Here you can find Kicad schematics and PCB layout for an 8 channel ADSR based on the CEM3310/AS3310 integrated circuit. It has a separate control board for all 4 parameters and offers gate and trigger input and CV output. Power is supplied by standard +12V/-12V Eurorack connector.
Leave 3310 ICs out and turn attack, decay and release control to maximum. Adjust trimpots until you measure -240mV on all three test points to ground.
https://www.synthxl.com/wp-content/uploads/2018/04/Crumar-Trilogy-Schematic.pdf#page=13
http://www.muzines.co.uk/articles/electro-music-engineer-cem-3310/3647