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the
matched blocks IC305B is then driven by the predistorted output of 10304.
To provide more detail: The output of 10304 is first attenuated by R334, R335,
C311, and then applied to the input of the feedback element IC305A. The output
of IC304 is predistorted as necessary to force the current output of IC305A to
precisely
and linearly cancel the audio input
into the " virtual ground" summing
junction of 10304. This same predistorted voltage is also connected to the input
of
IC305B.
Thus
the
output
of
10305B
is
an
undistorted
current,
which
is
converted to a voltage in current- to- voltage converter IC306A, R341, R376, C312.
The output of IC306A is the output of the VCA.
Because
IC305A
is
in
the
feedback
loop
of
10304,
the
gain
of
the
VCA
is
inversely
proportional
to
the
gain
of
IC305A.
Thus,
if the
control
current
is
applied to the control port of IC305A ( through R333), then the VCA behaves like
a two- quadrant analog divider.
In the case of the " Master" VCA, a fixed current is applied to the control port
of IC305B through R339, R340, CR301 to fix the gain of IC305B. CR301 provides
temperature compensation.
Second- harmonic distortion is introduced by differential offsets in either IC305A
or
IC305B. This distortion is cancelled by applying a nulling voltage directly to
the input of IC305B by means of resistor network R336, R337, R338.
If
the
VCA
is
not
perfectly
balanced, " thumps"
due
to
control
current
feedthrough can appear
at
the output. These are equivalent
to
multiplying the
control current by DC. If a correct DC offset is applied to the VCA input, then
this
equivalent
DC
multiplication
can
be
nulled
to
zero
and
the " thumps"
eliminated. Such an adjustable DC offset is provided by R331, R332.
R329,
R330, 0310 are
frequency- compensation components to prevent
the VCA
from oscillating supersonically.
2.d) Compressor Control Circuitry: ( on Card # 5)
The
output of the " Master" VCA is applied lo a voltage in/current out fullwave
rectifier- with- threshold, 10507A, 10505, R519, R520, R521, C503, CR503. This is
essentially an opamp with discrete class-B output stage. A bias voltage of - 12V
on the "+" input of 10507A holds the voltage at this opamp's "-" input at - 12V by
feedback
and
provides
appropriate
bias
conditions
for
the
rectifier
to
prevent
saturation. R521
determines the rectifier's transconductance. 0503 provides DC
blocking between the nominal ground potential of the input side of R521 and the
-12 volts at IC507A's "-" input.
A negative- going voltage at the input side of R521 pulls current away from the
"AC virtual ground" al IC507A's "-" input. An equal current must therefore flow
into "-" input by turning on the NPN transistor whose base is connected to the
output
of
IC507A.
Because
of
the
class-B biasing,
this
assures
that
the PNP
whose base is connnected in parallel to the NPN is off.
A collector current essentially equal to the NPN ' s emitter current flows into the
NPN from the output terminal of the rectifier. Part of this current comes from
the rectifier load; part comes from the fixed collector current of the top PNP
transistor. This PNP creates the threshold of limiting by saturating and diverting
all class- B output
stage
collector current away from
the load until
the output
stage current exceeds the nominal PNP collector current. When the output stage
current exceeds the PNP collector current, the PNP comes out of saturation, and
B-3
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