Orban 424A Operating Manual page 38

A
voltage proportional
to
the current output
of
IC7A
is
developed
across
R58
because
of
the feedback action
of
ICI2A. (C2I
stabilizes
I
C
1
2
A
against
oscillations.)
Feedback
forces 1C
1
2A's
(-)
and
(+)
inputs to
be
at
the
same
voltage.
Thus, the
same
voltage
which appears
across
R58
also
appears
across
R57, and
current flows
in
R57
in
proportion to the ratio
between
the values
of
R58
and R57.
This current
flows out
of
the
(+)
input
line
of
ICI2A
into the
emitter
of
Ql.
Because
Ql's
base current
is
small
compared
to
its
emitter
current, essentially
the
same
current flows out of Ql's collector into the gain-control port of ICI3A.
The
base
of
Ql
is
grounded;
its
emitter therefore
sits at
+0.6V. This forces
both
(+)
and
(-)
inputs of
ICI2A
to also
sit
at
+0.6V, and
assures correct bias
voltage
for
IC7A's collector.
CR9
protects Ql
from
reverse base-emitter voltage
which could otherwise cause
junction
breakdown and
latchup
of
the entire current-inverter
circuit.
3)
Compressor/Limiter Control Circuitry
General:
The
compressor/limiter
is
a
feedback
circuit.
That
is,
the gain-controlled
output
of the compressor/limiter
is
sensed,
and
is
used
to
develop a gain-control
signal
which
is
applied
to
the compressor/limiter 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.
Rectifier With
Threshold:
The output
of
1C 14
in
the
VCA
is
applied
to a rectifier
with threshold. This
circuit
has a current
input
and current output.
Its
current gain
is
adjustable with R79, the
ATTACK
TIME
control.
ICI4's output
is
gain-controlled but predistorted, and
its
peak
level rises
faster
than the
peak
level of the
VCA
output
as
VCA
clipping
is
approached. Using
this
signal as the control source to the rectifier
makes the compressor/limiter
circuit
automatically
resist
VCA
clipping, as
the error
signal
used
in
the
feedback
control
loop
becomes
rapidly larger as clipping
is
approached, thus effectively
speeding the
attack
time and
reducing overshoots
at
the
VCA
output.
Any
DC
offsets at 1C
I
A's
output are blocked by C24.
Network
R85,
C23
provides
a
highpass
shelving
rolloff in
the
bass,
making
the control circuit
less
sensitive to
bass. This
matches
the equal-loudness curves
of
the ear (the ear
is
less
sensitive to
bass than
to
midrange
frequencies),
and prevents obvious modulation
of
midrange
material by
bass.
A
jumper
is
available to defeat this
rolloff
if
the
424A
is
to
be
used
as
a true peak
limiter
which must be
equally sensitive
to
all
frequencies.
The
output
of
the shelving network
is
applied
to
a conventional precision halfwave
rectifier ICICIB,
R84, R83,
CR26, CR27. The
output
of this rectifier
(which inverts
its input)
is
applied
to
the
summing
junction
(a virtual
ground)
of
ICIOA
through
R8I with
a relative gain
of 2x.
The
input of the half-wave
rectifier
is
also applied
to
ICIOA's summing
junction through R82, with
a relative gain of
lx.
The
current
input
to ICIOA's
summing
junction
is
thus
a
full-wave
rectified signal
with a
relative gain of
lx.
When
there
is
no output from
rectifier ICIOB, threshold current
is
removed from
the
ICIOA summing
junction
through R80.
This current flows through
CR25, and
ICIOA's external
PNP
output
transistor (pins
10,
II,
and
12
of IC3)
is
turned
off.
When
the
current output
from the
rectifier
exceeds the
threshold current,
CR25
turns
off
and
the output
transistor
is
turned on through
CR24,
thus permitting
output
current to flow
from
the
circuit into the following timing module.
34