Solving RFI complaints--Part III
By John Shepler
Shepler is an electronics engineering manager and broad cast consultant. He has more than 20 years of experience in all phases of electronics.
An audio component that glitches and distorts at home but works just fine in your shop should clue you in to one probable cause: radio frequency interference (RFI). The last two Audio Corner columns described external and internal methods of eliminating RFI. In this final part of the series, we'll discuss desensitizing the amp and bypassing the power line.
Transistor and op-amp circuits
When input filtering doesn't do the job, you need to desensitize the amplifiers with more filter capacitors. This will not affect the audio response but will reduce the amplitude of the RF interference so that the amplifiers will not overload.
Figure 1 shows sample circuits for transistors and op-amps. A 5pF to 100pF capacitor from the inverting to non-inverting inputs of the IC will short out the amplifier as far as RF is concerned. As before, the capacitor value is dictated by the impedance seen at the amplifier inputs.
For transistors, the capacitor should be connected between the base and emitter as close to the transistor as possible. For both ICs and transistors, start bypassing at the first amplifier stage and continue until the problem goes away.
Often, only one or two capacitors are needed.
Figure 1. Filter capacitors can be used as an RFI bypass for transistors
and IC amps. A 5pF to 100pF capacitor from the inverting to non-inverting
inputs can be used with ICs. For transistors, the capacitor should be connected
between the base and emitter as close to the transistor as possible.
On rare occasions, diodes are used in audio circuits for gain control or voice compression. It may be necessary to add small bypass capacitors across each di ode to eliminate the RFI. Power-line bypassing Even if you have excellent shielding and your input and output cables are well-grounded, RF can sneak into the equipment through the power cord.
Sometimes a 0.01pF capacitor from each side of the line to the chassis is all that is needed. For stubborn cases, you can buy power-line filters in small metal cans for $8 or so.
Some outlet strips with built-in surge protection also have RFI filtering. You can install one of these close to the equipment and tightly coil up the amplifier's power cord to prevent it from acting as an antenna.
Opportunities
Many service shops have no experience with solving RFI complaints and shy away from this business. This creates opportunities for those who can exorcise the RF demons out of audio and other equipment. As you become comfortable solving these problems, you will want to advertise your services to radio and TV stations, ham and CB equipment dealers, and others who sell electronic equipment. These people are always getting RFI complaints and will be overjoyed to refer the business to you.
A simple plug-in filter or a couple of capacitors are worth quite a bit to people plagued by strong RF fields.