AUDIOCLINIC (Mar. 1992)

Suppressing R.F.I.

Q. I am having an intermittent CB radio interference problem. My house is on a heavily travelled road, and my surround sound loudspeakers blast with this interference every time a CB-equipped car passes by. My receiver is a Pioneer VSX-5300, which contains a surround amplifier. About 30 feet of 18-gauge wire connect my rear speakers to the receiver.

The main, or front, loudspeakers are unaffected by these transmissions. If the surround circuits are switched off, those speakers also then don't pick up the interference. Putting the receiver into "mute" does not prevent the interference from being heard. Do you have any suggestions as to what's happening and how to cure it?

-Larry B. Craven, Raleigh, N.C.

A. In instances like yours, the most common way radio frequency interference gets into a sound system is by way of the loudspeaker wiring. It acts as an antenna. True, it enters a late amplifier stage at its output, but the amplifier's feedback loop provides a good path for the signals to find their way into an early stage of the equipment, where they can then be rectified and amplified.

This is probably not the scenario in your case, because you explained that the receiver's surround circuits have to be turned on before you can hear the interference. If, however, your receiver is designed so that the surround channel's power amp is turned off along with the surround circuitry, that would explain everything.

If the interference comes from the speaker cables, this can be cured by bypassing each loudspeaker output terminal to chassis--yes, even the ground, or common, terminals. Use about a 0.01-uF disc capacitor at each terminal, keeping all leads as short as possible. You can also use ferrite beads. These beads can be obtained either split or in one piece with a hole through which the cable must pass.

There are various types of these beads, and you want beads designed to be most effective at 27 MHz (if the interference is truly coming from CB transmitters). Keep the beads as close to the amplifier as possible. You may need to use several beads on each cable.

If the speaker cables are not the source of the trouble, then you may have to place beads on input cables or on power-line wiring. Even shielded cable can permit interference to get into the center conductor because the cable is really not 100% shielded.

Replacing Tweeters

Q. I have had a quality pair of two way loudspeakers for about five years and have been pleased with them. Recently, however, for reasons that I am not sure of, the tweeters were dam aged. This has brought about some questions about the repair of these loudspeaker systems.

I am assuming that replacement drivers are available from the maker, but I am curious about the results that I might be able to obtain by using different drivers than the originals. I would keep the low-frequency driver and the crossover network intact. Aside from the obvious considerations of finding drivers that match in impedance, sensitivity, and physical dimensions, what else must I consider for the results to be acceptable?

-Timothy E. Gosman; Somers Point, N.J.

A. My first thought is that, unless you have a way to audition the new drivers, you may find after the new drivers have been installed that they do not perform as well as the original ones did. They may be peaky, or they might have less openness and transparency than the maker's choice of tweeter.

It is essential that the crossover point for the replacement tweeters be the same as, or perhaps a bit lower than, that of the original tweeters. You want to be sure that the transition between woofer and tweeter is as smooth as possible, at least as good as it was using the original tweeters.

Differences in cone placement with respect to the plane of the woofer could also make a difference in the sound, which again could mean that the best sound would be produced by using the original tweeters designed for use in the system.

"Unturned-Off" Hum

Q. I have noticed that my daughter's boom box emits a noticeable hum when plugged into the wall outlet al though all functions are turned off. After that, I checked all of my radios.

These are clock radios of everyday quality, and all of them emit this same hum, even though they are turned off. I must say that, with some of them, I have to place my ear right against them to hear the hum. Are these "humming" radios okay? I have been told by some friends that they might not be safe.

-Robert E. Olsen; Madison, Wisc.

A. Most boom boxes that I have heard don't really turn off when the power switch is off. This is because these switches are located in the B+ which feeds the d.c. to the various components within the box. This is the only way--or at least the cheapest way--to wire such equipment. When these devices are plugged into the power sockets, voltage is applied to the power transformer at all times.

Thus, the hum you hear is not coming from the loudspeakers but from the cabinet and is heard because of mechanical vibrations of this cabinet. Any power transformer, when it is fed from a power line, emits some mechanical vibration.

With voltage applied to the power transformer, the power supply is al ways producing voltage, even though this voltage goes nowhere when the radio is off. For greater reliability, unplug radios of this design when they are not to be used for extended periods of time.

Why are boom boxes wired as I've described? It is because they also operate from batteries. Placing the power switch as described provides a convenient way for you to disconnect the batteries.

Clock radios also use power transformers. Both the radio and the clock are supplied with voltage by these transformers. Therefore, the transformer must be connected to a power source at all times if the clock is to keep time.

Your children do not have to give up their boom boxes, and you don't have to throw out your clock radios. There are no significant safety problems associated with these pieces of gear.

(Source: Audio magazine, JOSEPH GIOVANELLI, Mar. 1992)

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