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the
difference
between
the
PNP
collector
current
and
the
rectifier
output
curent is delivered to the rectifier's load. The PNP transistor's collector current
is fixed by its emitter resistor R519, and by its base voltage ( determined by the
setting
of the CLIPPING control, R560). The CLIPPING control thus varies the
collector current of the PNP, and therefore the threshold of limiting. In PROOF
mode, CR503 and R520 parallel R519, increasing the collector current, and raising
the limiting threshold by approximately 14dB.
If
the
voltage
at
the
input
side of R521
goes positive, then
the bottom PNP
transistor turns on, and its neighboring NPN turns off. The collector current of
the
PNP
is
inverted by the dual NPN current
mirror,
and the current
mirror
output
is summed into the output port of the rectifier, where it is also subject
to
the
action of
the PNP
threshold transistor
as
described immediately
above.
The output of the left " Master" rectifier is " OR"ed with the output of the right
"Master" rectifier by means of two diode- connected transistors:
Q506 3:
0508.
Because the output of each rectifier is in the form of a current, voltage drops
across
the " OR"ing
diodes
do
not
affect
the
accuracy
of
the
rectifier.
Any
number of rectifier outputs could be " OR"ed to this point.
The
output
of
the " OR"
circuit
is
applied
to
a proprietary
circuit
which
computes
the
VCA
control
voltage.
Release
time
control
for
the
slow
"compressor" function is provided by R516, R515.
This " compressor"
function
may
have
either
a LINear ( release
proceeds
at
a
constant number of dB per second) or an EXPonential ( release starts slowly, then
speeds
up)
shape.
Support
for
the
EXPonential
release
shape
is
provided
by
IC503A,
IC504,
and
associated
circuitry.
IC504
implements
a sample- and- hold
circuit
which " remembers"
the
control
voltage
at
the
beginning
of
each
new
release
cycle.
The
hold
capacitor
is C502,
and
the
sampling
switch
is
Q503.
IC503A and associated circuitry detect each time attack current
flows into the
"Master"
release
time
module
from
either
the
rectifiers- with- threshold
or
the
loudness controller, and switch Q503 ON when this occurs, updating the sample-
and- hold. ( The details of how the sample- and- hold interfaces to the release time
module are beyond the scope of this document.)
A JFET switch 0505 is provided to slow the compression release when ordered
to
do
so
by the gating circuitry ( described immediately below). In the case of
the " Master"
compressor,
a pair
of
resistors
R517,
R518
whose
Thevenin
equivalent
voltage is approximately - 3.2VDC
is connected to one side of 0505.
When 0505 is ON, these resistors are swamped out by a low impedance drive on
the
other
side
of
the FET.
When
0505
is
OFF,
these
resistors
pull
a small
current
from
the
timing
module,
forcing
the
gain
reduction
voltage
to
move
slowly towards - 3.2V ( corresponding to 10dB G/R).
The output of the control voltage module varies between 0 volts ( maximum gain)
and
approximately - 10
volts ( minimum
gain=maximum
gain
reduction).
Thus
release is inhibited by applying a voltage of greater than + 10 volts to the anode
of CR502 which forces 0505 off. Release is enabled by applying a voltage of less
than - 10 volts to the anode of CR502. This reverse- biases CR502,
and 0505 is
forced
on
by R514's forcing 0505's gate to be
at
the same potential
as
its
source.
In the case of the " Bass" compressor, IC5038 attempts to make the "Bass" VCA
control voltage ( as read on the TOTAL BASS G/R METER) follow the " Master"
VCA control voltage ( as read on the TOTAL MASTER G/R METER) by feedback.
When
the
GATE
is
OFF ( i.e.,
when
0505
and
Q510
are
ON),
this
feedback
B-4
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