AUDIOCLINIC (Apr. 1990)

Shifting Stereo Image

Q. When I turn up the volume on my stereo system, I hear sound from the left speaker first. As I raise the volume, the sound balances out and I hear the "voice" move to the center, where it belongs. Is this normal?

-Joseph Barbera, Cheektowaga, N.Y.

A. I have seen some less expensive systems in which what you have described is usual. The shift occurs at the very lowest settings of the volume control. Because most listening is done at higher volume, the condition can be overlooked. It occurs because of differences in resistance between the two potentiometers of the volume control.

One potentiometer governs the level of the left channel, the other governs the level of the right. If the equipment is considered high end, a shift of stereo image should not occur; if it does, you should have the system's volume control replaced.

Shielding TV Sets From Magnetic Fields

Q. What material can I get to shield my television set from the magnetic fields in my loudspeakers? I notice that when I have the set near the loudspeakers, I get distortion in the picture.

-Barry Pearl, Little Neck, N.Y.

A. One of the best materials used to shield equipment from magnetic fields is Mumetal. It is very expensive when bought in sheets large enough to shield a TV set from fields generated by a loudspeaker system. However, the simplest and least expensive solution is to move the TV set away from the offending speakers. Moving them by just 1 or 2 feet is usually all that is required to reduce the strength of the fields to a point where they won't distort the picture.

I realize that in some installations, there is no way to move the TV or the loudspeakers. In those cases, some kind of shield must be placed between the speakers and the TV set, as you mentioned. Mumetal works well because it is "permeable" to magnetic fields. In other words, it tends to short the fields around the TV set because such fields prefer the easier path through the permeable material. Fortunately, more common materials also have high permeability. Sheet steel is quite good and is readily obtained because it forms the cabinets of many home appliances, such as washing machines and dryers. However, care must be taken to ensure that no one cuts himself on the edge of such a shield-especially because a large sheet will probably have to be cut to obtain a sheet of the proper size, thus leaving some rough edges. Use a file to smooth these out a bit. Although I have not tried using sheet steel specifically for this purpose, I can't see why it won't work just fine.

Fiber Optics in Home Audio

Q. Does a fiber-optic cable transmit a better signal than a coaxial or standard phono cable? If so, why do very few preamplifiers or integrated amplifiers have such inputs?

-Ralph Nelson, Hayward, Cal.

A. Fiber-optic cables employ light to carry signals, and conventional, coaxial cables do this by means of electrical conduction in copper. If you examine the connectors used to feed fiber-optic cables, you will quickly realize that the fittings are completely different from those that are used for attaching coaxial cables.

To pass information to a fiber-optic cable requires that the information modulate the output of a laser or an LED. The light is sent along the fiberoptic cable. At the far end of the cable, the light enters a demodulator which recovers the information.

Fiber optics have primarily been used in audio for digital signals only, because the fiber-optic system provides the wide bandwidth needed to handle digital information. So far, the only preamps, amps, and receivers with optical connections are those which have built-in D/A converters. Fiber optics can carry analog signals, but their only significant advantage in analog audio would be their immunity to electrical and magnetic interference. Analog fiber-optic systems are therefore now beginning to be used in some custom car stereo systems, to alleviate the noise problems inherent in car installations.

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( Audio magazine, Apr. 1990, JOSEPH GIOVANELLI)

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