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Separates vs. Receivers
Q. What is the difference between a preamplifier and a power amplifier? What are the advantages and/or disadvantages of a receiver as opposed to separate components?
-George Swytnyk, Chicago, III.
A. A preamplifier incorporates most or all of the sound system's basic controls, such as volume, source selector and tone controls, together with an RIAA-equalized, low-level, phono cartridge input circuit. Its output is usually a low-current signal of about 1 volt.
A power amplifier turns the preamplifier's signal into higher current signals with enough power to drive your loudspeakers. It requires no controls, though it may have a few. An integrated amplifier combines the circuits of a power amplifier and a preamplifier into one single box.
A receiver combines an integrated amplifier with an FM or FM/AM tuner.
Receivers are more compact and less expensive than the separate components they replace. But separates let you choose the best component of each type for your use (e.g., an ultrasensitive tuner with a medium-power amp, or a high-powered amp with a less sensitive tuner), and you can replace components one by one as your requirements change or sufficiently better ones become available.
Truth in FM Stereo
Q. All my local FM stations but one leave their stereo pilot signals on from sign-on to sign-off, even when broadcasting mono material. Why?
-Louis Goldstein, Grove Hall, Mass.
A. In the early days of stereo FM, when broadcasters had few stereo recordings to play, the FCC required that stations turn off their pilot signals when the material being broadcast was monophonic. Now, virtually all recordings are in stereo, and it's rare for anything to be broadcast monophonically except announcements and occasional old performances.
Because of this, and because (especially in small stations) the engineer in the control room may be busy doubling as announcer, record-puller and phone answerer, the FCC has relaxed the requirement.
No harm is done except with stations whose stereo signal is marginal at your location, but which could be received monophonically. And you'd have to switch your tuner to mono for such stations, even when they were really broadcasting in stereo.
Tying the Knot
Q. Would joining several speaker wires to form a longer one affect the signal traveling to the speaker, assuming sufficient wire gauge?
-Kin P. Gee, New York, N.Y.
A. Splicing short cable sections to make up one, long cable should not really cause problems if you make really good splices, preferably soldered, so that the two wires of the splice are intimately bonded. Also be certain to insulate the splice to prevent shorts.
Buzz in Headphones
Q. Why do I hear a buzz in my headphones but not in my speakers when I turn on my electric blanket? Is it a ground problem? And if so, how do I correct it?
-Jim Buck, Flemington, N.J.
A. When listening to headphones, you don't hear ambient noise which might otherwise mask noises in the signal. And with most stereo systems, the volume control is set quite low for headphone listening, bringing the maximum signal down closer to the amplifier's noise level.
Headphone junction boxes with volume controls and resistors may help by letting you raise the volume setting.
You might also eliminate the buzz by placing an a.c. line filter between your stereo system and your power outlet, or between the blanket and the outlet.
Use a transient-type line filter (such as those sold by GE or Radio Shack) if the noise is produced by high-voltage spikes. Otherwise, you'll need LC (inductor-capacitor) a filters or other 60 Hz, low-pass filters in the line.
I do not believe that grounding has any influence on your problem.
Q. The other night, I was listening to an FM station at 96.9 MHz. Then I turned the dial and picked it up again at about 107. Why?
-Darryl Frank, Conewango Valley, N.Y.
A. It's probably a signal-overload problem; you're apparently close enough to the transmitter, or have a good enough antenna, to get more signal from this station than your tuner can take.
This happens because of the nature of the almost universal superheterodyne receiver setup. A local oscillator circuit in your tuner beats against the frequency of the desired signal to produce an intermediate frequency (i.f.) of 10.7 MHz. If the incoming signal is strong enough, though, it can beat with harmonics of the local oscillator to produce images of that station at one or more additional frequencies within the FM band, a problem known as spurious response.
Perhaps realigning your tuner can reduce the problem. I suspect, however, that the best approach is to attenuate the signal at the antenna terminals so that it is no longer strong enough to create the conditions you describe.
Q. Your magazine reviewed a Sony synthesized tuner, and based on that review, I bought one. I ran A-B tests of my old receiver's tuner section versus the new tuner. Certainly, the Sony unit was everything your reviewer said it was.
It may be that I am now accustomed to that tuner, but I feel it has aged in the last two months. It is now performing in less than the optimum manner.
From my experience with tube equipment, I know that aging can and does occur. Could such aging possibly have occurred with this transistor tuner as well? If it has, are there any simple, internal adjustments which can be made to "peak" the circuits back to optimum performance?
- Arthur Venitt, Jamaica, N.Y.
A. No question about it, equipment does age with time. Two months, however, does not seem long enough for the effects of such aging to occur.
Should it happen that something has indeed gone wrong with the tuner, no simple adjustments can fix it.
With increased familiarity with your new tuner, your judgments may have become more critical; distortion may be heard, you may now notice something wrong with the highs, etc. Or you may be growing aware of the poor quality of all too many FM broadcasts.
In an effort to obtain the maximum coverage from their transmitters, stations often process the audio signal in various ways, hoping for better signal-to-noise ratios or louder sounding signals than would otherwise be the case.
These results are obtained, but at the expense of overall program quality.
Today's tuners may well be better than is necessary to extract the best that many stations have to offer.
(Source: Audio magazine, Nov. 1982; Joseph Giovanelli )
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