Six-Band Limiter Control - Orban 8100A/XT2 Operating Manual

Technical Information
6-9
7) SIX-BAND LIMITER CONTROL
Card #A2
The output of each band ' s VCA (except that of the 6.2kHz band) feeds a voltage divider
which, in turn, feeds a dual-comparator IC. If the output of the voltage divider exceeds a
positive or negative threshold of 0.5 to 1.0V set by the CLIPPING control, the comparator will
generate an output pulse which is smoothed by the timing module to produce a control voltage
which reduces the gain of the VCA. The drive to the clippers following the compressors and
pre-emphasis/high-frequency limiters is thus determined by the setting of the CLIPPING control,
which simultaneously adjusts all comparator thresholds (and thus the average VCA output
level).
The timing circuits process the signal in logarithmic form, and have low-impedance outputs.
These drive exponential converters which provide control-current outputs for their respective
VCAs. This timing circuitry is proprietary, and is located within sealed modules.
The DENSITY control provides a DC control voltage which offsets the logging transistors in
the reference logarithmic converters in the VCAs. This control voltage can vary the gains
of all VCAs by as much as 20dB while maintaining extremely accurate tracking. Thus, left
and right channels remain correctly balanced.
Component-level description:
Other than component values, the control circuits
for all six bands are identical — except the 6.2kHz
band, which receives the control signal from the
3.7kHz band. Only the control circuit for band 1
(150Hz) will be discussed in detail.
The output of the band 1 VCA is applied to volt ­
age divider Rl, R2 which protects IC1 (a 711
dual comparator) from being driven beyond its
ratings. IC1 produces a positive-going pulse that
is approximately +4.5V if the output of voltage
divider Rl, R2 exceeds the comparator threshold
voltage generated by IC12 (on Card #A1) and as ­
sociated circuitry. The threshold of limiting is
thus determined by two factors: 1) the loss in
voltage divider Rl, R2 (the more loss, the higher
the voltage at the VCA output before IC1 turns
on); and 2) the threshold voltage applied to IC1.
This voltage can be varied from ±0.5 to ±1.0V
by the CLIPPING control R160 (on Card #A1),
which varies the threshold of limiting (and thus
the average output level of the band VCAs) by
feedback.
The output of IC1 is applied to unity-gain in ­
verter and level shifter QI, R5, R7. The wave ­
form at the collector of QI consists of pulses
which go 4.5V below the +12V collector supply
voltage. These pulses develop approximately 4V
across R6, the emitter resistor of Q2. The current
which flows through R6 is essentially identical
to that flowing from the collector of Q2 into the
timing module. The value of R6 determines the
attack time by scaling this cunent.
The timing module integrates this current and
produces a voltage which is buffered by unity ­
gain FET-input opamp IC13a. IC13a ' s output is
a dB-linear control voltage which determines the
gain of the band VCA (in dB).