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Co-Operator
TECHNICAL DATA
6-7
current.
This
current
is
converted
to
a voltage
in
current-to-voltage converter
IC107,
R128,
L GAIN
TRIM
R129, and Clll.
The
output of
IC107
at
pin
1
is
the
output of
the
VCA.
The
VCA
behaves
like
a
two-quadrant analog
divider
when
the control
current
from
Q101
is
applied
to
the control port (pin 6)
of
IC105b.
The
gain-control
current
injected into
this
control port
is
developed
by
the lev-
eler/compressor
control
circuitry.
The
gain
of
IC105a
is
fixed
by
the current
through
R125.
Second-harmonic
distortion
is
introduced
by
differential offsets in either
sec-
tion
of IC105.
This
distortion
is
canceled
by
applying
a
nulling voltage
directly to
the input
of
IC105a by means
of
resistor
network R122, R123, and
L
DIST
NULL
trimmer
R124.
The "thumps"
which can
occur
if
the
VCA
is
not
perfectly
balanced,
are
equivalent
to
multiplying
the control current
by DC.
An
adjustable
DC
offset
is
applied
to
the
VCA
input
provided
by
R
1
18
and
L
THUMP
NULL
trimmer
R119
for
nulling
this
equivalent
DC
multiplication
to zero.
C107, C108, R116,
R117
provide
frequency-compensation
to
prevent
the
VCA
from
oscillating supersonically.
The
basic leveler/compressor control current
is
transformed
into
a
gain
which
is
proportional
to
a
control
voltage
in
dB
by
exponential current converter
IC5 and
associated
components.
IC6b, IC5,
and
associated
components
(on
the
second page of
the
schematic)
form a
log/antilog multiplier
which
multiplies the current
flowing
in
R9
by
the
exponential of the voltage
on
the
base of
IC5
(pin
9).
The
current
gain
of
the multiplier
(and
thus the
output
current
of
the
exponential converter)
increases as the voltage
on
the
base of
IC5
(pin 9)
becomes more
positive.
Note
that
the
reference
-0.7VDC
source developed
at the
output of
IC6b
is
used
for
both
the
left
and
right
channels.
The
current output
of
the log/antilog multiplier
appears
on
the collector
of
ICS
(pin
1 1).
Since
it is
the
wrong
polarity
and
level to correctly
drive the
control-current port
of IC105b,
it is
applied
to
current
inverter IC4a,
Q101,
R149, R150,
Cl
16.
This
circuit
has a gain
of
6.66*,
and
operates as follows:
A
voltage proportional
to the current
output of
IC5
(pin 11)
is
developed
across
R150
because of
the
feedback
action
of IC4.
Cl
16
stabilizes
IC4a
against
oscillations.
Feedback
forces
IC4's
- and +
inputs
to
be
at
the
same
voltage.
Thus,
the
same
voltage
that
appears across
R150
also
appears across
R149, and
current flows
in
R149
in
proportion
to
the
ratio
between
the
values
of
R150
and R149.
This
current flows
out of
the
+
input
line
of
IC4a
into
the emitter
of
Q101.
Because QlOl's
base
current
is
small
compared
to
its
emitter current, essen-
tially
the
same
current flows out
of
QlOl's
collector into the gain-control port
of
IC
105b.
Since
the
base of
Q101
is
grounded,
its
emitter therefore
sits
at
+0.6V. This
forces
both +
and -
inputs
of IC4
to also
sit
at
+0.6V,
and
ensures
correct
bias voltage
for
IC5's
collector (pin
11).
CR106
protects
Q101
from
reverse base-emitter voltage
which
otherwise
could cause
junction
breakdown and
latch-up
of
the
entire current-inverter
circuit
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