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IC4B ( pin 7) to the negative rail ( due to its nominal non- inverting gain of 6.9),
and CR3 will be OFF. Under this condition, the gain of the HF VCA ( IC11, IC12
and associated components) is determined by the current flowing through the sum
of
R75,
R76,
and
R19,
which
is
constant
since
IC4B
has
been
effectively
disconnected by virtue of CR3's being OFF.
When the input
to the Hilbert- Transform
Clipper
exceeds 2V peak, the
voltage
on
pin
5 of IC4B becomes
more positive
than - 1.45V
and IC4B comes
out
of
saturation, turning CR3 ON and taking control of the HF VCA control current.
This
current is determined by the voltage drop across R19. Normally, pin 6 of
IC11A
sits at approximately - 13.5V, so the control current is determined by the
voltage at the cathode of CR3.
PROOF
mode is activated by turning Gil ON, thereby parallelling R68+R69 with
R73
and
increasing the threshold. This has
one subtle
and peculiar
side- effect
which needs to be understood. The input of IC4B is protected ( internally) by two
back-to-back diodes. Under certain conditions in PROOF mode, these diodes can
turn on, coupling the control- voltage signal at pin 5 into pin 6. This coupling can
modulate the gain control current of the VCA ' s even though pin 7 of IC4A is at
the negative rail ( which is its normal state in PROOF mode).
If sinewaves below 4kHz
are applied in PROOF
mode, this causes no problems
because the output voltage from the vector- sum generator is almost ripple- free.
However, above 4kHz substantial ripple appears on the control voltage because of
the change in operating mode of the circuit above this frequency. Enough of this
ripple can couple through the protection diodes of IC4B to produce as much as
1.0%
THD before deemphasis and 15kHz filtering ( i.e., as observed at pin 1 of
IC5A).
If observed at the output
of Card # 8 or # 9 ( after such deemphasis and
filtering), THD may be as high as 0.25% between 4 and 5kHz only. This increase
in distortion, as small as it may seem, is nevertheless entirely an artifact of the
PROOF
mode
and
does
not
represent
the
actual
distortion
capabilities
of
the
system
in
its normal OPERATE mode. We felt that it was important to explain
this
phenomonon
in
some
detail
because
its
generating
mechanism
is
very
obscure, and because the increase in distortion above 4kHz in PROOF mode only
might
otherwise cause serious confusion in a troubleshooting situation if it were
misinterpreted as a system fault.
To
continue the discussion of circuit operation: The output voltage of IC4B ( at
the
cathode of CR3) is buffered by IC6A and applied to a second- order unity-
gain constant- delay lowpass filter ( IC6B and associated components). IC6B drives
R40, whose value is identical to that of R19. Thus, under steady-state conditions,
equal currents flow through R40 and R19, applying equal gain- control currents to
the HF VCA and LF VCA ( IC1, IC2, and associated components).
[The HF and LF VCA's operate identically to those in the " Master" and " Bass"
compressors, and the reader is referred to ( 2.c) above.]
The input to the LF VCA is processed by a lowpass filter consisting of IC7 and
associated components. This filter is normally down 3dB at 3.0kHz and exhibits a
deep notch at 6.9kHz.
For equal
control
currents, the gain of the LF
VCA
is
normally
approximately
5.5dB higher than the gain of the HF VCA because the sensitivity of the inputs
on the filter cards (# 8 and # 9) driven by the VCA ' s are unequal.
B-10
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