Audioclinic (Q and A) (Aug. 1972)

by Joseph Giovanelli

Improving AM Reception

Q. Since New York's only C and W station recently went off the air, I have had to take to AM dx'ing--both to pull in skip stations and low power suburban stations.

Most of the AM radios and receiver sections that I have seen, however, seem to be mediocre at best. I have decided, therefore, that the most logical solution would be to purchase a good car radio and use it in my apartment.

They appear to be designed to work under difficult conditions with superior sensitivity, shielding, etc. What is more, the whip antenna can easily be mounted on my windowsill or even outside the window.

The question that I want to ask you is what to do about a power supply. I have seen so-called `battery eliminators" on the market which convert house voltage to 12V.d.c. However, will it be necessary for such a unit to match the internal resistance--or perhaps some other parameter-of a car battery? If so, will there be any way that I can modify the circuitry to achieve this?

- Edward Subitzky, New York, N.Y.

A. Car radios are really little better than any other type of AM set. They generally have better AVC systems because they are often called upon to operate adjacent to broadcast transmitters. They do have an RF stage which does give them a measure of sensitivity. Because of your location and because the likelihood that you will be surrounded by TV sets which radiate a tremendous amount of interference, I am not sure that the use of a car radio will solve your problem.

If you want to experiment with a car radio, the power supply problem is not a serious one. The standard battery eliminator will be fine for your purpose. Their internal resistance is low enough so as not to be troublesome.

I suggest that you obtain a communication receiver. Such a receiver, in addition to having shortwave bands, has good i.f. selectivity. This selectivity will eliminate some "hash" caused by the TV sets in your neighborhood and will also eliminate most of the 10 KHz whistles which will be heard at night. Good communication receivers also have "notch" filters which help to eliminate some of the TV "birdies." Although the fidelity of sound reproduction will suffer when heard over such a receiver, at least it does offer you the best chance of hearing distant signals without annoying background noise. (Highs are lost because of the sideband cutting created by the selectivity of the IF system in such a receiver.) I do not say that reception will be perfect, but stands a chance of being much better than that which you can obtain with any other kind of receiver.

Generally, the audio in communication receivers is as bad as that in TV sets. Therefore, it would be best to take the signal from the detector, rather than from the audio output of the receiver.

A few feet of antenna wire connected to the back of the receiver is probably all you will ever need. (Of course, if you are in a steel building, it might be well for this wire to hang out the window. As was true of your possible car radio installation, you can mount a whip outside the window, thereby making a sturdier installation.) You know that a long antenna is often used with communication receivers. Although such an antenna has its advantages, you do not necessarily have to have one. Man-made noise imposes limits of reception which often will make the added antenna length useless.

Although this was not your original purpose, you may find that shortwave listening-made possible because of your communication receiver-can be fun and can open up a new world of entertainment.

It should be noted, too, that some of the newer, top-of-the-line receivers are including AM sections which very nearly match their FM counterparts in quality of output.

Four-Channel Sound

Q. Some of the new four-channel receivers advertisements speak of four channel sound effect from two-channel sources with the addition of two more speakers. Does this include a two channel or four-channel tape deck?

-Sp/4 James C. Gunter, APO San Francisco, Cal.

A. There are receivers featuring four separate power amplifiers sections. Recognizing that at present there is not much true four-channel sound available, equipment is often included to enhance two-channel listening so as to make use of all four power amplifier sections. The enhancement is brought about by the judicious use of some form of matrixing and/or reverb.

When this enhancing circuitry is not included in the receiver, it is available separately from manufacturers such as CBS, Electro-Voice, and Sansui. A straight four-channel amplifier or receiver can readily be converted to play encoded discs or tapes merely by the addition of these add-on devices. The tapes or discs are especially processed masters which are actually two-channel. The phase relationships, however, are such that four channel output appears from the decoder. Although there are these especially encoded records which make maximum use of these decoders, conventionally recorded two-channel discs will often produce excellent four channel spatial effects when played these decoders.

There is also the Dynaco decoder which uses four speakers but with just two power amplifier sections as in conventional stereo. It is the interconnection of the speakers to the amplifier which is non-standard and produces the four-channel effect.

While it is possible to obtain four channel tape decks which can feed into the four separate amplifiers of a receiver/integrated amplifier, it is also possible, because of the synthetic or combining systems just described, to obtain good four-channel results from two-channel tapes or discs.

European Equalization

Q. I often purchase imported phonograph records, mainly from Germany and England. These discs would have the European equalization. My amplifier has RIAA equalization. Am I losing quality when playing back these discs? If so, is there any way to correct the situation?

-Louis Hone, Montreal, Canada

A. As far as I know, European discs are equalized just as ours are. Therefore, you should not encounter any problems in this regard when playing them.

(Audio magazine, Aug. 1972)

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