Sequential prophet 5 - Page 47

Figure 2-13
2020 Block Diagram (One Channel) Courtesy SbM
The SSM 2020 VCA (Figure 2-13) is used in many places in ihe Prophet for control of audio or other
CV levels. There are 7 VCAs per voice. Their inputs are ac- or dc-coupled according to the signals they
are intended to pass. All are operated in linear mode by tying pins 4.6,7 or 12,10,9 together. Pins 1
and 15 are current outputs. The VCA is essentially a multiplier where the difference between its
inverting md non-inverting inputs is multiplied by the CV. There should ideally be no output when
there is no input. In several places, trimmers ensure this by correcting for dc offset and CV leakage
(rejection).
The FILT and AMP ENV (U406. U401). GENs are based on the SSM 2050 ENV GEN (see figure 2-14). Elec-
Ironically-switched CVs charge and discharge C418 (C419) through an exponential voltage-controlled
resistor. The ADSR states are defined by internal logic clocked from the GATE and TRG inputs. An
ATK flip-flop (FF) is set by the TRIG pulse through C402 (C401) and reset by either -GATE or the attack
comparator sensing +10V OUT. Then the ATK state » GATE AND -ATK FF. and REL = -GATE. Each state
is characterized by a nominally exponential approach to a characteristic voltage: 0V tor AIK.
and REL.+13V for SUS.
R402 (R401) simultaneously trims TIME CONSTANT of the ATK. DEC. and REL periods.
The LOW-PASS FILTER is based on the SSM 2040 VCF. U478 (see Figure 2 75)
resonance. R407 and R406 function as shown in Figure 2-12.