Inadvertent Radio and Other
Noises
Last month,
we began a short series on troubleshooting an audio system with a discussion of perhaps
the most common extraneous noise: hum.
In addition to the 60Hz line frequency that shows up as
hum, we are surrounded by other strong electromagnetic fields that can affect our stereo
systems as well. Radio signals generally just pass by us unless we take some care to
provide a tuned circuit that will pick them up: a radio. But sometimes a fortuitous
combination of cable lengths and connections can create just such a detector
inadvertently, and the radio-frequency signals are converted to something we can hear.
These may be strong enough to drive a speaker directly,
even when the system is turned off, but it's more usual for such unwanted material to
enter the system through low-level circuits and be amplified subsequently. In some cases,
RFI may be picked up by the speaker leads (which can make dandy antennas), carried to the
amplifier's chassis, and then picked up by a low-level input. Usually moving your
equipment, even slightly, or changing the position and length of cables -- especially
speaker wires -- is enough to solve the problem. Line filters to remove signals that may
be conducted along the AC power cables offer another solution. In extreme cases, such as
persistent interference from communications equipment (CB or ham radios, for instance), an
approach to the originator may be necessary.
Depending on your equipment, RFI from a CD player might
disrupt your FM reception. You can easily check this: if the problem stops when you turn
off the player's power, that's it. Placing the two components as far apart as possible may
clear things up; if not, you will probably have to switch off the CD player when you want
to listen to FM, but that's not much of a hardship, as few of us want to listen to both
simultaneously.
Other extraneous noises tend to be specific to particular
components. Crackling noises that occur when level controls or switches are operated can
usually be fixed with a shot of contact cleaner; if not, a trip to a service center is
probably necessary.
Increased noise or reduced treble response in a tape deck
often means cleaning and demagnetization are due. In extreme cases, the heads may have
become misaligned, and that will require professional attention. If tapes made on another
machine sound particularly bad (particularly in mono), that may be what's wrong.
Next time, I’ll talk about what to do when the sound
quits entirely.
...Ian G. Masters
ian@mastersonaudio.com
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