When the
CHANNEL IN/OUT
switch is
in the
IN
position,
the
LF
equalizer
operates
exactly like the other equalizers,
except that
a) two capacitors C407, C4II are inserted in
series with the summing junction
to
isolate
IC40I
and
the switching
from possible pop-producing DC
offsets; and
b)
a
complementary-symmetry discrete-transistor
output stage is included within
ICAO I
's
feedback loop.
Discrete Output Stage:
The
output transistors
Q403,
Q407
are operated as emitter
followers in a
class-AB configuration. R443, R445
provide local DC feedback to
stabilize the
quiescent
bias current through the output stage. The
output stage
is
biased by diode-connected transistors
Q40I,
Q405,
which
are thermally coupled
to
their associated output transistors to
provide
thermal
feedback which
stabilizes the
output stage
against thermal
runaway.
Current limiting to short-circuit protect the output stage is
provided
by
CR405,
CR407
in
conjunction with
R443, R445.
When the
voltage
drop across
R443
or
R445
exceeds the
turn-on
threshold of
its associated diode (about
0.55
volts), then the
diode conducts, shunting the drive
current
away
from
Q403,
Q407
and into the load,
thus protecting
Q403,
Q407
from
burnout.
IC40I,
the driver, is loaded by
R439, R44I. The
junction of
R439
and
R44
1 is
boostrapped to
the
output of the
emitter
followers by C4I5, which forces
the
junction of
R439
and
R44I
to follow the
AC
voltage excursions
of the output.
Because the output stage has
a
gain
of
almost
+1,
this means that IC40I does
not
see a load of 5.4K
(R439+R44I),
but rather a constant current sink because
the
voltage
across
R439
(and therefore the current through it) does not vary. The
boostrap thus relieves IC40I
from the
task
of driving a relatively low-impedance
load, and reduces distortion.
The
output is connected to
the
outside world through
RF
suppression network
R447,
C4I9A, C4I9B. This
makes the
output impedance of the
equalizer
47
ohms
in
parallel with lOOOpF.
(Note: When the optional output tranformer
is installed,
R447
is replaced with
a jumper to assure best performance from the transformer.)
Channel In/CXit Switch: R429
assures that C407
and
C4II
do not build
up
a
DC
charge
which
could
cause a pop
upon switching.
Because
R429
is
connected between
true ground and the virtual ground produced by the summing junction
of
IC40I,
it
has
almost
no
other
effect upon circuit operation.
When
the
CHANNEL IN/OUT
switch
is
switched
OUT,
the (-) input
of
IC40I
is
connected
to
AC
ground
through C4II, R43I.
Simultaneously, the
(+)
input
of
IC40I
is ungrounded and connected
to
the wiper of the
GAIN
control
through C409. R433
assures that C409 does not pick up
a
DC charge which
could
cause
a pop, and
R435
assures that the
(+)
input
of
IC40I
is referenced to ground while the
CHANNEL IN/OUT
switch
is operating.
Under these conditions, IC40I has
a
non-inverting
gain of +l8dB,
making up for
the fact that 1C 10
1 is
no longer
in
the
circuit and assuring
equal gain in both
CHANNEL IN
and CHANNEL OUT
modes.
The entire circuit
is carefully arranged to avoid
pops
due
to
differences
in
DC
potential.
However,
because the program is interrupted
for a
few milliseconds
while
the switch
is
being operated,
a
click due to waveform discontinuity
may be
heard
if program material is being passed through the equalizer
as the
switch
is
operated.
22
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