Aphex Studio Dominator 700 - VCA Limiters

accomplished by switching Cll2 in parallel with Clll while
crossover tuning is accomplished by switching R123 across
The JFET switches are controlled by d.c. signals from the
panel selector switches.
9 . 3 VCA LIMITERS
high
R122.
front
Each crossover filter outputs its signal to an individual
VCA limiter stage. There are, then, three VCA limiter stages
per channel. Since all VCA limiters are identical, only the
left channel low band limiter will be described in full detail.
The heart of the VCA limiter is U303, a type 1537A voltage
controlled attenuator device. U301A, Band U302A, B compose the
input and output support circuits for U303. Input audio is
present on TP301 while output audio is present on TP302. Pin 8
of U303 is the gain control port and is fed from three sources
through a passive mixerjattenuator using R318 as the shunt
element. Zero volts on this terminal allows maximum gain of the
VCA circuit (lOdB). Negative voltage on pin 8 causes signal
attenuation.
One source of gain control voltage comes via the "Drive
Bus". R320 couples this control voltage to the VCA. The drive
bus sets the basic operating gain of all the VCA's by this
means.
The low and high band limiters receive an "Equalization"
control voltage from the front panel "EQ" controls. The mid
band limiter receives no such signal. The result is that the
basic operating gain of the low and high band limiters can be
adjusted for "boost" and "cut" relative to the mid band.
The final source of gain control is the
composed of U304 and associated components.
is generated by this circuit which attenuates
are detected above the limiter threshold
manner:
limiter sidechain
A control voltage
the VCA when peaks
in the following
A sample of VCA output audio is coupled to the (-) input of
comparator U304A. A reference voltage is coupled to the (+)
comparator input. The source of this reference voltage is the
ALT bus which supplies a base voltage of approximately 2.0VDC.
This value changes to a lower value when the Automatic Limit
Threshold circuit is operating as will be later described.
The output of U304A remains positive, clamped to about +0.6
volts by 0301, when the sampled VCA audio is low in amplitude.
When the positive audio peak exceeds the ALT voltage, the U304A
output swings to negative 13 volts. The negative pulse from
U304A passes through blocking diode D302 to charge C304 via R323
which determines the charging rate. Buffer U304B couples this
negative voltage to the VCA gain control port via R321, thus
attenuating the audio signal until the output amplitude falls
just below the ALT value. An extremely high compression ratio
30