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INSTRUCTIONS FOR ASSEMBLING THE DYNAKIT STEREO 70
DESCRIPTION
Your Dynakit stereo 70 is a complete dual 35
watt power amplifier kit which offers the highest
possible fidelity, at low cost and in a compact
arrangement.
It
uses a patented circuit of out-
standing performance characteristics, along with
top quality parts, including the new Dynaco Super-
Fidelity A-470 transformers --the finest available.
The measured specifications show that the stereo
70 is an amplifier of unequalled performance, and
its lis ten in g quality is also unrivalled by any
regardless of price.
This amplifier has characteristics and power
rating ideal for either stereophonic or monophonic
use.
The two non-interacting 35 watt channels
provide adequate power, in a stereophonic or two
channel system, for loudspeakers of any level of
efficiency. The channels can be paralleled for a
full 70 watts of low distortion output, suitable for
practically any monophonic use even where
several loudspeakers are in use in different areas.
The high power capability and low internal imped-
ance provide excellent damping for all types of
high quality speaker systems without need to make
adjustments to match specific speaker installations.
The design features of your Dynakit which con-
tribute to its superior listening quality include some
which are not revealed by the customary steady-
state laboratory measurements. For example, the
stereo 70 does not exhibit bounce and flutter when
pulsed with a transient signal. Thus a sharp per-
cussive signal like a drum beat or a piano note will
be reproduced with sharpness and clarity and
neit~er
overhang nor muddiness.
The Stereo 70, unhke
many other amplifier circuits, has been designed to
provide its specified performance on a loudspeaker
load, not just under laboratory measuring condi-
tions. Therefore, the connection of a loudspeaker
to the amplifier does not deteriorate its perform-
ance or listening quality.
In
addition, due to the
high quality of the output transformers used, the
Dynakit's power handling capabilities are maintain-
ed over the entire audio band without the sharp rise
in distortion which characterizes most amplifiers
at the extremes of high and low frequencies.
The Stereo 70 uses a circuit arrangement based
on that of the Dynakits Mark
n
and Mark III which
has become world famous as one of superior qual-
ity while essentially simple and trouble -free. The
new 7199 tube is used as a pentode high gain voltage
amplifier directly coupled to a cathodyne phase
inverter. All parameters are adjusted for minimum
distortion.
This type of phase inverter has the
unique advantage that its operation is independent
of tube aging so that no adjustments are required
in maintaining optimum performance.
An
internal capacitive feedback loop balances
the phase inverter at the highest frequencies, and
the a r ran gem e n t provides accurately balanced
driving signals to the output tubes.
These tubes
are type EL-34's (one pair for each channel) oper-
ating well below their maximum ratings. The out-
put tubes use a fixed bias arrangement which is
set through use of Dyna Biaset (pat pending) which
provides optimum linearity of the tubes and mini-
mizes the effects of unbalanced components. The
connection of the output tubes includes a small
percentage of screen loading which improves the
regulation of the output stage and makes it corr--
paratively uncritical of load impedance.
The imp e dan c e match and bias conditions
utilized in the output stage provide minimum dis-
tortion over a very wide dynamic range.
In
addi-
tion, 20 db of negative voltage feedback lowers the
distortion to an unmeasurable proportion at normal
listening levels and to less than 1
%
intermodulation
distortion at 35 watts output on each channel.
The use of a minimum number of phase-
shifting stages along with car e f u I transformer
design makes it practical to have unconditional
stability at both high and low frequencies.
This
feature of the design means that there is no tend-
ency toward motorboating or oscillation under any
condition of use.
The inherent lin e a
I'
it Y of the circuit, the
complete stability, and the prefabrication of all
critical wiring on the printed circuit assembly
make construction completely non-critical.
The
use of conservatively
0
per ate d, highest quality
components insures superior and de pen dab I e
operation over a long period of time.
GENERAL WIRING PRACTICE
Assembly of the Dynakit stereo 70 is quite
simple compared to general kit assembly require-
ments. This is true because all critical parts of
the Dynakit are factory assembled for you on the
printed circuit assembly. Other parts are put out
in the open through a free and uncluttered layout
so there is easy accessibility for wiring or trouble
shooting purposes.
Construction of your Dynakit
should not take more than about five hours because
of these simplifications.
Upon opening your kit, check the components
with the parts list.
Familiarize yourself with the
components; they can be identified by comparison
with the pictorial diagram and by specified color
coding. Proper color coding will be mentioned for
each resistor as it is used.
Tools r e qui
l'
e d for easy assembly of your
Dynakit are soldering iron (small tip) or soldering
1