Dynamic microphone
Mic to line preamp
Battery-operated Phantom Power Supply
Ohm meter Divide and Conquer

Tools of the trade... Headphone amplifier and mic preamp - both battery powered (courtesy Shure Bros.)

Drop the level of the main fader on the mixer. If the RFI drops in level, you have localized it to the front end of the mixer (at least ahead of the main potentiometer). If it doesnot 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.

Listen to the mixer output through a headphone amplifier (battery powered) and continue on. Shure makes one that has a transformer-balanced input (Model FPI2) - highly recommended for this purpose.

Check the 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.

Mixer Problems?

Pick the mixer up 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 are in doubt at all about the condition of the building electrical ground, have a qualified electrician check it out. It is 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.

You may also want to read these related stories: