Audioclinic (Feb. 1979)

Increased Treble

Q. I recently purchased an outboard Dolby FM unit and a 25uS de-emphasis switch, all to be used in the tape loop of my receiver. I returned the Dolby unit and, out of curiosity, connected the de-emphasis switch to the tape jacks. I was delighted with the FM results, the highs were much more distinct and the background noise in creased only slightly. The fact that the switch alone produces such an improvement, is this to be expected?

- Douglas D. Short, Raleigh, N.C.

A. The increase in highs you hear is to be expected when you use the de-emphasis switch. FM broadcast practice calls for a 75-uS de-emphasis to produce a flat frequency response from an FM tuner. The 25-uS de-emphasis produced by your switch means that there is now more treble than in the standard FM broadcast. This means that you have boosted the highs by about 10 dB, so you are not listening to a flat signal.

Cable Systems

Q. I recently hooked up my cable TV antenna to my receiver which allows me to pick up Washington D.C.

FM stations quite well, but only in mono as the background hiss is in tolerable in stereo. My high filter switch doesn't help; what can I do to reduce the stereo noise?

- Name Withheld

A. Part of your problem may be that the cable TV system does not intentionally supply the FM band, so it may be that to the extent your cable acts as an external antenna, you are picking up Washington stations. Connecting your cable system to the receiver with a 300-ohm twin lead, rather than a coaxial cable and a transformer, can cause the receiver to pick up signals directly and not necessarily via the cable. Furthermore such installations can cause radiation from the cable sys tem which is undesirable. I recommend that you check with your cable company to see if they do indeed import Washington signals which would tell you if you have any hope of receiving these signals properly over the cable, rather than using some other kind of antenna.

See if, perhaps, a simple dipole antenna will bring about an improvement in FM reception over what is available to you via your cable system.

Actually it is always best to use an out side antenna where permitted, rather than the simple dipole often supplied with the receiver.

No noise reduction system will work without adversely affecting the quality of the programming being received.

However, if one or more of the Washington stations use the Dolby N/R sys tem for their transmissions, you can use that means to provide less back ground noise on such stations.

It is possible that a really good signal-booster amplifier can be helpful.

However, this booster would have to have a better signal-to-noise ratio than the tuner with which it is being used.

If the cable is involved, though, there may be noise from this source riding on the signals which would negate the effects of the booster.

Tuning Meter Drift

Q. My problem is that when I listen to my speakers my tuning meter drifts, however, when I listen to my head phones the meter stays centered. What is the problem?

- David R. Barr, Wyandotte, Wis.

A. Perhaps the vibration of the speaker system is causing misalignment of some circuit elements resulting in a change of oscillator frequency.

Play the system softly and notice if the drift still occurs. It shouldn't. If it still occurs, check once more to see if it happens when you wear your head phones.

The most likely possibility is that operating your speakers requires more power to be taken from the output stage of your receiver. This means that more heating of the electronics within your equipment takes place which will cause changes in capacitance of the oscillator circuit leading to a gradual center-of-tuning drift.

Another possibility is that if you start out using the loudspeakers and then switch to the headphones, it could be that the tuner has been stabilized by this time so that no further drift occurs.

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

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