Orban 424A Operating Manual page 40

A
sample-and-hold
circuit
is
associated with the timing
module.
The sample
and
hold provides a reference for the circuit
in
EXP
release
shape
mode
(it
essentially
tells
the
circuit
where
the
release started).
Each
time
ICIOA
turns
its
output
transistor
ON,
it
also turns
on
the switching transistor
in
IC3
(pins
13, 14,
and
15).
This
pulls
pin
15
to ground,
and
pulls
the gate
of
Q5
to
ground, turning
Q5
ON
and
capturing a sample
of
a
control
voltage on C27.
This
is
buffered by
1C
1
1
A
and
fed
back
to the
timing
module
through a
resistor
network and
the
RELEASE SHAPE
switch
SI.
A
second section
of
the
ATTACK
TIME
control
R79b
is
provided
to
slow
the
release
time
at
very slow attack times,
lest
the release
become
faster
than the
attack and severe inaccuracies occur
in
the compression
ratio.
R79b
is
buffered by
ICI7A,
the output
of
which
is
fed
into
the
resistor
network associated with
SI.
Other
details of the operation
of
the
circuit
and
how
it
interacts
with the timing
module
are
beyond
the scope
of this
manual.
Threshold-Setting Circuitry and
RATIO
Control:
As was explained above, the
compression
threshold
is
modified by
the setting of
R79
in
order to
lower the
threshold as the attack time
is
lengthened.
The
threshold
is
modified by three other
factors.
The
first
factor
is
the gain of the
COMPRESSION RATIO
control
R92.
The
basic
compressor
circuit has
a high compression
ratio
(approaching
infinity-to-one).
To
achieve
lower compression
ratios,
the
VCA
gain-control voltage
is
fed
back
into the
threshold
circuit
through R9I and R92
in
such a
way
as to increase the threshold
as
more
gain reduction occurs.
The amount
of
feedback
is
adjusted by
R92 from
none
(infinity-to-one) to
enough
to
produce a
2:1
ratio at
compression
threshold.
It
can
thus be seen that the compressor output
level
increases
as gain
reduction
increases, lowering the
compression
ratio as desired. (Because
of
details
of
the
circuit configuration,
all
ratios increase
as gain reduction increases
beyond
threshold, yielding a "soft
knee"
curve.)
The
second
factor
is
the
position of the
COMPRESSION RATIO
control
R92.
The
network R88,
R89, R90, R9I,
R92
provides a
bias
current
for 1C IB
which
in
turn
determines
the basic compression threshold. This bias current
is
decreased
as
R92
is
turned counterclockwise towards
lower
ratios,
thus lowering the threshold
and
increasing
the gain reduction to avoid potential
VCA
overload as
its
output
level
increases.
The
circuit has
been
designed
so that
at
25dB (maximum
achievable) gain
reduction, the
VCA
output
is
approximately constant regardless
of
the position
of
R92.
The
change
in
VCA
gain-control voltage fed
into
the
circuit
through R9I
approximately compensates
for
the
change
in
current flowing through
R90 and
R88
as the loading
on
these
resistors
is
changed by adjustment
of
R92.
The
third factor
is
the
OUTPUT
TRIM
control R64. This adjusts the gain
of
the
VCA
and
simultaneously adjusts the compression
threshold to
keep the
amount
of
gain reduction approximately constant.
R64's wiper
is
coupled
to the
VCA
gain port through
R65, and
to the gain-control
input of log/antilog multiplier 1C
I
(and associated
components). The
input to
the
multiplier
is
the current output
of
the
RATIO
control
and associated
network
discussed above.
The
multiplier varies
its
gain (and thus the threshold voltage) by
20dB
as
R64
is
adjusted throughout
its
range.
The
gain
of
the
VCA
is
simultaneously
adjusted
by 20dB,
so that the
VCA's
output varies
by 20dB but the
amount
of gain reduction stays approximately constant.
36