While my MIDI module has one digital ADSR output, I realised later that I needed a second ADSR output to control the VCF (filter). Rather than just build a single additional ADSR unit, I've decided to build a dual unit so that both are functionally identical.
So after reviewing several designs, I have decided upon a custom design of my own, documented here:http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=synth_dual_adsr
This design has the following general features:
- Has hysteresis on the gate input to allow analog signals to trigger it
- Can be switched into "ganged" mode, since you'll often want both units triggered from the same Gate signal (this saves an extra patch cord)
- Fairly simple design
If I left out the Schmitt trigger circuit, this would be a very simple design. But I felt that the Schmitt trigger is a useful addition, allowing flexibility to use LFOs or other analog mixed signals to trigger a unit.
The Schmitt trigger gave me the most difficulty. I had done several iterations of using discrete transistors for this purpose. But after doing some Monte-Carlo analysis realized that the resistor values were quite critical, requiring 1% components. For me, this meant "special order" and potential for failure.
Using a normal split supply opamp for the Schmitt trigger presented other problems, since this requires work to keep the low output near ground potential, instead of going negative. The best compromise seems to be to use the LM3900 opamp, which is a Current Differencing Amplifier (CDA). The main attraction is the fact that it likes to operate from a single supply and swing to ground rather than negative. Creating a Schmitt trigger from these CDAs seemed to require the fewest number of components. Since you get four CDA units to a package, this worked out just right for a dual design.
The only other special innovation of this design is my emitter follower after the output TL072 opamp. Once you review the circuit, you'll see that the best that the charged capacitor C6 can be discharged is to within 0.6 volts due to the diode involved in the Release circuit. The emitter follower circuit corrects for this and reduces the ADSR output voltage (by 0.6 volts) to within about 100mV or so, depending upon the output level you've chosen.
If you wanted to economize, this dual design could accomplish that simply by sharing a single Gate input circuit. You'd of course lose some flexibility but it would eliminate the whole 2nd Schmitt trigger and trigger transistor circuits.