AUDIOCLINIC (Apr. 1988)

Current and Work

Q. Is there less amperage in the negative side of a speaker wire? I wonder about this because it seems to me that the voice-coil has to use amperage to do work in the form of moving the speaker cone.

-Lee Engelman, Oradell, N.J.

A. An electrical circuit is complete only when all of the electrons which start by entering one terminal (of a loudspeaker, in this instance) return to the other terminal. Thus, the amperage, which is the measurement of the number of electrons passing a given point in a second, must be the same for all points in the circuit.

Equalizer Center Frequencies

Q. The frequency response of my amplifier is 40 Hz to 20 kHz. My equalizer has a number of frequency controls, the lowest being for 60 Hz and the highest for 14 kHz. When I boost those two outer bands, what happens to the frequencies below 60 Hz and above 14 kHz? Are they boosted along with the lifters' center frequencies? What about everything between those outer extremes? Are they affected, or do they change only in accordance with their individual controls?

-Name withheld; Brooklyn, N.Y.

A. How an equalizer affects frequencies other than its specified center frequencies depends to some extent on its design. Your equalizer can probably help maintain bass below 40 Hz, but this depends on the width of the frequency range affected by its 60-Hz control. If this control affects a broad frequency range, it will likely boost frequencies down to perhaps 30 Hz.

The same can be said for the highest control on your equalizer; it will probably still be somewhat active at 20 kHz. Of course, it likely won't have the effect at 20 kHz that it will have at 14 kHz, its center frequency.

As to frequencies between the extremes, well, each of the frequencies shown on your equalizer panel is the center of a band of frequencies affected by that control. Chances are that the width of each band is such that there will be at least a small amount of overlap between one control and its neighbor. Thus, boosting the lowest frequency fully will cause some boosting in the range immediately above it.

Therefore, with your unit, if you boost the 60-Hz control substantially, you will probably find it desirable to cut the frequency band immediately above it just slightly. By doing so, you will keep that portion of the spectrum more or less where it would have been had you not boosted the 60-Hz band. The same can be said for the effect of boosting the upper frequency: You will probably need to cut the band just below the 14 kHz control if you add substantial boost at 14 kHz.

You will find that most adjustments will not be of the coarse nature we have just discussed. Moving any one of the controls just 2 to 3 dB will usually produce dramatic audible results.

Changes of this small size will have such a minimal effect on neighboring bands that no compensation need be made. As far as I am concerned, one should use an equalizer only sparingly.

Pilot Frequencies

Q. What are the 19- and 38-kHz products associated with FM equipment? Why is there a need to suppress them if the frequency range of FM stereo broadcasts does not extend beyond 15 kHz?

-Robert Beiswinger; Townbank, N.J.

A. In order to explain in any detail the matter of these frequencies, it would be necessary to fill more space than this column allows. Suffice it to say that, in order to transmit a stereophonic signal, it is necessary to generate a 19-kHz "pilot" signal. This is sent via the same transmitter which supplies the desired program. The tuner requires this signal so that the stereo decoding circuit can lock onto the stereo information. The tuner uses the 19 kHz signal to create a 38-kHz carrier which is used to reconstruct stereo information.

If these two frequencies were allowed to appear at the output of the tuner, serious problems would occur.

The higher audio frequencies would combine with the 19-kHz pilot signal to produce a kind of background swish which is often referred to as hash. It is at least conceivable that harmonics of the 19-kHz signal would "beat" with any SCA signals that might be present, again producing very annoying hash.

An attempt to record from a tuner which does not suppress these frequencies would very likely result in even more hash, because harmonics of the 19- and 38-kHz signals could beat against the recorder's bias oscillator frequency.

Tape decks with Dolby noise reduction commonly have multiplex filter switches which can be set to limit the highest recordable frequency to 15 kHz and to suppress all higher frequencies. This ensures that the Dolby circuits, whose actions are controlled by the high-frequency content of the signal, will respond only to upper audio frequencies and not to any 19- or 38 kHz tones which may escape the filters in the tuner. Otherwise, the Dolby decoding process might not work properly, and highs would be rolled off in playback.

Cartridge Coil Cracks

I am writing this in response to your answer to Mr. Cornell Coco about the expected life of his moving-magnet phono cartridge (July 1987). Thermal expansion and contraction can (and often does) cause microscopic cracks on the varnish used to insulate the coils of a moving-magnet cartridge. Moisture can then penetrate right to the surface of the copper. After sufficient buildup, electrolytic action can result, which will ultimately result in shorted windings. This, in turn, leads to weakening of the audio signal produced by the cartridge, lowering its impedance and introducing nonlinearities in frequency response.

I have handled thousands of cartridges and have seen many suffering from this problem. The remedy, of course, is to replace the cartridge when either channel is discovered to be weak or after eight to ten years of satisfactory service. This is just what one would do with a broken stylus or any other non-repairable item.

Incidentally, the same type of coil failure is often found in microphones, tape recorder heads, speaker voice coils, and electromagnetic pickups used on guitars.

-George Winter, Tuscaloosa, Ala.

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(Source: Audio magazine, Apr. 1988, JOSEPH GIOVANELLI)

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