Audioclinic (July 1984)

Individual Channel ECU and Controls

Q. I cannot understand why the makers of equalizers find it necessary or desirable to separate the controls for the left and the right channels. Every time an adjustment is to be made, it requires that much longer to insure that the left and right channels are balanced as well as equalized.

-Jim Quaderer, Palo Alto, Cal.

A. If you're using your equalizer as a more elaborate tone control, then it's more convenient to have both channels ganged. Five-band equalizers, which are mostly used for this, almost invariably have ganged controls.

If you're using it to correct for room or system problems, then you need to be able to equalize each channel separately, since the acoustical properties of the left side of a listening room may be quite different from those of its right side. Third-octave and half-octave equalizers, which are mostly used for this, invariably have individual controls for each channel.

The common 10-band equalizers may be used for either of these applications. Consequently, some manufacturers build them with ganged controls, while other firms make units with separate controls for right and left.

If you frequently re-use the same settings, you might take a tip from Soundcraftsmen and cut a cardboard template for the slider positions. This lets you restore all your slider settings in seconds, by moving them till they are stopped by the template's edge.

Switched Outlets

Q. Is it safe to plug my amplifier's power cord into the switched outlet on the rear of my preamplifier? My preamplifier's instruction manual says this outlet can handle 800 watts maximum.

The instruction manual for the power amplifier states that it can draw up to 1,000 watts. Also, the power cord has a three-prong plug. How can this be plugged into my switched (2-prong) outlet?

-Greg Bower, Fremont, Ohio

A. The connections you wish to make are marginally acceptable. Most of the time your power amplifier will not draw the 800 watts allowed by your switched outlet's wiring. The 1,000 watts will only be drawn on extreme peaks. From this standpoint, you can get away with the hookup. My only reservations have to do with the fact that if the power amplifier is capable of such a high, peak current, it must possess a low d.c. resistance. What this means is that sometimes (depending on the instantaneous line voltage at the time of turn-on) the current will be very high for an instant-before the back EMF of the power transformer stabilizes the current drain. Repeated surges will cause the on/off switch on your preamp to fail because of arcing.

The matter of the 3-prong plug is readily solved. You need an adaptor made to connect a 3-prong plug into a 2-prong socket. This adaptor contains a pigtail lead which represents the ground wire. This ground should be screwed to the most convenient ground on the preamplifier's chassis.

CD Players, Tape Recorders and Acoustic Feedback

Q. Can acoustic feedback affect CD players or tape recorders having wide dynamic range?

-Dennis A. Millage, Evergreen, Colo.

A. Although I have heard of a case of acoustic feedback which involved a CD player, I have been unable to re create it myself. I experimented with several players, selecting a quiet musical passage and turning up the volume, bass and treble controls on my amplifier. I never noticed even a tendency toward feedback.

As for this phenomenon's taking place with a tape deck, I have been able to produce that condition in some older machines, with microphonic heads and tubes. I have not been able to produce acoustic feedback with any recent tape recorders and players, cassette or open reel.

When to Replace a Cartridge

Q. How does one determine when a cartridge loses its magnetic strength and, hence, when it's time to purchase another one?

-Robert Hirose, Los Angeles, Cal.

A. It is unlikely that the magnetic flux in your cartridge will drop to a point which will produce changes in performance. Even if there was a loss of flux, the result would be a lowering of overall sound level, but the rest of the parameters for that cartridge would be unaffected. If the signal level dropped below the point where the background noise from your preamplifier became a problem, it would then be time to re place the cartridge.

Digital Audio

Q. Please explain to me what digital audio really is. In this connection, how does a CD player know when to slow down from 500 to 200 rpm?

-Tony Perkins, Los Angeles, Cal.

A. Sound consists of continuous waves in air, which older recording and transmission systems carry as continuous models, or "analogs" of the original waves. Digital systems record a series of discontinuous "samples," representing the signal voltage at each sampling point, in numerical (digital) form. Recording and transmission (whether analog or digital) are never perfect, but digital data can include extra signals which allow most of the errors that do occur to be corrected, so less of the music is lost due to these imperfections.

The CD player "knows" when to slow down by reading the signals on the recording and comparing them to its internal "clock." Because the slow down is gradual, matched to the changing diameters of successive tracks, the linear speed of the recorded track past the laser pickup remains constant, even as the circumference of each track increases (the disc is scanned by the laser from the inside out). The phonograph disc, by contrast, has a constant angular speed, which means that its information is crammed together at the inner grooves and wastefully spread out at the outer ones.

Typo Trouble

Due to a typographical error, Ohm's Law was partially misstated in "Reader Response: Power Amps and Speaker Impedance," in the March 1984 column. The relationship between power and voltage is: P = E^2/R (not E^2R, as we had printed it). Thanks to reader Bruce Rorem of St. Paul, Minn., for pointing this out.

(adapted from Audio magazine, July 1984; JOSEPH GIOVANELLI)

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