Audioclinic (Q and A) (Mar. 1972)

Cable Properties

Q. While investigating cable properties for use in some long runs from preamplifier to crossovers, amplifiers, and a remote control system, I became confused at the profusion of coaxial cables and their various properties. The main properties in question are (1) impedance, (2) capacitance from conductor to shield, (3) capacitance from conductor to conductor, (4) per cent shield coverage, and, of course, (5) wire size.

It appears that as nominal impedance rises, capacitance per foot decreases.

The question is: What are the most desirable characteristics to look for?

-Harry W. Kunz, Old Tappan, N.J.

A. The impedance of a piece of transmission line has no bearing on audio work. Cable of the type you have been examining is used for radio frequency work, where the impedance of the line becomes a factor. For your application, however, you need good shielding, reasonably low capacitance per foot, and flexibility. This latter consideration is helpful when wiring the system initially.

Fortunately for all of us, the output impedance of most equipment is quite low. We can accept, therefore, a considerable amount of capacitance per unit length of cable and still not suffer any loss of frequency response.

Good shielding is important in order to minimize hum. Therefore, in those circuits where this is a problem, it is well to use a cable whose percentage of shield coverage is high.

The wire size of the inner conductor or conductors of a shielded cable becomes important where the cable is to carry large amounts of current. The applications for which you plan to obtain cable are not at all stringent as regards the amount of current the cable must carry. Therefore, the wire gauge of the inner conductor can be quite small.

However, do not use the smallest possible wire diameter because it is too difficult to handle, and it can break if the cable is flexed too often. No. 20 or 22 gauge inner conductors work very well. You did say in your question that some of the cable is to be used for some kind of remote control system. This could involve current for relay control and the like. Therefore, consult wire tables to determine whether a given gauge conductor can handle the current you will use.

For all-around usefulness, use the ordinary "hi-fi" interconnecting cable.

This cable will not be suitable for microphone lines. Much of what has been said regarding shielded cable does apply to microphones. Very often, the microphones being used are high impedance. This means that high frequency losses will be greater than will occur in devices having low impedance. For this reason, microphones used for high impedance circuits should have less capacitance per foot than cables used with low impedance circuits. Another consideration is that shielded cable tends to produce noise when it is handled. This does not show up where the cable is fed from high-level sources.

However, when microphones or other low-level devices are involved, the handling noise produced by some cables can produce significant output. Special cable has been developed to meet this need. Cables of this sort are specified by their manufacturers as designed for microphones and other low-level audio signal applications.

Four-Channel Tapes from Discs

Q. Can four-channel tapes be made by taping from two-channel turntable and two-channel discs?

-Sp/4 James G. Gunter, APO San Francisco, Calif.

A. If the discs are encoded, they can be taped in the ordinary manner, using two-channel equipment, and the tapes can be played using the decoder and be heard as a four-channel product. Of course, if the tapes or discs being copied happen to be discrete four channel products, they must be copied on a four-channel recorder in order for the information to be received in its original form. Some recovery of the four channels of information could be obtained if you had a four-channel encoder. The four discrete channels would feed the proper inputs of the encoder, whose two outputs would be connected to a two-channel tape recorder. The tape could then be played through a decoder corresponding to the original encoding system, and you would once again have four channel output. Primary rear information would not be nearly so precisely pinpointed as it was on the discrete four-channel original tape or disc.

(Audio magazine, Mar. 1972, Joseph Giovanelli)

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