AM8103 – SH02 VCA & ADSR


AM8103 Module

Overview In 2022 I developed a set of Eurorack modules to replicate the Roland SH-2, which was originally launched in 1979. A fantastic sounding two VCO analog monster. The AM8103 module is an accurate replica of the ADSR and VCA circuits in the SH-2, made possible by the release of the AS662 chip 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 (INA, INB) with a mix potentiometer, the level of the output signal can be adjusted with the VOLUME potentiometer and there is an audio output (VCA OUT).

Min Attack Time = 1ms

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
  • GATE selects the external gate signal (GATE).

The original ADSR circuit is based on a dual Op Amp with transistor switches, it 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

AM8103 Rear PCB

AMSynths Design 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. but retains the fast 1ms minimum time  There is a manual momentary push button which triggers the ADSR, with a red LED illuminating when a gate signal is present. The 0 to +10v envelope output signal is available on the ENV OUT jack socket.

The module accurately replicates the originals attack curve when running on 12V power the original uses 15V. The envelope timing and sustain level resistors have been left unchanged and the envelope voltage amplified by 1.25 to provide a 0 to +10V output for Eurorack modules. This ensures AM8103 performs exactly like the original.

Min Release 2-3ms

The module also has a trigger input so that the envelope can be retriggered whilst the gate signal remains on. This enables fast lead lines to be played and is a feature of the original Roland mono-synth and 100M system. A separate AMSynths 6HP module (AM8210) converts a pitch CV signal to a trigger output, with low note priority. When a new pitch is played on the keyboard the envelope is retriggered.

Retaining the trigger feature and ensuring the correct attack curve is important, and unfortunately both these features are sadly missing from the Behringer 140 ADSR module.

The AM8103 module can be used in the three original modes, but without the need for a MODE slide switch;

  • GATE – patch a +5V gate signal into the input socket
  • GATE & TRIG – in addition, patch the AM8190 trigger into the input socket
  • LFO – patch a LFO signal into the gate input socket

Outcomes & Availability A prototype module was tested in July 2022 and the ADSR and VCA circuits immediately work. A few minor changes to the pot layout were made with production modules available from September 2022.

 

 

Copyright AMSynths 2022