From television to feature length films twenty-first American citizens have come to greatly rely on using media for entertainment. The moral of the story? Assume nothing.ДЄre the choir mics causing RFI problems? Most choir mics have a вЂњmoduleвЂќ that goes on the end of the line from the mixer.In our present day there is a wide variety of entertainment sources available. The manufacturer replaced the wire without question. 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). Ron Steinberg of Rentcom Communications told of an installation that was an RFI/EMI nightmare. If there are shorts between shields that canвЂ™t be located at either end, you may need to pull new wire. This is a time consuming process, and many installers overlook it. 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. Each pair of a snake cable should be individually shielded, and each shield should be isolated from the other shields. Using an ohmmeter, check for shorts between shields of pairs of multi-conductor cables. ItвЂ™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. If youвЂ™re in doubt at all about the condition of the building electrical ground, have a qualified electrician check it out. The only ground on a mic line should be at the mixer! Find the improper grounds and disconnect them. They will still work, but will be noisy since there is no common-mode rejection at the balanced input of the device.Д®lectricians often ground audio shields to conduits, jack plates, etc. Mic lines often get вЂњunbalancedвЂќ by a conductor getting shorted to conduit, etc. The building ground should be accessible on the 3-prong of the AC outlet that the mixer is plugged into. Using an ohm meter, check for shorts between any of the three conductors and a building ground. Using an appropriate outlet tester, check the AC socket that the mixer is plugged into to make sure that it is properly grounded.

If this changes the level of the RFI, consult the manufacturer of the mixer. The RFI will either get better, get worse, or stay the same. Pick up the mixer (if itвЂ™s not too large/heavy), and turn it. See the section on mixer inputs for some other things to try. 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. You may need an external phantom power supply for condenser microphones. Listen to each mic line individually through the headphone amplifier. 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. If the problem goes away when a specific channel is dropped, you have isolated the problem. If unplugging the mixer output stopped the RFI, the problem lies in the mixer and/or devices connected to it.Дrop the channel faders on the console one by one.

If there is still RFI at the output of the system, there is help later in this document. If it doesnвЂ™t go away, unplug the output of the mixer from the rest of the system.

If the RFI drops in level, youвЂ™ve localized it to the front end of the mixer (at least ahead of the main potentiometer). Start by dropping the level of the main fader on the console/mixer. Note that there are MANY more esoteric gadgets out there which can prove invaluable under many circumstances.Д«ut before we pull out the вЂњbig gunsвЂќ letвЂ™s look at some inexpensive tools that will locate most of the problems: YouвЂ™ll need some basic tools to troubleshoot RFI problems. Problems range from simple to complex, but more fall into the simple category. Start with simple tests and proceed to more rigorous ones. If more than one problem exists, these tests will help disclose that also. The key is to вЂњdivide and conquer.вЂќ ItвЂ™s essential that the problem be localized to one part of the sound system.

LetвЂ™s lay aside the вЂmagic fixesвЂќ and вЂњvoodoo methodsвЂќ and set forth a methodical procedure to deal with this problem, which is called Radio Frequency Interference (RFI). You turn on the sound system and you hear a radio station.

Old time radio sound board