Audioclinic (Jun. 1983)

Unusual FM Reception Problem

Q. I have three high-quality FM tuners. All perform well in my living room and equally well in my basement, except that each produces a loud hiss when receiving WNYC there. None of the tuners does this when receiving WNYC in the living room. All three tuners, when in use, are fed from the same Yagi antenna. I tried a good distribution preamplifier and a splitter.

The basement tuner sits on a shelf above the top of the stone foundation. The only electrical devices reasonably nearby are a refrigerator and the furnace. I turned both of them off, but the hiss persisted. I've also installed a line filter between the tuner and the wall outlet. This is not a matter of incomplete tuner limiting. The signal strength meters show that the levels are far above those required for total limiting. The condition is far worse when I use a distribution amplifier than when I use a simple splitter. And opening a knife switch in the portion of 300-ohm lead-in going to the downstairs tuner, so that there is a 1/4-inch gap, diminishes the hiss drastically-though not enough to make acceptable recordings.

-Morton N. Wekstein, Bronxville, N.Y.

A. The answer most likely lies in the 300-ohm lead-in you are using, or its routing to the basement. Perhaps it is wrapped around ductwork or pipes, fastened to a metal support column, or run through ductwork. Such situations cause signal losses and reflections. If such routing cannot be avoided, I suggest that you use a 75-ohm coaxial system. That should solve the problem.

If signal losses are too great overall because of long cable runs, balun and splitters, you will need to try a mast-mounted amplifier, not as a distribution amplifier but as a means of obtaining more signal. This amplifier must, how ever, have a better noise figure than the tuners have. If it does not have a better noise figure, reception will again be degraded, especially weak-signal reception. The coaxial system will necessitate a complete rewiring of the installation, including the living room even though reception in this room was always good.

Signal losses are not the only cause of hiss; multipath problems will also cause it. Such added signal reflections can be introduced by metal objects in contact with 300-ohm lead-in as well as those introduced into the antenna from surrounding buildings and/or terrain.

Mono Speaker, Stereo Amp

Q. What's the best way to connect a single speaker to a stereo amplifier or receiver without losing fidelity or dam aging the equipment? Would the same connections be used for tube and sol id-state gear?-Name withheld A. With either type of amp, just connect your speaker to one channel's output terminals, rotate the balance control all the way to that side, and use only that channel's input. Set the mode switch to stereo so no signal feeds the unused channel.

Third-Lug Theme

Q. I notice that all volume controls have three terminal lugs. What is the purpose of the third one?

-Robert Ades, Los Angeles, Cal.

A. A volume control, or potentiometer, consists of a resistor with fixed contacts at each end and a sliding contact between them. The signal is usually placed across the entire resistance of the volume control, and the output taken between the center terminal lug and one end.

Nickel-Cadmium vs. Alkaline Batteries

Q. I am considering buying a set of rechargeable nickel-cadmium batteries to be used in a portable stereo system, but a friend tells me they do not provide full power for nearly as long a time as alkalines. I know nickel cadmiums would pay for themselves in time, but since they supply 1.2 instead of 1.5 volts, does this affect the play speed? Which type is the better choice?

-Steve Kuske, Goodhue, Minn.

A. It's true that nickel-cadmium batteries do not provide as much voltage initially as alkaline batteries and that they won't supply their full voltage for as long as alkalines will. However, alkaline cells will gradually fall off in volt age over their lives, while nickel-cadmium types will tend to maintain their operating voltage during much of their discharge cycle. Thus, if such batteries perform well for a specific application, their performance can be relied on until the charge has been virtually exhausted.

Once alkaline cells have been exhausted, they must be discarded; no attempt should be made to recharge them. The nickel-cadmium cells can be recharged a number of times. (In fact, they can be cycled a lot more often than is true of dry, lead-acid cells.) The lower voltage produced by a battery of nickel-cadmium cells will seldom affect the performance of the equipment in which they are installed.

Even tape speed will not be adversely affected because of the recorder's speed-controlling system and because it probably was designed to allow for low-battery operation.

If you plan to use a portable stereo system over extended periods, you definitely should consider using rechargeable cells. Keep two sets on hand, one under charge and the other in use. Should you be away from power lines and should one set of cells run down, the recharged set will be immediately available.

Equipment Compatibility

Q. I once read an article which spoke of "equipment compatibility," claiming it would be unwise to purchase a music source capable of producing more signal than would drive the amplifier to full output. My power amplifier's manual states that "a 1-volt signal will drive each amplifier channel to its full output." I note that many tape decks have outputs of 1.5 volts, 1.8 volts, etc. Would they be incompatible with my present power amp?

-Samuel J. Neiditch, Highland, Cal.

A. If you're speaking of an integrated amplifier with a volume control, no problem; just turn down the volume control. If you're using a power amp with the tape deck (which is unusual), there will only be a problem if the amp lacks an input level control and the deck has no control for output level.

Gross mismatches (such as a 10-to-1 mismatch) can cause problems with distortion or noise, but not the sort of mismatch you're discussing here.

(adapted from Audio magazine, Jun. 1983; JOSEPH GIOVANELLI)

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