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Q. I have a direct-drive turntable which produces a high-pitched hum after I use it about 20 minutes or so, even though the hum is not audible through my speakers. What is my problem?-Willie Stanley, English town, N.J.
A. The hum produced by your turntable may be the result of loose laminations in either the motor or the power transformer or it may also be defective shock mounts. Because some time is needed before the "hum" is heard, heat is probably a factor in producing this sound. If the sound is not too annoying, I doubt that it is significant.
Q. I have some small electronics tools that have become rusted. What is the best way to remove this rust?
-Robert Watson, Dover, Del.
A. Tools with rust deposits can be readily cleaned by lightly spraying them with WD-40 and then lightly polishing them with very fine steel wool. If the rust is heavy, it will be necessary to reapply the WD-40 more than once and to use either sandpaper or emery cloth. For really heavy rust, use Naval Jelly but keep it out of any of the electronics.
Q. What is the advantage or significance of a speaker that can reproduce square waves?
-Eli Sammett, Flushing, N.Y.
A. There are a number of musical instruments which produce sudden "starts" or "stops" of sound. A square wave is an artificial means of doing the same thing. If such a wave shape is presented to a component such as a speaker, amplifier, tape recorder, etc., it would be nice if the output of the device would reproduce the shape exactly as it appears when it enters the input. Depending upon the nature of the difference between the output and input waveforms, one can tell if a speaker is having difficulties with sudden starts, sudden stops, or a combination of both.
If a waveform contains many harmonics, then it would tend to have the appearance of a square wave. If this waveform is presented to the input of a component and if the output waveform ends tend to be rounded off, this means that some of the higher frequency harmonics have been removed because of a frequency response that doesn't go high enough to encompass the energy contained in the waveform.
Q. Is impedance matching necessary only for optimizing the reproduced sound?
-Michael D. Muller, APO, New York
A. Generally speaking, when we drive a transducer we wish to supply a maximum of power. When we drive an electronic circuit, we want to transfer a maximum of voltage. For example, the impedance of a power amplifier is generally much higher than the output impedance of the preamplifier so that a maximum of voltage is transferred between the two devices. However, if we are concerned with a maximum transfer of power between these, or any two circuits, we would match impedances and it is only by doing so that a maximum of power is transferred from one component to another.
(Source: Audio magazine; Apr. 1978)
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