anywhere

NEXT TIME
BE SURE!
Foo!ears Peerless engineers have
.. ~.: ~ I; ~ .
designed transformers to meet
the most unusual and stringent
sp ec ificat io ns subm itte d by
c i v II ian man u f a cf u r e r san d
government contractors, and have
manufactured these transformers
with tight quality cont rol. .The
small difference in price means
a BIG difference in performance.
T
ODAv'S BEST AUDIO AMPLIFIERS are becoming
better and better; in fact, it is
quite possible that they are-from the
performance standpoint-as good as they
ever need to be. Distortion is down to negligible
values which cannot affect our ears;
frequency curves are flat as a pancake; and
output powers are more than enough. This
may sound a little like the statements that
were said to be made sometime in the late
eighties that all the inventions that could
be made were already made.
That is not the sense of the remark. A
good many more inventions and developments
need to be made in the audio fieldnot
to improve the possible quality of amplifiers,
but to improve the normal individual's
chances of obtaining an amplifier with optimum
characteristics for something less than
an arm and a leg. In other words, now
that we know we can have amplifiers with
fidelity better than the original music,' let's
simplify and cheapen the circuits and components
so that everybody can have one.
One of the paradoxical reasons for high
cost in amplifiers is that· the band to be
transmitted must usually be very much
wider than the band anyone can hear. A
few amplifiers are flat up to the apparently
ridiculous frequency of 100,000 cps. and
more where even a bat would need a hearing
aid. But this is required to provide for
the large percentage of negative feedback,
which is primarily responsible for high
audio quality. If there is attenuation of the
pass band anywhere near the ends of the
audible spectrum, there will also be phase
shift which spells death by oscillation to
* 255 W. 84th St., New York 24 N. Y.
1 Joke. No indignant letters, ple'ase!
RICHARD H. DORP
the feedback loops because of reversing
phase at certain points.
It seems to the writer that John M.
Miller has struck out along the right path
in his patent No. 2,652,458, which is assigned
to Bendix. The patent covers an
"amplifier with positive and negative feedback,"
but there is as much horse sense
as engineering in the approach, and more
than immediately meets the eye.
The advantages of negative feedback are
well known-reduction of distortion of all
kinds, including frequency, and ioudspeaker
damping. In the last couple of years the
advantages of adding positive current feedback
wi thin the amplifier have come into
the open; with it some of the offset in gain
caused by negative voltage feedback can
be reduced and the output impedance can
be brought right down to zero, or even
negative values, for almost perfect speaker
damping. The rub is that the usual method
of applying optimally large amounts of
positive and negative feedback requires
highly expensive output transformers and
wideband coupling in all the stages.
Mr. Miller's amplifier has both negative
and positive feedback loops. His trick is
principally (a) to recognize that with a
transformer of good but not premium quality,
the negative feedback wilJ, after the
extremes of the audible band, turn positive :
and (0) to simply incorporate a very elementary
network in the positive feedback
loop so that when the negative feedback
tends to become positive and the positive
feedback tends to become negative and keeps
the amplifier form oscil1ating.
Figure 1 shows the schematic diagram of
an amplifier designed according to the invention.
The amplifier proper (which be-
0 .3
SELECT Cx FOR
MINIMUM VALUE AS
PARASITIC
.SUPPRESSORS
Next time let us quote on your
transformer requirements.
_PEERLESS
Electrical
,"''''''"
Products
'~Uilt
9356 .S!lnta Monica Blvd., Beverly Hills, Calif.
161 Sixth Avenue, New York 13, New York
1
.-----...:.----
·POSITIVE
FEEOBACK,--,:;~;=.3_-4----+--o+2B5V
Fig. 1.
NEGATIVE FEEDBACK ..
o
'" 0
'" '"
2
AUDIO ENGINEERING • . DECEMBER, 1953