AUDIOCLINIC (Aug. 1993)

Connecting a Subwoofer

Q. I have apparently backed myself into a corner in attempting to add a single sub woofer to both outputs of my 22-watt stereo system. I had hoped I could have accomplished this via some chokes, but I have the impression that trying this would damage my amplifier. I’d like to know what I can do to solve this problem without buying a separate amplifier for the subwoofer. I would appreciate your comments and suggestions.

—Gerald L., Alexandria, Va.

A. The crossovers built into or supplied with many subwoofers have inputs for both stereo channels. The low-frequency information from the two channels is then summed to feed the subwoofer driver. If you’re building a subwoofer system from scratch, look for a driver that has two voice-coils, each of which can be connected to one channel of your stereo system. Considering the modest 22 watts you have available, you should probably look for a subwoofer that has its own built-in amplifier; failing that, you should get a sub- woofer of the highest possible sensitivity.

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Frequency Response vs. Frequency Limits

Q. I recently purchased a test CD so I could evaluate my stereo system. I was mainly interested in checking frequency response. My loudspeakers are only supposed to respond to frequencies from 50 Hz and above. Why, then, when I played the test disc, could I hear all frequencies down to almost 20 Hz?—Bret A., Sauk Rapids, Minn.

A. Your speaker’s spec for frequency response shows that it should be reasonably flat (not deviating greatly in amplitude) from 50 Hz to the upper end of its stated range. But response does not just die out beyond the stated limits. The further you go beyond those limits, the softer the sound gets, until it eventually becomes inaudible. If your speaker were flat to 20 Hz, those low bass tones would sound considerably louder than they do now. Also, you may be feeding your speakers more power than they normally get. Test signals on CD are often made at maximum level (0 dB); few, if any, notes on a CD would actually get that loud. And people have a tendency to turn the volume control up when listening to test CDs, especially at frequencies where the system’s response drops off. If that’s the case with you, be careful—it’s possible to damage your speakers if you overdrive them. If you hear distortion, turn the volume down.

Cable FM Reception

Q. I am thinking of getting cable service to get better FM reception from some of my favorite stations. I am not interested in the television portion of the cable service; I am looking for a device that will make it possible to receive cable on my hi-fi system. It would be a box that would be connected between the cable and my FM tuner, so I could use my tuner in the usual way. Is there such a device? Where can I get one?

—R. Ross, Odessa, Tex.

A. First, find out what FM services your local cable TV system offers. Not all cable TV systems carry FM stations, and not all the systems that do require a converter box between the cable and the FM receiver. You will, however, need a switcher to select cable or over-the-air signals. Radio Shack and most electronics stores carry these. So that you get the proper switcher, note whether the antenna you use for normal reception is a 75-ohm coaxial with a screw- on F connector or a 300-ohm flat twin-lead requiring screw terminals. (Cable systems almost invariably use F connections.)

You won’t always get better FM reception from cable; some cable companies are rather sloppy about FM. You may also find that stations have been shifted to new frequencies on the cable or even that some stations you can normally receive aren’t carried by your cable system.

Poor AM, Both Home and Road

Q. My home and car stereo systems are considered to be of moderately high quality, but I get poor AM reception in both—particularly in the car. Both tuners have problems with hum and whining noises. At home, AM is unusable when I have my TV set on or am trying to videotape. In the car, the sound quality is sometimes reasonable, but at other times it is simply awful. What’s wrong with my tuners?

—Gino C., Buffalo, N.Y.

A. Let’s take your home system first. Both TV sets and VCRs are notorious for wrecking AM reception. Harmonics of the horizontal oscillator beat against the de sired AM station’s carrier to produce noise within the AM band. It would take much work to shield your VCR and TV to pre vent radiation of undesired signals. So your best solution might be to place a short AM antenna on your roof, as far from your TV antenna as you can place it. Check your local library for a good book on amateur radio to get details on how such antennas are constructed and how to protect them, when necessary, from lightning. Or contact the American Radio Relay League ( 225 Main St., Newington, Conn. 06111) for a copy of The ARRL Antenna Book, for $23, including postage (plus 6% sales tax for Connecticut residents, and 7% for Canadians). The antenna designs shown will be for short-wave bands, but formulas in books about ham radio can help you calculate correct lengths for the AM broadcast band (530 to 1,600 kHz). In most locations, it pays to add lightning arrestors to any out door antenna; I’m told a good source for those and for information about them is Industrial Communications Engineers ( P.O. Box 18495, Indianapolis, Ind. 46218).

As to the variable AM reception in your car, try to note when and where it’s good and when it’s not. If it’s fine when your engine is off but not with the engine running, check your antenna, antenna lead, and the grounding of your radio and of the antenna’s base. I’ve also found that getting a tune-up and replacing ignition wires has reduced some engine noise. After that, try interference suppressors, available from Radio Shack and car stereo specialists.

If the problem occurs only in certain places, then it’s caused by local noise sources, such as high-tension power lines, which you can’t do anything about.

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