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follower.
The
2.2uF
compensation
capacitor
C511
so
limits
the
slew
rate
of
IC515 that only the slow component of gain reduction is permitted to drive the
COMPRESSION meter.
The LIMITING
meter
is
connected
differentially
between
the
outputs
of
IC515
and
IC516B.
It
thus
indicates
the
fast
component
of
gain
reduction
as
the
difference
between
the
slow
component
and
the
peak- held TOTAL
component.
"Bass" band
gain reduction
metering reads
the sum
of the " Master"
and " Bass"
control voltages through R535, R534, in the same proportions that are applied to
the
input of the " Bass" exponential converter. In the interests of simplicity, the
"Bass" TOTAL G/R metering signal is not electronically conditioned.
2.f) Gating Circuitry: ( on Card # 5)
The
gating
detector consists of a peak detector followed by
a comparator. IC
opamps are employed for both functions.
The left and right input signals are summed in R551, R552, and lowpass-filtered
at
3kHz by means of C509. The lowpass-filtered sum is amplified by means of
non- inverting
amplifier IC508B, whose gain
is
variable from
0 to approximately
40dB by means of R550, the GATE THRESHOLD control. Low frequency response
of IC508B is rolled off with C510 to prevent low- frequency noise from inhibiting
the gate.
The
positive
peak
output
of
IC508B
is
detected
by
CR512
and
C504.
R549
determines the recovery time of this peak detector.
The
output
of the peak detector is applied to comparator IC508A. R547, R548
create
a reference
voltage
of + 1.9
volts.
If
the
output
of
the
peak
detector
exceeds
this value, then the output
of IC508A
is
driven towards
the
negative
power
supply, and the gate is inhibited. Otherwise, the output
of IC508A rests
close to the positive power supply, and the gate is enabled. In this condition, the
GATE LED is lit by current supplied through R545, CR510.
Hysteresis
to
assure
clean
switching
is
provided
by
positive
feedback
through
R546.
In PROOF
mode, CR511 applies + 15 volts to the "-" input of IC508A to inhibit
the gate, thus permitting all VCA ' s to recover to full gain.
3) Phase-Corrected Lowpass Filter/Preemphasis: ( on Card # 6)
Phase
correction
for the preemphasis and fourth- order lowpass filter
associated
with
it
is
provided
by
a fourth- order
allpass
filter
IC601,
R601,
R602,
R603,
R604,
R605,
R606,
R607,
R608,
R609,
C601,
C602,
C603,
C604.
The
overall
magnitude response of the filter between the card input and the filter output at
IC6018 is flat, gain is OdB, and the phase response varies from 0 to 720 degrees.
The operation of the filter is difficult to explain in words, and is best left to a
mathematical analysis.
The fourth- order lowpass filter is in fact quasi- fourth- order. The first section of
the filter is generated by a conventional second- order multiple- negative feedback
active
lowpass filter IC602A, R610, R611, R612, C605, C606. ( See, for example,
Wong and Ott: Function Circuits, op. cit.). However, the second section has been
combined with the preemphasis, and transformed from a purely lowpass form to a
peaking bandpass equalizer.
B-6
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