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6-4
TECHNICAL DATA
Orban Model
464A
Circuit
Description
On
the
following pages, a
detailed description
of each
circuit's
function
is
accom-
panied
by
a
component-by-component
description
of
that
circuit.
Keywords
are
highlighted throughout
the
circuit
descriptions
to
help
you
quickly
locate the
infor-
mation you
need.
The
circuitry
is
described
in six
major
blocks: input
buffer,
VCA
(voltage-controlled
amplifier),
leveler/compressor
control,
high-frequency
limiter,
PEAK OUTPUT LEVEL
metering,
and
power
supply.
Whenever
circuitry
is
duplicated
for the
left
and
right
channels,
only
the
left
channel
will
be
described.
The
schematic
at
the
end of
this
section also
shows
only
the
left
channel
(left
channel
components
are
numbered
100 through
299;
right
channel
components have
corresponding
numbers
in the
300-499
range;
power
supply
and
shared
components
are
numbered
0-99).
1.
Overview
The
block
diagram on
page
6-21
illustrates
the
following
overview of Co-Operator
circuitry.
The
signal,
which
enters the
Co-Operator
in a
balanced form,
receives
moderate
RF
suppression, then
is
applied
to
a very low-noise
opamp
configured
as
an
"active
transformer".
The
current-controlled
gain block
used
in the
Co-Operator
is
a proprietary class-A
voltage-controlled amplifier
(VCA).
It
operates
as
a
two-quadrant analog
divider
with gain
inversely proportional
to
a current injected into
a
first
gain-control
port,
and
is
cascaded with
a
two-quadrant analog
multiplier
with gain
directly
proportional
to
a current
injected into a
second
gain-control
port.
Any
"thumps"
due
to control
current
feedthrough
are
eliminated
by
applying
DC
offset
the
VCA's
input.
The
leveler/compressor
is
a
feedback
circuit:
the output
of
the leveler/compressor
is
looped
back
to
develop
a
gain-control signal
that
is
applied
to the
VCA.
This
arrangement
results
in
superior
stability
of
characteristics
with time and
temperature,
extremely
low
distortion,
and optimized
control-loop
dynamic
response.
The
proprietary leveler/compressor
timing
module
generates a control
signal
that
enables die
Co-Operator
to
achieve natural-sounding
control
and very low modulation
distortion.
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
to
proceed
at
a
constant
rate (in
dB/second)
regardless
of
the absolute gain
reductioa
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.
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