Sequential prophet 5 - Page 45

CV at pin 12 drives the first exponential current generator transistor, which charges the CAP (C415
or C404) through the current mirror. The direct integration yields a sawtooth. The falling edge results
from the one-shot discharging the CAP when triggered by the comparator. The sawtooth is 0-10V >
across pull-downs R453 or R444.
•
U460-1 (OSC IB, U456-7) maintains exponential control characteristics by forcing the transistor
current to equal the reference current established by R449 (OSC B. R441). D402/R450 (D401/R439)
compensate for the internal buffer's input bias current at the low end and the bulk-emitter resistance
effect of the "logging” transistors at the high end. by providing extra base drive at high current to the
sweep circuit. U480-1 (U456-7) will always be more negative than U463 (U459)-13. R414 (R412) HI
V/OCT controls how much the base will be "pulled-down”.
RT402/R452 (RE401/R442) mount directly to the 1C to counteract the effects of increased internal
transistor leakage with increasing temperature.
Integrator C415 (C404) is polystyrene for temperature stability and low leakage.
C416 (C405) compensates the current mirror, reducing possibility of high-frequency oscillation in
the control circuit.
Variable width pulse output is derived by internally comparing the sawtooth to the pulse width input
(pin 9). Control sensitivity is 10%/V (100% (5) 0V decreasing to0% at 10V). Pulse output Is0-7.5V across
pull down R454 or R443. When switched on, OSC A's HARD SYNC input (pin 7) senses OSC B's saw¬
tooth edge, forcing immediate discharge.
The sawtooth and psuedo-triangle output pins are both emitter-followers from the internal saw¬
tooth. On U459 OSC IB (and U371 LFO — see SD312), U457-7 (U372-14) produces the actual bipolar
triangle by subtracting the sawtooth from the half-sawtooth obtained by biasing the psuedo-triangle
emitter follower and amplifying by X2. (R306) TRI SYM adjusts the bias to set equal slopes.
On U371 LFO (see SD312), Q308 helps discharge the large-value integrator C335, required for low-
frequency operation.
Figure 2-12 graphs the ideal VCO (and VCF) V/OCT function, and the corrections possible with the
trimmers provided. As can be seen, the adjustments overlap. INIT FREQ raises or lowers the line.
V/OCT, actually the CV summer gain, changes its slope and HI V/OCT (not present on VCFs) adjusts
the function above 3 kHz.
U495-1 in OSC IB's KEY 1 CV path decreases the effect of U440-10 OSC B KBD S.
(Note 3 shows a buffer modification installed on some Prophets to prevent OSC B mistuning due to
the added load of the TRI CONV and OSC A SYNC S.)
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