Audioclinic (Sept. 1984)

Protection-Circuit Problem

Q. I have a receiver which I purchased some time ago. Every once in a while a relay will click and the sound disappears from my loudspeakers.

However, the signal is still present at the input of my tape recorder. Sometimes, the relay will again click and the sound returns. At other times, I can bring the operation back to normal by a sharp rap on the side of the unit. Does this sound like some kind of protection-circuit problem?

-Robert W. Thompson; Glendale Heights, Ill.

A. The relay is part of a protection circuit. You may be driving your amp beyond its limits, thereby activating the protection circuit (and the relay) when the temperature on the heat-sink rises to an appropriate point. The fact that the tape recorder is still fed by its correct signal is at least partial proof that your protection circuit is triggering.

Sometimes these circuits will "take off"--even when the receiver is not being overdriven or when there is no signal fed into it. This may represent a malfunction of the protection system. It may also signify there is a lack of good ventilation around the equipment, which would result in a gradual rise in heat-sink temperature. This, in turn, would trigger the protection circuit.

Using Timers

Q. How safe are timers when used with a stereo system? I have always understood that such timers have the same effect as pulling the a.c. power cord out of the wall outlet with the system turned on.

-Rocky Ciesick; Panama City, Fla.

A. Cutting off the power has virtually the same effect whether it's done by pulling the plug, shutting off the system's master switch, or interposing an external switch, such as a timer. The more components switched on or off at once, the bigger the surge on the a.c. line. Therefore, it might be more advisable to use the timer to turn on only the components which need to be on at that time-for example, just the tuner and the tape deck, when you are taping programs off the air.

The only problems I have found with using timers occur when using mechanically controlled tape decks or tape decks and phonographs with idler-wheel drive systems. Mechanically controlled tape decks use your finger pressure on the "Play" key to push the pinch roller against the capstan.

On such decks, the roller must be left in position for the period between setup and timer actuation, which can permanently flatten the roller at that point.

(This does not happen on solenoid operated decks, whose pinch rollers are moved into position electrically.) In some decks and turntables, transport drive idlers will also be engaged under pressure, again possibly producing flat spots. Flat spots on the pinch roller or drive idlers cause wow and, sometimes, physically noisy operation.

In general, if a tape deck has a timer-recording switch or switch position, it is safe to use it with a timer. Ali other components are also safe, except for idler-driven phonographs (which are becoming less and less common).

Stylus Life

Q. How many hours can a stylus play records before you must replace it?

-Robert Hirose; Los Angeles, Cal.

A. Stylus life varies markedly, depending on such factors as stylus shape, tracking force, anti-skating force, the overall loudness of the discs being played, and the amount of foreign matter the stylus encounters in its travels. There are also variations in the durability of the individual gems from which styli are ground. Use the manufacturer's recommendations as your guide, and have your stylus checked from time to time (especially when it nears or passes the end of its rated life) on a stereo microscope (one with independent left and right optical systems). Good stereo microscopes are expensive, but many dealers have them.

If the tonearm is handled carelessly, so that the stylus is dropped onto a disc or the turntable, the stylus may need replacement after virtually no playing time at all.

Muting Again

In the October 1983 "Audioclinic," you fielded a question concerning the -20 dB muting switch found on many amplifiers and receivers. The owner's manual supplied with my receiver stated the following about this switch:

1. It extends the useful range of the volume control. With high-output signal sources, with efficient speakers or with sensitive headphones, you may find that the sound is too loud over most of the range of the volume control, i.e., you are restricted to using only settings near the lower end of the control range. By engaging the low-level switch, you can use the full range of the volume control for normal listening.

2. It provides optimum signal-to-noise ratio for low-level listening in quiet environments. For example, if you are listening to soft music late at night and the surroundings are quiet, the low-level switch minimizes the already low residual of the preamplifier and tone control circuits.

3. It provides a convenient, temporary cut in volume (as your response in Audio stated).

-Joe Mazzini; Montgomery Creek, Cal.

Digital Turntables

Q. Are there such things as digital turntables? If not, why are more and more phonograph records being digitally recorded if they are just to be played back on analog or conventional phonographs?

-William Scaramazca, Swedesboro, N.J.

A. The phonograph is an analog system, and even digitally-recorded master tapes must be converted to analog form for phonograph disc mastering. But studios are making more and more of their masters digitally, both in preparation for the release of these recordings in such digital formats as the Compact Disc and to get better quality in their LP releases. Digital masters have less noise and distortion and more extended high- and low-frequency response than analog masters, and they are not gradually degraded by constant recopying in the process of converting multi-track originals to two track stereo releases. (There are, however, some who feel that digital recordings sound harsher.) The Compact Disc player might be considered the "digital turntable" you asked about. But it, of course, plays only digital CDs--some of which were made from analog masters.

(adapted from Audio magazine, Sept. 1984; JOSEPH GIOVANELLI)

= = = =