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The
output
of
IC201A
is
applied
to
seven
parallel
bandpass
filters ( IC201B
through IC208A). Bandpass response is achieved by cascading a lowpass filter with
a highpass filter. Except
for the top band (which uses
a second- order lowpass
filter), the first filter in each band is a third- order unity- gain positive feedback
filter. Explanations of such filters can be found in almost any handbook on active
filters.
Each
of
the seven highpass filters is equipped with a Class-B transistor output
stage.
Each
output
stage
is
loaded
with
a resistor ( R211,
R218,
R225,
R232,
R239,
R246, R252). The resistor associated with a given filter draws a current
proportional
to
its resistance from
the
filter's output
stage. The same current
flows
in
the
collectors
of
the
output
transistors:
positive- going
currents
flow
through the NPN transistor while negative- going currents flow through the PNP
transistor.
This provides a convenient means for obtaining a full- wave rectified current: A
current mirror ( 0215 and the PNP transistors in IC209) inverts the sum of the
currents through the NPN's and adds it to the sum of the currents through the
PNP s. This sum ( a full- wave rectified current containing contributions from both
the NPN's and the PNP's) is then applied to another current mirror ( the NPN
transistors
in IC209) whose
output
is
finally
applied to
the
line
going
to
the
"Master" timing module on Card 115 ( pin K of Card 112).
A positive- going threshold current is applied to the same line by means of 0216
to
establish the threshold of loudness control: If the full- wave rectified current
does not exceed the threshold current, no output current flows from pin K of the
card,
and
0216
saturates.
The
threshold
current
is
determined
by
the
voltage
drop
across R253 divided by the value of R253. The voltage drop is determined
by the voltage on the CLIPPING BUS, supplied from Card # 5. Thus the threshold
of the Loudness Controller changes in proportion to the threshold of compression
as determined by the setting of the CLIPPING control on Card # 5.
The
output
current
from
the
Loudness
Controller
can
be
totally
suppressed
by
turning
0217 ON and diverting the output current to ground, thus defeating the
Loudness Controller. 0217 is controlled by the Loudness Controller ON/OFF logic
circuitry ( IC210 and associated components).
ON
and
OFF
modes
can
be
selected
by
passing
current
through
opto - isolators
IC211
and IC212 respectively. Current
limiting and RFI suppression are provided
by resistors and feedthrough capacitors within the filter box; rectification for AC
control signals is provided by CR207.
IC210B
and IC210C are cross- coupled as a bistable
multivibrator ( flip-flop).
This
flip-flop
changes
state
whenever
a transistor
in
one
of
the
optoisolators
turns
ON,
pulling
its
collector
down
to - 15
Volts.
Local
switching
by
means
of
the
momentary ON/OFF switch on the
subpanel
also changes the state
of the flip-
flop.
Initiali7ation
circuit
CR206,
C243,
R260
assures
that
the
system
always
comes
up
in
the
mode
selected
by
jumper " B"
on
power- up.
Upon
power- up,
the
transition
of the
negative power
supply
from 0 to - 15 Volts
is coupled through
C243
and CR206 to either IC210B or IC210C, as selected by jumper " B". Under
steady-state conditions R260 pulls the anode of CR206 up to 0 volts. CR206 then
effectively disconnects the power- up circuitry.
When
the Loudness Controller is ON, pin 10 of IC210 is at - 15 Volts and pin 4
is
at
0 volts.
The
opposite
is
true
when
the
Loudness
Controller
is
OFF.
IC210A
and
IC210D
are
connected
as
inverting
buffers
to
drive
0217
and
the
ON/OFF
LED
driver
circuitry ( 0218
and
associated
components)
respectively.
B-15
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