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6-8
TECHNICAL DATA
Orban Model
464A
4.
Leveler/Compressor
Control
The
leveler/compressor
is
a
feedback
circuit:
the gain-controlled
output of
the lev-
eler/compressor
is
used
to
develop
a gain-control signal
that
is
applied
to
the
lev-
eler/compressor
gain-control port
of
the
VCA.
This arrangement
results in
superior
stability
of
characteristics
with time and temperature, extremely
low
distortion,
and
optimized
control-loop
dynamic
response.
The
output
of
opamp
in
the
VCA
is
applied
to
two
rectifiers
with
threshold.
One
serves as the leveling
rectifier,
and
the
other
is
the
compression
rectifier.
The two
rectifiers
have
different
thresholds
and
attack times.
The
rectifiers
feed the leveler/compressor
timing module,
which
contains proprietary
circuitry that
outputs a control voltage
with
dynamics
appropriate
to
achieving
natural-
sounding
control
and
very
low
modulation
distortion.
The
output
of
the
module
can
be wired
in a logical
"OR"
circuit
with
other
such
modules
to effect stereo
tracking
of an
arbitrary
number
of
channels.
The RELEASE TIME
control allows
a 5:1 variation in the basic release
dynamics,
which
are
determined
by
the
timing
module on
the basis
of
the past history
of
the
input.
The RELEASE SHAPE
button,
when
set
to
HARD,
causes recovery proceeds
at
a
constant
rate (in
dB/second)
regardless
of
the absolute gain reduction.
When
the
button
is
set to
soft, recovery proceeds
at
a constant
rate
from 25
to
lOdB
gain
reduction,
and
then progressively slows
as the
gain reduction approaches
OdB.
A
gating detector monitors
the
level
of
the
Co-Operator's
input
signal,
and
activates
the gate
if this
level
drops
below
a threshold
set
with
the
gate THRESHold
control.
The GAIN
REDUCTION
meter
consists
of
ten
comparators with
current regulators
at
their
outputs.
The
comparators
are
arranged
to
produce
a
meter
with
a linear
scale.
The
ten
LEDs
in the
bargraph
are
connected
in
series.
Component-level
description:
The
output of
ICI06
in
the
VCA
is
applied
to
two
rectifiers
with threshold
in
IC109.
The two
halves
of
IC109
are
both conventional
full-wave
recti-
fiers.
IC109b
is
the leveling
rectifier;
IC109a
is
the
compression
rectifier.
Threshold
currents
are applied
through
R159, R166. Attack
times
are de-
termined
by R167,
R169
(compression)
and R160,
R162
(leveling).
Any
DC
offsets at
IC106's output
are
blocked
by Cl
18,
Cl
19.
The
output of
the
timing
module
is
a
low-impedance
unidirectional voltage
source, positive-going
with
increasing gain reduction
(scale factor
of approx-
imately 0.4V/dB).
OV
corresponds
to
OdB
gain reduction.
Approximately
+10V
corresponds
to
the
maximum
available gain
reduction (25dB).
The
gain control voltage
is
averaged over
the
long term (30 seconds
or
so)
by R153,
Cl
17.
This average
is
buffered
by
ICllOb,
and
appears
at
pin
7
in
low-impedance
form.
Under
gated conditions
(when
the gating
FET
Q103
is
turned off
by
pulling
its
gate
towards
-15
volts),
the gain control voltage
is
forced
to
move
slowly towards
this
average
by
being routed
to
the
timing
module
through
R154.
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