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Timing Module: The
collector
of
IClla
feeds
the
compressor/limiter
timing
module.
This
module
contains proprietary
circuitry
which
receives the input current
from
the
rectifier
with
threshold,
and
outputs a control voltage with appropriate
dynamics
to
achieve
natural-sounding control
and
very
low modulation
distortion.
The
output
of
the
module
is
a
low-impedance
unidirectional voltage
source
which
can
be
wire-"OR"ed
with
other
such
modules
to
effect
stereo
tracking
of
an
arbitrary
number
of
channels.
The
RELEASE
TIME
control
R69
allows
a
20:1
variation
in
the
basic
release
dynamics, which
are
determined
by
the
timing
module
on
the
basis
of
the past
history
of
the
input.
When
attack current flows
into
the timing
module from
the
collector
of
IClla,
Q3
is
turned
ON.
This turns
IClle
ON,
pulling
its
collector
and
the gate
of
Q2
to
+15V,
thus turning
Q2
OFF
and
suppressing the release process
while attack
is
occurring.
Threshold-Setting Circuitry
and
RATIO
Control:
As was
explained
above,
the
compression
threshold
is
modified by
the
setting
of
R74
in
order
to
lower
the
threshold
as
the
attack
time,
is
lengthened.
The
threshold
is
modified by
two
other
factors.
The
first
factor
is
the gain
of
the
COMPRESSION RATIO
control
R62.
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
R61
and
R62
in
such
a
way
as to
increase the threshold
as
more
gain reduction occurs.
The amount
of
feedback
is
adjusted by
R62
from
none
(infinity-to-one) to
enough
to
produce a
2:1
ratio at
compression
threshoid.
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
R62.
The
network R63,
R64, R65, R61,
R62
provides
a
reference voltage
for
ICBb which
in
turn
determines
the
basic
compression
threshold.
This
reference
voltage
is
decreased
as
R62
is
turned counterclockwise
towards iower
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
R62. The
change
in
VCA
gain-controi
voltage
fed
into
the
circuit
through
R61
approximately compensates
for
the
change
in
current flowing through
R63
and
R66
as the loading
on these
resistors
is
changed
by adjustment
of
R62.
Gain Reduction Meter: The
gain-controi
voltage
at
the
output
of
the
timing
module
is
fed
to
a
peak-detector
circuit
(consisting
of
IC9 and
associated
components) which
in
turn drives the gain reduction
meter.
The
gain-control voltage
is
applied
to
the
(+)
input of
lC9b.
The
output
of
IC9b
drives a
peak
detector
network C2D, C19,
CR15, CR14, CR13, CR12.
When
lC9b's
(+)
input
goes
more
negative than
its
(-)
input (indicating that the
output
of
the
meter
driver
circuit
needs
to
be updated),
IC9b's output goes negative and charges
both
C20
and
C19
to
approximately
equal voltages through
CR15
and
CR12.
IC9b's
output
stays
negative
and
continues charging
until
C20
is
charged
to
the negative
peak
value
of
the
gain control
voltage.
IC9a, configured
as
a voltage-follower,
buffers
C19
and
provides
a
negative
feedback
voltage
for
IC9b.
33
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