AUDIOCLINIC (Q and A) (Dec. 1976)

Record Grove Deformation

Q. I recently read that after playing a record you should wait at least an hour before replaying it. Is there anything to this?

-T.C. Williams, Alamosa, Colo.

A. Vinyl material, used as the main ingredient in phonograph records, is soft. The force exerted on the groove walls by the stylus, especially at high frequencies, can be tremendous. This force results in a deformation of the groove walls. Vinyl, however, has a memory and will slowly return to its original position after a time.

When the disc is played over and over again, however, the material will not have an opportunity to "spring back" into its normal shape. The longer it is prevented from doing so, the less likely it can ever completely return to its original condition.

Deformations of the kind we are discussing will ultimately result in distortion at high frequencies, much like the sound of a worn stylus. This deformation will produce audible effects even though the disc will still play with little background surface noise.

Microphones and Transformers

Q. On most commercially made PA mixers, there are both to-Z and Hi-Z mike inputs. Is the purpose of these transformers on the Lo-Z inputs to step up the signal from a Lo-Z mike for the Hi-Z inputs of the mixer/preamp? What then is the relationship of the mike, the line transformer, and the two inputs?

-T. Young, Thomaston, Conn.

A. A Lo-Z mike with a long run of cable will need the transformer for feeding into the Hi-Z input because of the need for higher input voltage and to insure against loss of the signal.

With a long run of cable, Hi-Z mikes should not be used because with lengths of 20 feet there will be a high frequency loss of up to 6 dB at 10 kHz.

If you have a Lo-Z mike and mixer with the option of using low impedance inputs, by all means use them.

This will mean that you don't need the transformer. The transformer may actually be located in the mixer already, or at least the functions of the transformer are taken care of by appropriate circuitry.

Hiss In a Reverb Amplifier

Q. I have a reverberation amplifier which produces a bad "hiss" when the "reverb time" control is turned up to its halfway point or higher. If the reverb is off but the power is still on, there is no "hiss." Once the reverb has been turned either to its "on" or to its "record" mode, and with the "reverb time" control turned up as described, the "hiss" starts again. I've tried changing my inputs and tried grounding arrangements, but this "hiss" is always there. Is there anything I can do to overcome this problem?

-Sgt. Herm Rosario, APO, San Francisco, Cal.

A. When the reverberation control is advanced, what actually takes place is that a signal from a high-gain amplifier is mixed with the "non-reverberant" signal. High-gain amplifiers can often be noisy, either because of inherently poor design or defective circuit elements. The most common problem is a noisy input transistor. We suggest, therefore, that you change this transistor. If the circuit is still "hissy," see if you can locate a transistor which has equivalent electrical characteristics, but an inherently lower noise level.

While any amplifier generates noise, if there is a sufficient amount of signal present, the noise will be masked. Thus, there is the possibility that you are not feeding a sufficient amount of signal into your reverberation system. Check the specs on the unit and the source to see whether this is the case.

If you have a problem or question on tape recording, write to Mr. Herman Burstein at AUDIO, 401 North Broad Street, Philadelphia, Pa. 19108, USA. All letters are answered. Please enclose a stamped, self-addressed envelope.

(Audio magazine, Dec. 1976, JOSEPH GIOVANELLI)

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