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59
POWER
AMPLIFIER
The
signal
from
the tone
circuit
is
coupled to the base
of
transistor
Ql05L.
Transistors
Ql05L
and
Ql06L
form
a
differential
amplifier
circuit
that
maintains exact zero
DC
voltage
at
the
output of the amplifier.
Feedback
from
the output
is
coupled through
resistors
Rl65,
and
Rl67,
and
capacitor
Cl33.
Whenever
the
output attempts to
go
negative
or
positive,
it
causes
transistor
Ql06L
to conduct
greater
or
less.
This causes
transistor
Ql05L
to
conduct
greater
or
less,
thus
controlling
the operation of
transistors
Q108L,
Ql09L,
QllOL,
and
Qllll.
The
signal
from
the
collector
of
tran-
sistor
Ql05L
is
coupled
directly
to the
base
of
transistor
Ql07L.
The
signal
from
the
collector
of
Ql07L
is
coupled
through
resistor
Rl75
to
Ql09L
and
through diode
Dl03L
and
resistor
Rl73
to
transistor
Ql08L.
Diode
Dl03L
and
resistor
Rl73
provide a
controlled
forward bias
on
driver
transistors
Ql08L
and
Ql09L.
This in turn provides bias
for
output
transistors
QllOL
and
Qllll,
so
that
class
AB
operation
is
maintained.
Capacitor
Cl37
provides the extra voltage
swing
necessary to
saturate
driver
transistor
Ql08L
and
output
transistor
QllOL
on
the
maximum
positive
signal
swing. Capacitor Cl4l provides the extra
voltage
swing
necessary to
saturate
driver
transistor
Ql09L
and
output
transistor
Qllll
on
the
maximum
negative signal swing.
Resistors
Rl85
and
Rl87
provide .
DC
stabilisation
for
output
transistors
QllOL
and
QlllL. Coil LlOl,
resistors
Rl93
and
Rl89,
and
capacitor
Cl45
provide
stable
operation under
all
load conditions.
The
output
of
the
amplifier
is
connected
through the speaker fuse to the speaker
switch
and
then to the speaker terminal.
POWER
SUPPLY
The
transformer-operated
power
supply
contains
two
full-wave
rectifier
circuits
that
supply voltage to the out-
put
amplifiers,
and
a voltage doubler
circuit
which
supplies voltage to a
zener-regulated supply for the pre-
amplifier.
Two
AC
receptacles
are
provided
on
the
rear
of
the chassis to furnish switched
and
unswitched
power
for the equipment.
Line voltage
is
present
at
the unswitch-
ed
receptacle
whenever
the
power
cable
plug
is
connected to
an
AC
source.
Line
voltage
is
present
at
the switched
receptacle only
when
the
power
switch
i s in the
"on"
position.
Line
voltage passes through the fuse
(2-ampere for 120-volt
operation,
l-
a
mpere
for
240-volt operation) to the
primary windings of the
power
transform-
er.
For
120-volt
AC
operation,
the
primary windings are connected in
parallel.
For 240-volt
AC
operation,
the primary windings
are
connected in
series.
A voltage-doubler
circuit
composed
of
diodes 0109,
and
0110
rectifies
the
AC
voltage. This voltage
is
filtered
by
capacitor Cl5l
and
coupled through
resistor
Rl79
to the
collector
of
transistor
Qll4. Resistor
Rl80
and
zener
diode Dlll
sets
the reference voltage for
the base
of
Qll4.
With
the base
of
transistor
Qll4 held
at
30
volts
DC,
the
voltage
at
the
emitter
of
Qll4 will
be
held
at
29.4
volts.
This voltage
is
coupled to the
preamp
1
ifier
circuits.
Diodes
0105, 0106,
form
a full-wave
rectifier
that
supplies
positive
DC
voltage to the output amplifier
circuits.
Diodes
0107
and
0108
form
a full-wave
rectifier
that
supplies negative
DC
voltage to the output
amplifier
circuits.
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