AM8103 – SH02 ADSR & VCA


AM8103 Mockup

Overview In 2022 I developed a set of Eurorack modules to replicate the Roland SH-2 that was originally launched in 1979. A fantastic sounding two VCO analog monster. The AM8430 module is an accurate replica of the ADSR and VCA circuits in the SH-2, made possible by the release of the AS662chip by Alfa Rpar.

EuroRack Module:   10HP wide
User Manual:             
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Description The original VCA is based on the BA662 OTA chip, which I have replaced with the new AS662 chip from Alfa Rpar. This chip is a dual OTA, so only one side of the chip is used in this module. The VCA has two audio inputs without any attenuation (INA, INB), and an audio output (VCA OUT) with a rotary volume control. There are no external CV inputs, as the VCA is controlled by the on-board ADSR. A small slide switch selects how the VCA is controlled; HOLD sets the VCA to a constant maximum level, ENV selects the on-board ADSR, and GATE selects the external gate signal.

The original ADSR circuit is based on a dual Op Amp with transistor switches and is a different topology to the contemporary Roland 100M. It has the following specifications:

  • Attack: 1ms to 2.5 s
  • Decay: 2ms to 10s
  • Sustain: 0 to 100%
  • Release: 2ms to 10s

The AMSynths circuit replicates the original ADSR with a 1MA potentiometer for the Decay, rather than 500KA. This increase the maximum attack time to 5 seconds. A small slide switch selects whether the ADSR is triggered just by the GATE external input or by both GATE and TRIGGER. There is a manual momentary push button which also triggers the ADSR, and a red LED shows when a gate signal is present. The ADSR output is available on the ENV OUT jack socket.

Outcomes & Availability The AM8103 module is at the development stage.

 

Copyright AMSynths 2022