Dynaco ST-35 Service Manual
INSTRUCTIONS FOR THE ASSEMBLY AND USE OF
DYNAKIT STEREO 35 POWER AMPLIFIER
SERIAL NUMBER
14635013
This number must be mentioned in all communications concerning Dynakit.
Price $1.00
046414
3916 POWELTON AVE
PHILADELPHIA 4 PA
CONTENTS
Description 3
General Wiring Practice 4
Mechanical Assembly 5
Wiring instructions 6
240-Volt Transformers 9
Installation 9
In Case of Trouble 10
Part List 11
Schematic Diagram Back Cover
SPECIFICATIONS
Power Output:
Frequency Response:
Power Response:
Intermodulation Distortion:
Hum and Noise:
Sensitivity:
Input Impedance:
Output Impedance:
Damping Factor:
Minimum Channel Separation:
Power Consumption:
Tube Complements:
Size:
Shipping Weight:
35 watts continuous, 45 watts
IHFM Music Power (both channels)
±1db from 10 cps to 40,000 cps.
20 cps to 20,000 cps without
exceeding 1% distortion within
1 db of 17,5 watts each channel.
Less than 1% at 17,5 watts each
channel. Less than 0.1% at
average listening levels.
Inaudible; more than 80 db below
rated output.
1 volt input for 17.5 watts output (each channel).
500,000 ohms.
8 and 16 ohms.
10 from 20 cps to 20,000 cps.
80 db.
100 watts, 50/60 cycle a.c.
7247 (2), 6BQ5/EL84/7189 (4),
silicon diodes (2).
4" high, 13" wide, 5½" deep.
16 lbs.
INSTRUCTIONS FOR ASSEMBLY AND USE OF
THE DYNAKIT STEREO 35 POWER AMPLIFIER
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 frequency 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 distortion 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 condition 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 amplifier 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 conservatively rated power transformer, after testing, is completely 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 reproduction of transient signals. Specifications commonly quoted
for amplifier refer only to performance with pure sinewaves 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 measurements help to evaluate the transient performance of circuits, 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, tambourines, 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 everything 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 naturalness of sound; this was the design objective of the Stereo
35, and it has been achieved in a unit of remarkably low
cost.
3
GENERAL WIRING PRACTICE
Assembly of the Stereo 35 is exceptionally simple when
compared to that the other kits. The circuit board are supplied with all components mounted, and the remaining parts
arranged on the chassis in an open, uncluttered way that
makes wiring quick and easy. The construction of the Stereo
35 should take no more than few hours.
When you unpack the kit, check the components against
the parts list first. You can identify unfamiliar components
by matching them to parts illustrated in the pictorial
diagram supplied.
Have the proper tools at hand before beginning to build
your kit. You will need a pencil-type soldering iron of 30to 60-watt rating; a long-nosed pliers; diagonal cutters; and
a screwdriver. If you have a soldering gun, it should be used
with care, especially when working on the circuit board,
because of its higher than necessary heat output. Although
not essential, a wire-cutting and stripping tool will help
considerably; these are available for less than a dollar.
The only procedure involved in building a Dynakit
which requires a bit of technique is soldering, and this is
quite easy to master. There are four steps to making a good
solder connection:
1. Make a good mechanical connection.
2. Heat both parts of the connection with the iron.
One of the best ways to make a good mechanical connection
is to bend a small hook in the end of the wire, and then to
crimp this hook onto the terminal to be connected. The
amount of bare wire exposed at the end need to be exactly
¼-inch; however, if it is too long, there is danger of the
excess touching another terminal or the chassis. There is
no need to wrap the wire around the terminal more than
one time, as this makes a connection that is much more
difficult to remove if an error has been made
3. Apply solder to the connection until it melts and
runs.
4. Allow the connection to cool undisturbed.
ALL SOLDERING MUST BE DONE WITH ROSIN
CORE SOLDER.
There is no warranty on any equipment in which acid
core solder has been used. Make sure that the solder you
use is plainly marked "Rosin Core". If you have solder on
hand of doubtful origin, it is wise to obtain a new roll of
50/50 or 60/40 rosin core solder.
Whenever a connection is to be soldered, the instructions
indicate this by the symbol (S). If this symbol is not
shown after a step, further connections must be made to
the same point before soldering.
A number of steps in the instructions begin,"Connect
one end of a wire...", with the length of the wire specified.
In each case, first cut a piece of wire to the correct length
from the roll supplied with the kit and then remove about
¼" of insulation from each end before making the connection. The leads on components should be trimmed as they
are used, the length chosen being that
nection to be made from point to point without strain on
terminals or components. The lead "dress", that is the
manner in which the wiring is arranged as it goes from one
point to another, should follow that shown in the pictorial
diagram as closely as possible. Care must be exercised to
see that uninsulated wires do not touch each other, and
cannot do so through vibration or sagging, unless of course,
they are connected to the same point. It is especially important that uninsulated wires and component leads or terminals do not touch the chassis or bottom plate accidentally.
Check your work after each step, and, when you are satisfied that it has been correctly done, mark the space provided and go on to the next step. Examine the pictorial
diagrams often; if you check your work methodically, your
amplifier should work as soon as the wiring is complete.
which permits a con-
To transfer heat from the iron to the wire and terminal,
the tip of the iron should be kept brightly tinned with
solder. If this is properly done the first time the iron is
used, the tinning may be maintained by wiping the tip with
a cloth or sponge every few minutes while soldering. When
correctly tinned, the tip will heat both parts of the connection almost immediately. Solder should then be applied
directly to the parts to be soldered. as shown in the middle
illustration above, and both iron and solder removed as
soon as the solder flows freely.
The circuit boards of the Stereo 35 are supplied with all
components (resistors and capacitors) already mounted
and soldered in place. The circuit boards are connected to
the other sections of the amplifier channels by soldering
wires to eyelets on the boards. These eyelets, which are
numbered for identification, are filled with solder already.
To solder a wire to them, they are first heated with the tip
of the iron, and the end of the wire inserted as soon as the
solder in the eyelet flows. A correctly made connection looks
like the illustration at the right, above, which shows a smooth
transition from eyelet to wire.
4
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