ADSR Envelope Generator

I recently got PCBs back for the second iteration of the ADSR envelope generator. I was originally inspired by the designs that Rene Schmitz published, which have their origins in a 555-based circuit by Johnathan Jacky (see the notes in the YuSynth version). The 555 core is very clever and encapsulates the state logic in the 555's internal latch and threshold circuits. I've gone through a few iterations to get to this version, but I think this one is really solid and adds something new (at least as far as I can find for the 555-core circuits): a trigger. More below the fold.

Originally, I followed the schmitzbits.de design and while it worked well, it didn't seem to decay well back to 0V. I tried a bunch of things (e.g. adjusting the buffers to minimize Vce,sat. and pull the output down more), but the "off" state was still stuck around 0.2-0.3V. I was checking the build notes from Eddy Bergman and was relieved to find that he'd run into the same issue and, even better, had tracked down a solution: the precision ADSR by Kassutronics. This is a great solution: replacing the passive diodes with a (precision rectifier)[https://en.wikipedia.org/wiki/Precision_rectifier] eliminates the offset from the weak current through the diodes as they turn off.

That became the v1 circuit, which works really well. However, when I compared it to a few other non-555 designs (e.g. the MFOS ADSR), I noticed another feature that looked interesting: a trigger input. That motivated me to try and design a trigger while keeping the module based around simple discrete ICs. I came up with a solution using a single extra logic IC: the quad NAND gate CD4093 with Schmitt-trigger inputs. This has just enough gates to implement an extra SR latch and OR a pair of signals into the reset of the 555, so this could likely work as an add-on module to the existing 555-ADSRs. The only inelegant part of the design is that it does require a few extra buffers implemented with BJTs, but overall I'm happy with the result.

As of this note, I have some art done for the faceplate and will send that off soon. Updates to follow.