Aug - AmericanRadioHistory.Com

have either good stability or good distortion characteristics,
not both.
If choice there must be, the writer agrees that absolute
stability is even more important than lower distortion, for:
z. An unstable amplifier in the bass region may "motorboat"
(put -put at certain settings of bass control). At best,
it will tend to muddy any sudden bass pulses, and put an
unhealthy and continuous pulsation on woofers.
2. An amplifier having treble oscillations may blow out
certain super tweeters. In any case, treble transients will be
clouded over, and your lovely tweeter may reproduce nothing
but an unpleasant overlay of screech.
3. When an amplifier oscillates in any fashion, part of its
total power is devoted to the oscillation, severely limiting
the power available for musical reproduction.
On this whole matter of stability, the writer would like
to relate an incident that he wouldn't believe, frankly, if he
hadn't been present. On a listening test of two preamplifiers,
both complete- feedback units, with identical intermodulation
measurements, and equalization within 1/2 db
of each other, it was noticed by the four critical listeners
present that one preamplifier made records sound a bit
more harsh than the other, and definitely "broke up" more.
It was then decided to investigate with instruments to see
whether there could be any other difference between the
two preamplifiers. Using the square wave (a laboratory device
to measure performance on things that affect an amplifier
in the same manner as music), the testers noticed that
both preamplifiers were the same - except that the offending
one had a slight oscillation on top of the oscilloscope -
trace of the square wave. From the placement of the oscillation,
it was determined that it was occurring at about
6o,000 cycles. After correcting the circuit malfunction, and
thus the oscillation, the testers listened further, to find that
the preamps now sounded identical! Moral - any oscillation
will adversely affect listening pleasure!
The various methods of measuring stability are too involved
for the scope of this article. However, you are advised
to steer clear of amplifiers which in their literature sometimes
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hint that they might be unstable under certain conditions.
Particularly if you are not looking for a bargain, but have
come to realize that good amplifiers must be in the upper
price brackets, you should insist on detailed information on
stability.
Aside from elementary circuit mistakes, the stability of a
particular amplifier is almost entirely dependent on the
quality of the output transformer. A good output transformer
is expensive, so the writer has always cautioned
people to steer away from the amplifiers that seem to give
you just as much for one half the cost - such amplifiers
almost invariably have either high distortion or very poor
stability. No one is sadder to behold than the phonophile
who has just spent all sorts of money on an expensive
speaker, to get rid of the boom and screech of his former
speaker - only to be told by a testing laboratory that it was
his oscillating amplifier that had been at fault all the time.
More amplifiers suffer from problems of stability than any
other problem - so, take care!
"This is fine," you may say, "But what if I have already
bought my dream amplifier, that has in theory an ample
power reserve, good frequency response, exceedingly low
distortion, and admirable stability? How can I try to keep
it in good condition, in my home? Are there any tell -tale
signs of deterioration that I might be able to correct ?"
The following suggestions are not intended to be a repair
guide. Actual amplifier repair is a job for competent service
personnel, with proper equipment and know -how. Nor are
the suggestions an adequate substitute for periodic tests on
distortion -analyzing equipment. For instance, the intermodulation
analyzer is the most potent diagnostic guide
your local audio specialist (let us hope you have one) can
employ. It is the one sure way you have of knowing
whether your amplifier is meeting its original specifications
of performance. Use it, even if it means shipping your
amplifier back to the factory every year. Unfortunately, the
human ear, if it is buffeted enough, can come to accept distorted
sound as "right." The "ear" mentioned below is,
necessarily, one that hasn't been worn down by a steadily
deteriorating amplifier.
One of your most potent diagnostic tools at home, then
can be your own ears. The ear can establish by direct comparison
the lesser of two degrees of distortion. It can also
hear bass or treble instability, spurious noises, and general
sonic hash. Let us begin with symptoms and cures in the
preamplifier.
Every reputable amplifier has an instruction manual.
Read it - the manufacturer knows his own product's idiosyncracies,
weak points, and cures better than you do. For
instance, he will tell you where the "hum- balance control"
is. He may even mention juggling tubes from one socket
position to another (make sure the same tube type was in
each before you interchange) . Generally, a noisy (hummy,
hissy, or microphonic) tube is better in a later stage, i. e.,
farther away from the input sections. One precaution -if
your preamplifier has DC on the filaments, turn off the set
before you move tubes around.
If you have noisy volume- or tone -controls, you can
often quiet them with radio servicemen's preparations, or,
in a pinch, with an Continued on page 86
36 HIGH FIDELITY MAGAZINE