Sequential prophet 5 - Page 43

POT VALUES
U328-1
Waveform 2-2
VDAC
V: 2V/div
H: 1 rrn/div
SYNC: U426-6, S/H 14
The DAC has been covered above. The software first latches data with CSC. waits 20 ms for the DAC to
settle, strobes the S/H with VDAC for 30-40 ms. and allows the DAC to settle again to 0. This occurs 41
times in each loop. See Waveform 2-2.
It may be remembered that R337 need not be as precise as the rest of the R-2R ladder elements, since
the MSB is not used for FREQ CVs. Also, the OSC FREQ CVs themselves are inverted. That is. the
MSUMs require the OSC A and INIT CVs to operate 0 to -5V for a 5V octave frequency increase.
Not mentioned above. U364-1 switches the ac-coupled 440-Hz reference into the noise circuit,
following U364-15 MIX NSE VCA. U367-8 filters U366 NSE GEN output to PNK NSE for modulation.
The LFO is based on the 2030 also used in the voices, discussed in paragraph 2-23. This application
differs by having R378/80 fix PW at 50%. and Q308 discharge C335.
The MSUM stage consists of simple op-amp summers and BIFET buffers connected through pre¬
cision-matched resistors.
The GLIDE circuit, active only in UNISON mode, consists of Q309 exponential converter. U379 trans¬
conductance amplifier. C348, and U378-1 BIFET buffer. Increased GLIDE CV delays the effectiveness
of rapid UNI CV changes.
U379 is wired for unity gain through U378-1. so its voltage output. GLIDE OUT CV. should equal the
input. UNI CV. D303/02 clamp the difference between these to not exceed 0.6V. C348 and U378-1 are
similar to a SAMPLE/HOLD (S/H) circuit, except C348 is under more or less constant charge through
U379-6. Negative current accompanies a downward UNI CV shift.
To understand the effect of Q309. consider the case where the control current at U379-5 is 0. No
current will be available from U379-6. C348 holds its charge, maintaining the same GLIDE OUT CV.
though UNI CV may fluctuate wildly. With a slight increase. C348 will be able toeventually match slow
changes in UNI CV. Increased control current will accelerate the circuits’ ability to equalize differ¬
ences between UNI and GLIDE OUT CV. With maximum control current. C348 can track instantan-
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