9710 VCA 23
000216
When a Trigger is used instead of a Gate many of the same things happen but the Decay and Sustain
portions happen so quickly that you can think of them as being skipped. C22 couples Trigger pulses to
steering diode D3 so that the Q8/Q9 bistable is set and to R101 so that Q15 is turned on briefly to reset
the IC7:C comparator for retriggering.
For an LFO function, Q14 inverts the state of the IC7:C comparator, which is high as long as the Output
voltage is greater than the Sustain level. When S1 is closed the collector voltage of Q14 serves as a Gate
that goes on when the Sustain voltage at the end of the Decay phase is reached.
VCAs and Balanced Modulator (see schematic fig 7)
The L and R VCAs are designed around an LM13600 type Dual Operational Transconductance Amplifier
(IC5). Taking the L VCA as typical, signals applied to the OTA's inverting input (pin 4) by way of the
voltage divider R37 and R35 are converted to a bidirectional current from pin 5 which is then converted to
a voltage by IC6:A and R51. The darlington output structures that are part of LM13600s are not used
because of the unavoidable output offset voltage they produce.
The "gain" of the OTA is controlled by a current into pin 1 and as this current increases, a constant level
voltage input on pin 4 produces increasing output currents from pin 5 and consequently larger output
voltage from IC6:A. The control current for the L VCA is produced by opamp IC3:D, Q1 and associated
components. This current is proportional to the sum of the voltages from the Pan control R109 (coupled
by R10), the CV
L
input (coupled by R11) and an offsetting voltage that assures the VCA can be turned
fully off (R17).
The Panning function is produced by increasing the gain of the L VCA while decreasing the R gain and
vice-versa. While the voltage on the wiper of the Pan control goes from 0 to 12V and increase the L gain,
IC3:B and associated components outputs a voltage that is the inverse of the Pan control setting for
decreasing R gain. Similarly, the circuitry around IC3:A provides an inverse of the CV
L
input.
The L and R VCA outputs are passively mixed by R116 and R117 and become the A VCA input.
The terms "Carrier" and "Modulation" can be confusing when applied to a physical device like the 1496
Balanced Modulator (IC4) because they're information theory terms. In terms of function, there is no
difference between these inputs. Another name for a Balanced Modulator is "Four Quadrant Multiplier",
which means that the output is proportional to the product of voltages on the two inputs and that the sign
is preserved - if either of the inputs goes negative the output goes negative and if both inputs go negative
the output goes positive. When a Balanced Modulator is used as a VCA it is equivalent to using only two
of the four quadrants and one of the inputs (the Control Voltage) is constrained to only positive voltages.
As this voltage increases the output voltage swing increases.
The Carrier input is buffered by the opamp summer built around IC6:C which also provides a +5V offset
from ground required for proper operation of the BM.
Q3 and Q4 form an exponential converter circuit that sinks a 10dB/V exponential current into the
collector of Q4 in response to the 0 to 10V output of the ADSR. Inserting an external input plug into J6
causes the Ring to ground the junction of R66 and R63 which disables the expo circuit. External Modula-
tion inputs are always treated as a linear input allowing Balanced Modulator operation.
NOISE
The 9710 Noise source uses the common approach of reverse-biasing the emitter-base junction of
transistor to the point of break-over and amplifying the white noise that results from avalanching. The
transistor that supplies the noise is Q5 with Q6 and Q7 configured as a two stage discrete amplifier.
