Audioclinic (May 1977)

Removing Record Scratches

Q. I have a large collection of 45 rpm records, and quite a few of them are so badly scratched the groove keeps repeating itself. Because some of these discs are treasured collector's items, I wonder if there is some process to remove the scratches from the records?

-John Heiselman, Brooklyn, N.Y.

A. I do know of a method which is sometimes successful in keeping a repeating groove from repeating. It's a bit drastic so care must be used.

Use a cartridge with a conical stylus assembly and capable of an eight gram tracking force. Set the tonearm on the disc at a point just after the scratch. Then slowly rotate the turntable backwards so the stylus approaches the scratch from the opposite direction. If you are lucky, the stylus will plow through that area and remove enough of the damaged groove so the disc will play once again without repeating. There will always be some noise at that spot, but the disc should play without repeating.

Improved Overload Characteristics

Q. A few years ago a phono overload figure of 100 mV was supposed to be excellent, but now some companies are advertising values in excess of 500 mV. Does one conclude that a figure of 100 mV is not high enough? What maximum output voltage is to be expected with a Shure M91ED, with a nominal output of 5 mV?

-G.S., Honolulu, Hawaii.

A. A 100 mV overload figure is a good one, and I cannot imagine many instances where a cartridge having the voltage output you mention could reach this kind of signal level. The higher overload levels in today's equipment probably reflects the better transistors now available rather than a real need for the added headroom. As the cartridge output falls, because of the smaller moving mass, there is even less requirement for this high overload capability. What does appear important is to have better and better signal-to-noise ratios in the early stages of equipment.

I cannot conceive of a cartridge with a nominal output of 5 mV putting out more than 30 to 40 mV on some hot discs. By nominal output I refer to that output produced by a cartridge when it plays back a signal recorded at a frequency of 1 kHz with a velocity of 5 cm/sec.

Speaker Synthesis

Q. I am a 17-year-old girl trying to understand the BIG technical world of audio. I am only a beginner and I hope you won't think my questions are too elementary. What is the difference between a woofer and a tweeter in a speaker system? How can a person tell which is which?

-Soraya Cates, Tiptonville, Tenn.

A. You should not be concerned as to whether your questions are too elementary or not. All of us who work in the audio field had to start from the beginning, knowing nothing at all.

Both a woofer and a tweeter are loudspeakers which are combined to form a system. The woofer produces the lower tones, and the tweeter produces the higher ones. The tweeter is physically much smaller than the woofer since it has to move back and forth much faster, to produce the higher tones, as fast as 20,000 times per second. If it was larger, it couldn't move that rapidly.

The woofer is large and massive, compared to the tweeter. It requires a great deal of power to reproduce the low frequencies, and a large, rigid cone is needed to withstand that much power.

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, May 1977; Joseph Giovanelli)

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