Sequential PROPHET-600 - 8-2 PRECAUTIONS

Pitch accuracy amid varying temperatures is the main challenge of VCO design. As it
heats and cools, the exponential control current generator charges the timing
capacitor more or less quickly, altering the oscillation period (pitch). The stability of
the Prophet-600s oscillators results from the 3340 including a temperature
compensation (TEMPCO) circuit which has the same temperature characteristics as
the exponential generator, but which counteracts drift by making the necessary
adjustment through the precision multiplier stage. (The oscillator accuracy depends on
the TUNE system.)
Referring to the Voice 1 schematic, R424 adjusts OSC 1A scale by determining the
current gain of the precision multiplier through the TEMPCO circuitry. Pull-down
R423 helps with stability. On the SUM input, R447 sets the basic range of operation,
and R448/C463 compensate the precision multiplier. High-voltage switch U429-1
applies the P-MOD OSC A CV, when enabled. R449 (pin 14) converts the current
output of the precision multiplier into a drive voltage for the exponential current
generator. R450 (pin 13) sets the reference current for the exponential generator.
R451/C464 compensate the generator. C465 (pin 11) is the timing capacitor which is
the main determinant of oscillator frequency range. It is a polystyrene type for low
leakage and best stability.
When closed, the OSC A SYNC switch U429-I1 couples the falling edge of OSC B's
sawtooth through C466 and R453, which turns on Q401. The internal sawtooth buffer
connected to pin -10 is thus pulled down to -5V, resetting it. This hard-syncs OSC A to
B. The S SYNC output through R452 merely provides a bias for the transistor.
Except for sync, OSC B is implemented identically. Further details of the CEM 3340
can found in the Data Sheets section.
Notice that there is no analog switch on the A or B pulse outputs, pin 4. To turn the
pulse off the computer simply drives the pulse width all the way to dc. Because of
interaction with the sync circuit, the computer must use *5V to disable the OSC A
pulse. For OSC B, the pulse disable voltage is 0V so that a dc offset is not created for
either POLY-MOD destination.
The sawtooth waveshape switches operate from ♦ 15V, to pass the lOV-level sawtooth.
The triangle switches operate at ♦ 5V. To prevent frequency shift, these are arranged
to present a constant impedance. When off, U430-1 connects the output to ground
through R433. OSC B has two destinations: audio and POLY-MOD. At U430-12 R454
approximates the parallel resistance of R435 and R434. The -5V applied to the 4053s
lowers their input impedance. Also with regard to switches, sheet E (at the right)
shows D409 and R406 creating a —6V bias for all 4016-type switches, allowing them to
pass slightly negative voltages
The OSC B POLY-MOD level is controlled by VCA U434-2, buffered, and switched to
OSC A FREQ by U429-1, or to the filter by U430-4.
2-14 INTEGRATED MIXER/FILTER/AMPLIFIER
The oscillator outputs are ac-coupled to VCAs in the 3372 which are controlled by
separate VOL A and VOL B CVs. These voltages determine the MIX levels to the filter.
2-16 TM600A 7/83