Audioclinic (Aug. 1977)

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Record Grooves

Q. I assume that the record industry has standardized the specification for the pitch of a record cutting lathe, also the number of grooves per inch along the radius of the record. What is that number and what is it called?

-Mark Hutchenreuther, Oxnard, Cal.

A. There is no standardized number of grooves per inch in a cutting system, and it is not possible to standardize this parameter. What is often done to make maximum use of the space available on the disc is that groove geometry and the number of grooves per inch are made to vary in accordance with the amount of volume present on the disc at any given time.

In a case where the dynamic range is wide and where there are long passages of soft playing, the number of grooves might be compressed during those soft passages to better than 350 per inch. During the loudest passages they would expand to 150 grooves per inch. This would enable a good match between the maximum permissible recording time, recording level, and the bass response.

FM Drift

Q. What would cause my receiver to go off frequency even when the AFC has been switched on? This does not happen on weekends, only during the week, and as late as 10 p.m. I live in a business-residential area in downtown New York City. What can the problem be?

- Bernard Friedland, New York, N.Y.

A. Are you sure your receiver drifts off frequency? Perhaps something else is wrong. If the equipment has a tuning meter, you can see this at a glance. Of course, even without a meter, if retuning the dial improves audio quality, then frequency drift is the problem.

The chances are, though, that your equipment is functioning normally and the problem is really one of power line voltage variations. If the voltage drops below a certain critical point, the regulators in your equipment will become ineffective, and frequency drift and reduced power will be the likely results of this. I suggest that you check to see just how much voltage change takes place using an a.c. voltmeter which costs between $5 and $10.00. Your power supply company should be consulted in the event this condition does exist.

Also, the probable reason that your equipment works best on weekends is because the industrial users are not consuming enough power to cause a serious voltage loss in your home.

There are devices known as "constant voltage transformers" which can be used to steady line voltage, but these are quite expensive. But this condition should not go uncorrected because, as you have seen, low voltage can adversely affect the performance of a sound system.

Speaker Phasing

Q. I wish to wire two identical 4-ohm speakers in series. Please explain how this is done so the speakers will be in phase?

- Stanton Voliman, Bronx, N.Y.

A. To wire two identical speakers in series and know they are in phase, connect the common of speaker A to the common amplifier output terminal, the hot terminal of speaker A is connected to the common terminal of speaker B, and the hot terminal of speaker B is connected to the hot amplifier output terminal.

If this hookup is to be made with two or more channels, each channel must be wired in the same manner.

All the speakers will then be in-phase as long as all amplifiers are in the same phase relationship. This will be true if all speakers employed are of the same make and model, but if various speakers are used then the phase relationships will have to be checked.

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

(Source: Audio magazine, Aug. 1977, Joseph Giovanelli)

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