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through
R536 is turned off and has no effect because it is swamped out
by a
low
impedance appearing on pin # 2 of the "Bass" timing module. However, when
the GATE
is ON, 0510 goes OFF, closing the IC503B feedback loop. Under this
GATED condition, the gain of " Bass" VCA quickly becomes equal to the gain of
the " Master"
VCA,
and
both bands
eventually
gate
to - 10dB (" 0"
on
the
G/R
meter) as the "Bass" VCA follows the " Master" VCA by feedback through IC503B.
(This occurs accurately regardless of the setting of the BASS COUPLING control,
correcting a problem in the old 8180A in which the " Bass" band would only gate
to - 10dB
if
the
BASS
COUPLING
control
was
adjusted
fully " wideband".)
The output of the release time module is a low impedance voltage source. It is
applied to exponential converter circuit IC501, IC502, R501, R502, C501 through
pad R504, R505 and R506. The collector current of either matched transistor in
IC502 is an almost perfect exponential function of its base- emitter voltage. The
scaling factor of the converter is stabilized by forcing a constant current through
the
left-hand transistor by means of IC501.
This current
is determined by the
current injected into ICSOl's "-" input through R502. The base of the left-hand
transistor
is
grounded;
the
emitters
of
the
matched
transistors
are
connected.
Thus, assuming a perfect match between transistors, the collector currents of the
two
transistors
will
be
equal
if
the
base
of
the
right
transistor
is
grounded.
Varying the base voltage on the right-hand transistor varies its collector current
exponentially about the nominal current in the left-hand transistor. This nominal
current
determines
the
quiescent ( no
gain- reduction)
gain
in
the
VCA's.
The current output at the collector of the right-hand transistor is connected to
a matched pair of resistors, one of which feeds the gain control port of the left
VCA, and the other of which feeds the gain control port of the right VCA. This
is
a " current divider" and is analogous to the familiar resistive voltage divider.
The
operation
of
the " Bass"
control
loop
is
similar
to
the
operation
of
the
"Master"
control
loop.
However,
provision
is
made
to
mix " Master"
control
voltage
into
the
input
of
the " Bass"
exponential
converter
through
BASS
COUPLING
control
R533,
and R530.
When R533 is
fully
clockwise, the " Bass"
exponential
converter
is
being
fed
as
much " Master"
control
signal
as
the
"Master" exponential converter. In the absence of output from the " Bass" release
time module, " Bass" and " Master" VCA 's will thus track exactly.
Because
the "Bass" rectifier
is
always
connected
to
the
output
of
the "Bass"
VCA,
exceptionally strong
bass
will
exceed
the
threshold
of
bass
limiting
and
cause
an
output
from
the " Bass"
release
time
module,
thus
momentarily
decreasing the gain of the "Bass" VCA below that of the " Master" VCA. This is
the low- frequency equivalent of familiar high- frequency limiting.
(NOTE:
The " multiband
feedback
compressor
with
crosscoupling
into
dB- linear
VCA s" concept is protected by U.S. patent # 4,249,042.)
2.e) Gain Reduction Metering
Gain
reduction
metering
in
the " Master"
band
is
provided
by
three
meters.
The first, TOTAL G/R, is driven by a peak detector IC516, R561, C512, CR516.
C512 captures negative- going peaks and discharges slowly through R561. To avoid
being
loaded
by
the
meter,
C512
is
buffered
by
voltage- follower IC516B.
The
discharge time of C512 is sufficiently slow to permit the mechanical movement
of
the TOTAL G/R meter to rise to the actual peak level of the gain control
voltage, thus accurately displaying it.
COMPRESSION
is
indicated
by
passing
the
output
of
the
release
time
module
through
a grossly
overcompensated
301A
opamp IC515
connected
as
a voltage
B-5
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