Orban 424A - 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