AUDIOCLINIC (Apr. 1991)

Receiving Distant, AM Broadcast Signals

Q. As a youngster I remember listening to the family Zenith console radio and picking up favorite programs.

On occasion my father would do a "station search," trying to receive distant stations. It was not uncommon to pick up many states without the use of an outside antenna.

Do the new, digital equipment or any of the new AM radios have the capability of clearly receiving faraway AM stations and at what distances?

-Juan K. Ferry, Weymouth, Mass.

A. Most AM radios or tuners have the capability of receiving distant broadcast signals. In many respects they are better equipped to do so than older sets were.

The range over which such stations can be heard is not determined by the sensitivity of the radio. Rather, it is limited by background noise-natural or man-made. I can listen to Cuba almost nightly, on a frequency of 830 kHz. Cuba is more than 1,000 miles from my home. I have heard many stations at even greater distances.

Listeners to distant AM broadcast signals had an advantage years ago. There were fewer stations to receive. This means that fewer stations shared any given frequency. Today the AM broadcast band is so packed with stations that it becomes almost impossible to separate one from another without using a very directional antenna.

Co-channel interference is just one part of the problem. If a tuner is to have any measure of high-fidelity sound, it must have wide bandwidth. This means that its ability to reject adjacent channels is poor. Some tuners provide narrow bandwidths as an alternative, and this does help. But many stations transmit audio frequencies above 10 kHz, so even a receiver capable of good adjacent-channel rejection can't completely eliminate interference. A 10-kHz audio signal from an undesired adjacent channel will produce a side band that will fall exactly on the channel to which you are tuned. These side bands will be heard as a kind of "chatter" along with the desired program. If the desired program is weak in signal strength and the adjacent channel is strong, the strength of the sidebands will override the desired signal. Sometimes single-sideband techniques help, but that form of signal reception is only found on shortwave receivers, not on AM broadcast tuners.

Input/Output Loops

Q. In audio, what is meant by a "loop"?

- Eric Wong, New York, N.Y.

A. You probably know how to connect a tape recorder into your audio system so that you can monitor its output as well as that of the source being recorded. What is happening is that, somewhere in the signal chain, a link of that chain is broken, and a new link--the tape recorder-is added. This method of interrupting the flow of a signal in order to insert an additional device is known as a "loop." Such external processor loops can be used for other devices than tape recorders. For example, we often insert an equalizer in such a loop so that all program sources can be affected by it.

Piezo Tweeters

I read with interest your reply to Mr. Roosevelt Anderson, Jr.'s inquiry about piezoelectric tweeters in the June 1989 issue. I find, though, that I take exception to some of your views.

It is true that certain small piezoelectric tweeters exhibit a lack of smoothness of frequency response-most notably the popular, round Super Horn, which does have a nasty peak at about 3 kHz or so. However, this is not the case with all such tweeters; some even have a surprisingly flat response.

This very same piezo Super Horn was used in the famous Dahlquist DQ-10 speaker system to cover the very highest frequency range, from 12.5 kHz and up. That avoided the 3-kHz peak. This loudspeaker system is one of the most transparent I've ever heard, certainly surpassing anything in its price bracket. The piezo tweeter is a key ingredient in this speaker's lightning-fast transient response.

-Alan B. Chambers; South Bound Brook, N.J.

Hum from Exciter Lamps

Q. I need to use a 12 V storage-battery charger to supply power to an exciter lamp on a 35-mm movie projector. Five amperes at 10 V are required by the lamp.

I tried this charger and got a very bad hum; it used a half-wave rectifier. I replaced the rectifier with a full-wave bridge rectifier and plenty of capacitor filtering. The hum was reduced but not sufficiently. I added more capacitors (in the thousands of microfarads), but still no good. What's wrong?

-Charles Schmitz; Canal-Winchester, Ohio

A. I have never worked with a 35-mm projector and was surprised to learn how much current is required to light up the exciter lamp. I have some experience with 16-mm projectors; the filaments in their exciter lamps run on about 4 V and much less current. As in your case, these filaments respond almost immediately when voltage changes are applied to them. In other words, the brilliance of the light is modulated by the a.c. ripple component of any poorly filtered power supply. Rather than using brute-force filtering in my projectors, I obtained exciter power from the output of an r.f. oscillator. The a.c. variations of such an oscillator are much too fast for the filament to respond. Even if the filament could act, the frequency would be too high for us to hear.

It is possible to build an r.f. oscillator which is capable of the 50 watts you require. A couple of TV horizontal-output tubes would do it, but you stand more than a good chance of causing r.f. interference to some services. I suggest, therefore, that you continue on your present course. Try adding a 1-ohm resistor in series with the output of the charger and the filters. Of course, the lamp would be connected across the filters, as you now have it connected. The resistor can lower the overall voltage to the required 10 V and can reduce the hum by turning the capacitive filtering into an RC filter.

If the hum is still present, use a regulated supply; a regulator chip driving a couple of 2N3055 transistors in parallel should be fine. You may need somewhat more voltage than can be supplied by the charger, however. National Semiconductor and other firms offer application notes on such regulator chips, and I suggest you check them for complete circuit details.

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(Source: Audio magazine, Apr. 1991, JOSEPH GIOVANELLI)

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