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DESCRIPTION
The Stereo 35 is a dual power amplifier of deceptively
simple appearance and circuitry. Behind its seemingly
conventional design is an extremely sophisticated circuit
configuration representing the most recent advances in
power amplifier engineering and construction. This high
degree of technical refinement, combined with the conserva
tive use of premium-grade components, has resulted in a
typical Dynakit design offering superb performance at low
cost.
Each of the two channels in the Stereo 35 is rated as a
17,5-watt amplifier. Although not evident from this simple
rating, the power stated is available on a continuous basis
at any frequency from 20 to 2,000 cycles per second. A
"music power" rating would make the Stereo 35 a 45-watt
amplifier, although even this rating would non disclose that
full power is available at low distortion throughout the
audio spectrum. Nor would such a rating indicate that the
Stereo 35, even when driven to power output levels beyond
its rating, is able to handle these overloads gracefully, with
minimum detriment to the sound. As a result, the Stereo
35 can, if necessary, duplicate the sound intensity of
amplifiers with much higher power ratings when called
upon to do so.
The heart of the Stereo 35 is the special Dynaco Z-565
output transformer, a patented design tailored specifically
to this circuit. The transformer is free of resonances, has
extended frequency response (from 6 to over 60,000
cycles), and is wound by special techniques on a carefully
designed core to insure low distortion over a wide fre-
quency spectrum. Transformers of this caliber have never
previously been used in low-cost equipment of moderate
power rating.
The transformer provides an optimum match between the
loud speaker load and the output tubes used. These tubes
are supplied as a matched group with the kit, and are
used in a circuit configuration which gives low inherent dis-
tortion and high stability of their d.c. operating point.
The output stage is preceded by a composite voltage
amplifier and cathodyne phase inverter (a 7247 tube with
direct coupling between sections). The phase inverter used
is unique; its operations is independent of the age of con-
dition of the tube. The phase inverter stage is therefore
able to maintain its capabilities permanently with no need
for adjustment or balancing. Gain in this part of the ampli-
fier is augmented by a feedback connection from cathode
to cathode.
Two negative feedback loops, one mostly resistive and
the other purely capacitive, are carried over the amplifier
to provide 20 db of negative feedback. This provides low
distortion and noise, a high damping factor, and further
benefits. The feedback loop is unconditionally stable under
all load arrangements, so that the Stereo 35 is suitable
for use with any loudspeaker, including electrostatic types
The values of components selected for the Stereo 35 have
been carefully determined, to be certain that the operating
conditions for each stage have been set at the center of
the optimum range. The use of close tolerance parts
provides additional assurance that these settings will not
shift, and will remain accurate, so that every amplifier will
meet its specifications. This is very important to the kit
builder, as it gives immunity from the variations to which
much electronic equipment is subject. Heavy-duty circuit
boards give additional stability and reproducibility of
characteristics, assuring a level of performance which takes
full advantage of the capabilities of the design. The con-
servatively rated power transformer, after testing, is com-
pletely sealed in a special encapsulating material which
prevents core vibration and also serves to dissipate heat
effectively.
One of the outstanding aspect of the design of the
Stereo 35 is the attention which as been paid to the repro-
duction of transient signals. Specifications commonly quoted
for amplifier refer only to performance with pure sine-
waves as the signal content; however, music and speech
are actually composed of non-repetitive sounds which are
non-sinusoidal in character. It is this irregular, or transient
type of waveform which the amplifier will usually be called
upon to reproduce. Pulse tests and square wave measure-
ments help to evaluate the transient performance of cir-
cuits, and these have been applied to derive the correct
operating parameters for the Stereo 35. The success of this
approach is particularly evident in listening tests where the
source material includes percussive sounds-drums, tam-
bourines, piano, cymbals, etc. The Stereo 35 reproduces
these sounds with a transparency and absence of blurring
which is only obtained when an amplifier has extended
frequency response, without bounce, flutter or overshoot.
In addition, the Stereo 35 recovers almost immediately
from overload, so that the tones immediately following
are not affected by the overload signals. Thus, even whit
low-efficiency speaker systems, where an occasional peak
signal might overload the amplifier, reproduction of every-
thing except the highest peak of the heaviest passage is
effortlessly reproduced.
It is this characteristic subtlety of design approach
which distinguishes the Stereo 35 from other equipment
with superficially similar specifications. These differences
provide a perceptible improvement in clarity and natural-
ness of sound; this was the design objective of the Stereo
35, and it has been achieved in a unit of remarkably low
cost.
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INSTRUCTIONS FOR ASSEMBLY AND USE OF
THE DYNAKIT STEREO 35 POWER AMPLIFIER