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This is a rough simulation of the VCA and AD envelope Roland used in the TR-808 drum machine. It is elegantly crude, perfect for a drum machine. There are four sections in this circuit: trigger logic processing, attack phase smoothing, passive envelope generator, and swing-type VCA.
The NPN/PNP pair and surrounding circuitry comprise the trigger processor. The top pulse wave is the global trigger, which, in the 808, is sent to every voice when the CPU sends a trigger out. The trigger level is set by a potentiometer called "accent level," meaning it must go through an analog "AND" gate where it is compared to an instrument timing pulse, which is not global. I recommend altering this part of the circuit for eurorack DIY purposes, as this is meant for interfacing with the 808's CPU.
The processed pulse then goes through a smoothing circuit. The physics are complex, so we can just think of it as a single pole RC filter that applies only to the attack stage of the trigger. As far as I can tell, this is just to get rid of any annoying clickiness that would sound atrocious.
This then goes through a simple diode attack/decay envelope generator. In the real 808, there are three separate envelopes, one of which uses a transistor core. However, for simplicity, I stuck with the envelope generator for VCA 3.
The envelope is then sent to the CV control for the swing-type VCA, the star of the show. I'd recommend watching Moritz Klein's video on designing a DIY hi-hit circuit, as he covers the VCA's quirks in sufficient detail, but in short, this is an extremely simple single transistor amplifier with built-in clipping due to its highly sensitive biasing. Normally, the collector voltage would be held stable, but that is not the case here, as it is used as a CV input. Of course, there is significant DC offset, but that is rectified by a resonant highpass filter stage after the amplifier. The diode is here to ensure strange behavior does not occur at 0 volts, which will happen otherwise.
Thanks to Moritz Klein for his excellent video explaining and simplifying the circuit, as well as Warner et al.'s ICMC/SMC 2014 paper "The TR-808 Cymbal: a Physically-Informed, Circuit-Bendable, Digital Model." I highly recommend looking into these resources if you found my derivative explanation helpful. And of course, all this would not be possible without the brilliant engineers at Roland who designed the TR-808 to begin with. These drum machine circuits are works of art, and their elegant beauty places them near the peak of synthesizer design.
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