TECHNICAL DESCRIPTION
The
DYNA
Stereo Control Amplifier, SCA-35, is acom-
lete stereo preamplifier
and
stereo power amplifier com-
ed on asingle chassis with asingle power supply.
The
SCA-35 uses new
circuitry
and
new components in aunique
arrangement
which provides highest
quality
performance
ill
acompact
and
moderately priced unit. Superficial ex-
amination
of
the
circuit
of
the
SCA-35
might
indicate
an
apparently
simple
and
conventional design. However,
closer
scrutiny
will reveal
an
extremely sophisticated
and
carefully refined design.
There
are
four basic sections of
the
SCA-35:
(1)
the
stereo preamplifier; (2) the dQal tone control section;
(3)
the stereo power amplifiers,
and
(4)
the power supply.
Each
of these utilizes novel circuitry
with
premium grade com-
ponents conservatively operated.
The
four sections combine
in
an
inter-relationship which develops the maximum poten-
tial of these components.
The
Preamplifier
The
preamplifier section of
the
SCA-35
is
a
printed
cir-
cuit
module
in
which a12AX7
tube
is
used
for
each
stereo
channel. All low level
inputs
are
fed to
the
input
stages of
these tubes via aportion
of
the
selector switch.
High
and
low level magnetic cartridges
and
crystaVceramic cartridges
are
introduced to the
input
switch through a
PEC
(packaged electronic
circuit).
The
switch also selects a
tape
head
input.
These
low level signals
are
amplified
'through two stages
(the
two halves of
the
12AX7).
The
amplification of
this
tube
is increased almost to
its
theoreti-
cal
maximum
by
the
application of positive feedback from
cathode to cathode.
Anegative feedback loop containing
the
proper
equaliza-
tion circuits to
shape
the
response of phono
and
tape
head
is carried
around
the
12AX7. Operating
parameters
are
selected to enable this stage to
carry
signals
at
distortion
levels below 0.1
%.
A 4 millivolt magnetic cartridge will
produce
one volt
at
1000
cps
at
the
output
of this section.
The
output
of
the
12AX7 is fed to
another
section of the
selector switch, which
at
this
point
can
also select higher
level
inputs
such
as
radio
and
tape
preamplifier.
The Tone Controls
Dual tone controls
are
used
in
anetwork' which is com-
pletely passive;
it
has
no distortion
or
noise.
This
network
is also a
PEC
in
which
all
components
are
encapsulated.
Associated with
the
tone controls
are
the
filter switch,
the
balance control,
and
the
volume control. Signal levels to
this section normally
are
about
one
volt (which is also the
normal
output
of
the
preamplifier
stage).
However,
the
design is such
that
signals of
any
magnitude
can
be
handled
without possibility of overloading the power amplifier
inputs.
he
Power
Amplifiers
Each
power amplifier is on its individual
printed
circuit
module.
These
are
truly
unique circuits
in
which optimum
use is
made
of
the
components.
The
first stage is apentode section of a7199 tube.
This
is
directly coupled to acathodyne
phase
inverter. Negative
dc
feedback is
taken
from cathode to screen to stabilize
the
dc
operating
point
of
this
pair. Apositive feedback loop
from cathode to
cathode
augments
the
amplification of the
stage.
The
cathodyne
inverter
has
the
unusual
advantage
that
its
phase
inversion capabilities
are
dependent
solely
on
the values of its associated resistors.
The
accuracy of phase
inversion is unaffected
by
tube aging
and
similar variations.
The
phase
inverter drives a
pair
of 6BQ5
output
tubes.
Careful investigation
has
determined
the
precise operating
parameters for
the
output
tubes
and
output
transformer.
The
output
transformer designed for the circuit
(DYNACO
Z-565) provides maximum linearity
in
this configuration.
This
transformer represents
an
advance in
the
state
of
the
art
as
it
permits
the full power of
the
output
tubes
tJ
be
handled
throughout
the
audio spectrum
in
a
unit
of very
small size.
It
also permits ahigh
order
of feedback with
complete stability of operation
under
all amplifier load
conditions.
Contrary
to traditional usage, asingle cathode resistor
is
used
for
all
four ou.tput tubes.
This
improves the per-
formance of
the
output
stage,
but
it
makes
it
desirable to
match
the
four
output
tubes.
The
output
tubes supplied
are
carefully
matched
for this circuit.
The
Power
Supply
The
SCA-35 is powered
by
apower transformer
and
full
wave rectifiers using silicon diodes.
Separate
heater
wind-
ings
are
included
in
the
power transformer to provide means
for individual
hum
adjustments
on
each
stereo channel.
The
power transformer is impregnated with
an
epoxy
resin which serves the
dual
purpose of
heat
dissipation
from
the
core
and
prevention of noise
and
vibration.
GENERAL
WIRING
PRACTICE
Assembly of the SCA-35 is exceptionally simple when
compared to
that
of other similar kits.
The
circuit boards
are
supplied with
all
components mounted,
and
the remain-
ing
parts
arranged
on
the chassis
in
an
open,
uncluttered
way
that
makes wiring quick
and
easy.
The
construction
of
the
SCA-35 should
take
no more
than
afew 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
diagrams supplied.
Have
the
proper
tools
at
hand
before beginning to build
your
kit. You will need apencil-type soldering iron of 30-
to 60-watt rating; long-nosed pliers; diagonal cutters;
and
ascrewdriver.
If
you have asoldering 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, awire-cutting
and
stripping tool will help
considerably; these
are
available for less
than
adollar.
7
J
