Envelope generator in my Polysynth is rather typical and as simple as I could make it. All parameters are voltage
- Control voltages:
- -4.5..+4.5V for times, and 0..+4V for levels. -4.5V gives longest time, +4.5 the shortest
- Time range:
- from 2.5ms to about 60s. Attack time may be shorter if Attack level is set low
- Power supply:
- +/- 5V, supply current not measured. Higher voltage (+/-7V) for opamps would improove performance
How does it work
Gate rest level is +5V. Anything between +2 and -5V sets sr flop through C2,R9 network. U3C-4 is low and opens
transistor T3. Voltage at (+) of the OTA rises and OTA charges up the C3. Meanwhile U3A-3 is high and opens switch
U4B which connects Attack control voltage to 'exp' network D1,2,3,R5,6,7. Other switches are closed if you look at
control logic. U2A serves as a buffer, so when C3 is charged up to Attack Level, the comparator U2B changes state
to high and resets sr flop (U3A-3 is low and U3B-4 is high). T3 closes, and voltage at (+) of OTA is determined
only by Sustain Level voltage and envelope output voltage. Now U4D switch is open and others are closed. OTA gain
is now set to value determined by Decay Rate control voltage. C3 is discharged exponentially to Sustain Level
When gate goes off (to +5V) U3C-10 goes high opening U4C switch and T2 transistor. OTA's (+) input is
near 0V (few mV) and C3 is discharged exponentially to 0V at rate set by Release control voltage.
C1 is useful
when gate goes off before attack peak is reached. It resets sr flop and EG goes directly into release stage.
Otherwise release rate would be the same as attack.
The use of U2C inverting amplifier make sense only together
with other modules. In this particular synth EG voltage is actually a reference voltage for sampler's DAC, so it
becomes a component of audio signal (it is somehow similar to VCA control voltage bleedthrough). U2C removes this.
Hope this description was helpful in any way. After all this is very simple circuit.
- U3: 4001 - quad NOR
- U4: 4016 - quad analog switch (might use 4066)
- U1: TL084 - quad opamp (may be replaced by any FET input opamp)
- U2: CA3080 - OTA
- T1, T3: any pnp transistor (BC557)
- T2: any npn transistor (BC547)
- D1: any diode; D2, D3 - those parts, as well as R5, R6 and R7 may be replaced by exp converter, or single
10k resistor depending on your control needs (linear, exponential, or any kind)
- C3: 1 uF tantalum. Use different value if you don't like 2.5ms-60s ADR time range
- R1, R2 divider set the feedback rate for OTA