Audioclinic (Q and A) (Apr. 1971)

A Matter of Tracking Force

Q. I own a good record changer, but I am having trouble with its anti-skating adjustment. I use a cartridge fitted with an elliptical stylus, which tracks a 1.5 grams. I also have the antis skate control at this same 1.5 gram force. The problem is that the stylus tends to take long thin pieces of vinyl from some records and very little from others. Sometimes so much vinyl (not dust) collects on the stylus that I must stop the table to remove it.

Shouldn't the anti-skating device prevent this? What effect will this "vinyl stripping, have on my records? What can I do about this? I balanced the tonearm correctly and I bought a new stylus. The condition still continues. The antiskate adjustment is stable and will not move when set. Is this problem unusual?

-Howard Wong, Woodside, N.Y.

A The problem with your record changer is unusual. No vinyl should ever be removed by the stylus. I can only wish that your problem really is dust.

It must be that you are tracking at a greater force than you realize. Do not depend on the gram scale on your changer. Obtain an independent force gauge and try it. The anti-skating control does not have much to do with this situation. This device is designed to correct some distortion and to limit long-term record wear.

If you are unable to repair this changer, send it back to the manufacturer's service agency. You will ruin your records in short order if you continue using this equipment in its present operating condition.

Condenser and Dynamic Microphones

Q. What are the differences between condenser and dynamic microphones?

-Arthur Darrow, Albany, N.Y.

The dynamic microphone is very similar to a loudspeaker. In fact, many intercoms use loudspeakers to double as microphones. The dynamic microphone consists of a very low-mass diaphragm fitted with a coil. This coil is suspended in a magnetic circuit. This equipment is so constructed that sound pressure striking the diaphragm causes it to move in accordance with that pressure. This motion, in turn, results in the coil's moving. The turns of the coil thereby cut the flux lines of the field . The voltage is then passed along to the appropriate amplifying equipment for signal processing.

The capacitor, or condenser, micro phone is much like an electrostatic speaker. (The dynamic and the electro static speaker and headphones were discussed in "Audioclinic," January, 1971.) The capacitor mike has a very low mass diaphragm, but with no coil attached. This is so arranged as to be one plate of a capacitor. Because the diaphragm can move as sound waves strike it, the distance between it and the other plate of this capacitor will vary in accordance with the sound pressure striking the diaphragm.

There are various schemes by which changes in capacitance cart be translated into electrical output. One approach is to make the capacitance between the two plates in the microphone a portion of a tuned circuit. This circuit is then placed in a configuration such that it will oscillate. As sound strikes the diaphragm, the frequency of this oscillation will vary because of changes in the resonant frequency of the tuned circuit.

The output of this oscillator will ultimately feed some kind of FM detector.

Another scheme is to place a large resistance in series with the capacitor and apply a voltage across this combi nation. As the diaphragm moves, the amount of charge on the capacitor will vary, causing a variation in voltage across the resistor. The variations of voltage across this resistor can be extracted and amplified. As was true of the dynamic microphone, this scheme results in a variation of voltage which is determined by the sound striking the diaphragm.

Minimum Response Requirements

Q. What does the frequency response of a music system need to be in order to faithfully reproduce music?

- Spl 4 Don Niemczuk, APO San Francisco, Calif.

A. The minimum frequency response that is required for adequate sound reproduction is a highly debatable subject. Certainly we would like a system to be flat from 40 to 15,000 Hz. How ever, there are experts in this field who believe that an amplifier should by capable of reproducing flat to at least five times the highest audio frequency required. This would require a response flat to about 75 KHz.

(Audio magazine, Apr. 1971, Joseph Giovanelli)

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