INTRODUCTION
The Heathkit
Amplifier
model
W-5M
was designed to
fulfill
the
performance requirements
of the
most critical
audiophile,
at the lowest
possible
cost. Up-to-the
minute design techniques
were
used
throughout to
reduce distortion
and at the
same time, increase
power output, thus allowing
the
connoisseur
of
music to
enjoy a
level
of realism
never before
obtainable.
The
full
dynamic
range
of all
types
of audio
material can
be handled
with ease because
of
the high
peak
power
handling
capabilities
of the
W-5M.
Just
a
few
years ago, tuners,
phonograph
pickups
and
speakers
were considered
as
the stronger
links
in the
audio chain
and the
amplifier
as the weakest.
The audio field
has
advanced tremen-
dously
in the
past five
years however,
and now the positions
are
reversed.
When
the
amplifier
is constructed
and
adjusted in accordance
with
the
instructions,
the
Heathkit model
W-5M
Amplif-
ier will faithfully
reproduce
all
program material fed to
it,
providing
optimum
performance from
all
other parts
of
the high
fidelity
system.
It is logical
to assume that
best performance will
be
obtained
with
the highest
possible
quality
accessory components.
The Heath
Company
cannot
recommend
specific components, but it
is
suggested that reference
be made to
catalogs and
magazines concerned
with the audio field.
In
general, the
higher the price of
the
components, the higher
the quality,
but
this
is not
necessarily
always
true.
Careful shopping will usually locate very
satisfactory
items
at a price
to fit
the
budget.
CIRCUIT DESCRIPTION
Thebasic circuit of
the
W-5M
amplifier
is
straightforward and simple.
Signal
from
the program
source
is
fed into the input jack, which is
coupled
to
the
grid of
the
input
12AU7
tube through
a
.1
ju.fd
condenser. Signal amplification takes
place
in the
first half of
this
tube and
the output is
directly
coupled to the grid of the second half
of
the tube.
Phase splitting or signal inversion is
accomplished
in
the
second
half of
the first
12AU7,
which
is a
split load type
of
inverter.
Signal
at the
cathode
of
this
stage
follows the grid,
while
the
plate
voltage will swing in the
opposite direction.
Coupling
to the grids of the second
12AU7
push-pull
driver
stage is through
.1
/Ltfd
capacitors, one connected
to
the
cathode and the
other
to the plate of the phase
inverter.
Amplification of the
signal takes place once again in the
12AU7
driver stage
and
signal
is taken
out through two
1
jifd
condensers,
one connected to
each
plate of the
driver.
The opposite end
of these
condensers
is
connected
to
the grids of the
output tubes. Here,
the signal
voltage
var-
iations are converted
to large current
changes
in
the output
tubes
and
the output
transformer
,
which is
in the
plate circuit.
AC
current variations in the high impedance primary
of
the
trans-
former are
passed
to the
low impedance
secondary
and
to the
speaker
line
connected
at
this point.
Feedback is
applied
from
the
secondary
of the
output
transformer back to the
cathode
of the input
12AU7
stage
to reduce
distortion and lower the
output resistance
of
the
amplifier,
thus
improving
amplifier
control over
loudspeaker
performance.
Inverse
feedback
also
improves
the frequency
range of
the
amplifier.
In
order
to reduce
harmonic
distortion at low
frequencies
it
is
essential that
the
plate
current
of
the
output tubes
be
balanced. An
exclusive
balancing circuit
is
incorporated
in
your
Heathkit
amplifier to
make
the
balancing
operation
easier
and much
more
accurate.
Precision
balanced
resistors are
connected in
the cathode
circuits
of the power output tubes.
When the
current
in
each
tube is
balanced,
the voltage drop across
each precision
resistor
will
be
the
same
and
the
resultant
potential will be
zero if measured
at both
cathodes at
the
same time.
A great advan-
tage is
gained,
since
the
current
in both
tubes is
measured
simultaneously
instead
of
one
at a
time.
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