Audioclinic (Q and A) (Feb. 1972)

Equipment Fed in Parallel

Q. Is it possible to connect tuner and tape deck outputs in parallel, and then to the input of a power amplifier? Is it possible to connect the input of a tape deck and amplifier in parallel, and then to the output of a tuner?

-Joel Masser, Highland Park, Ill.

A. You should not connect a tuner and a tape deck in parallel, and then feed the input of a power amplifier or preamplifier. By so doing you stand a chance of losing both output and low frequency response.

Keep in mind the fact that, in addition to each piece of equipment feeding into the power amplifier, each one attempts to feed the other. It is well for a tape deck or tuner to "see" a high impedance. Because the tuner "sees" the low impedance of the tape deck's output, and because the tape deck "sees" the low impedance of the tuner's output, these devices do not look into the high impedance they require.

With the arrangement you are proposing you stand to lose at least 6 db of output, and perhaps more. You may find that the tuner works fine but that the tape machine does not perform well, or vice versa. Further, if the size of the coupling capacitors in the output circuits of the tuner and tape deck are comparatively low values, bass response will be lost because of the excessive loading applied to them. (This comes about because of the reactance of the capacitors' being higher than the impedance into which they look.) Most of the time there would not be a need to connect a tuner and tape deck to an amplifier; they would not be used simultaneously. Therefore, a simple switching system should be used. If more flexibility is required, a simple jack field can be wired so that all devices can be connected together as is done in telephone switchboards.

On the other hand, if you want your tuner to feed into both your power amplifier's input and your tape recorder's input in parallel, this is fine.

The impedance of both the tape machine's input and that of the amplifier is high enough so that no loading of the tuner's output will result. This still holds true even though the tuner's output looks into the combined impedance of both the amplifier and the tape deck's input circuits. If each of these input circuits is 200 K ohms, then the combined impedance of the two units will only be 100 K ohms, high enough not to cause excessive loading of the tuner. If you really wish to be safe, check the manual for your tuner and note the minimum impedance it should look into. Then check the impedance of both your tape machine and amplifier.

If their combined value is higher than that of the tuner's recommended minimum, you can make the connections as desired.

Of course, if your amplifier has tape monitoring provisions, this is the best way of setting up your equipment. It, permits greater flexibility without the need for changing connections.

Skating and Tracking Forces

Q. I read somewhere that as stylus force increases, the less the need for antiskating correction. Then, why do manufacturers of arms and players increase the corrective force as tracking force increases, instead of decreasing it?

-Elliott Dennis, Brooklyn, N.Y.

A. If you have a tonearm which exhibits lots of skating force and if this arm tracks lightly, it is possible that the skating force will cause skipped grooves at times of high groove modulation. Actually, it is the combination of skating force, light tracking force, and the vertical component of the groove modulation which causes groove jumping. When more tracking force is added, skipping does not occur; the additional tracking force has overcome the tendency of the vertical groove motion to throw the stylus out of the groove. This does not mean, however, that the tendency to skate has been suppressed. It simply means that its effects have been overcome at least in terms of skipped grooves. In reality, the amount of skating force has actually increased.

One of the factors which produces skating force is the friction between the stylus and the groove walls. As tracking force increases, friction also increases, thereby producing an increase in skating force.

(Audio magazine, Feb. 1972, Joseph Giovanelli)

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