Audioclinic (Jul. 1979)

FM Noise Problem

Q. I have a reception problem with New York City FM radio station WQXR (96.3 MHz), with which I hope you can help. I receive a perfectly clear, noise-free signal from every other local station with the single exception of this station, in which case I experience a background noise similar to SCA "chatter" or "swishing." However, this noise disappears when my tuner is switched to mono.

-Alfred Cocchini, Glendale, N.Y.

A. The i.f. of most tuners is 10.7 MHz, and if you multiply this frequency by nine, you come up with a frequency of 96.3 MHz, the frequency on which WQXR broadcasts.

When your tuner is receiving any station, the i.f. system radiates energy to some extent. It not only radiates the 10.7.MHz i.f., but also the various harmonic frequency multiples of 10.7 MHz. The 9th and 10th harmonics of 10.7 appear to be the only ones which can interfere with FM reception. In your case, the 9th harmonic has sufficient energy to be picked up by the front end, perhaps by a small piece of the twin lead used to feed signal from the antenna terminals to the input of the front end. This internal signal mixes in the front end with the external signal to form a beat; the audible effect of this beat is less apparent in mono because there are no pilot or other higher frequency signals to form other beats. In the stereo mode, there will be other beats, all dependent upon the instantaneous deviation of the desired signal. Because of the various possible beats, and because they are constantly changing, the effect is that of background noise rather than a pure tone.

My own tuner exhibits this problem, and I cure it by slightly mistuning the station, which does not degrade the sound in the slightest. The result is that the i.f. is now something other than 10.7 MHz, just different enough to prevent this beating effect from being audible, but not so different as to result in the signal being out of the bandpass of the i.f. system. I think you should write to the maker of your tuner to see what he can suggest by way of better shielding for the i.f. sys tem, so that its energy cannot be picked up by the front end.

Transient Pulses

Q. I hooked up an equalizer to my amplifier at its preamp-out, power-amp in jacks. Whenever the amplifier is on and the equalizer is turned on, there is a loud pop and the speaker fuses blow. This also occurs when turning the equalizer off while the amp is still on. The problem, however, doesn't occur when the equalizer is turned on first. Can I just leave the equalizer on all the time, or is there a better solution?

-Dennis Fischer; Denver, Colo.

A. You should consider putting the equalizer into the tape loop, rather than between the pre-amplifier and power amp. I know that this can be a problem when a tape recorder is al ready connected to this set of jacks. However, most equalizers have tape-loop facilities, so this is probably the best solution.

Test Equipment Impedances

Q. I recently purchased electronic test equipment in order to test and repair my audio equipment. Suppose I want to connect my 50-ohm sinewave oscillator output to the 100-kilohm in put (AUX) of my amplifier. Would the impedance mismatch cause problems?

-Michael Bartholomew, Hellertown, Penna.

A. Keep in mind that most of the time we don't match impedances in audio, contrary to all the talk being thrown around. This is true for the audio oscillator used to test the power amplifier; there is no need to match impedances. Matching impedances produces a maximum transfer of power between two circuits. With the circuits we are discussing, we are more interested in transferring maximum voltage, which doesn't occur at the point of maximum power transfer.

(Source: Audio magazine, Jul. 1979; Joseph Giovanelli )

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