AM8103 ADSR & VCA

Description This module is an accurate replica of the ADSR and VCA in the Roland SH-2 analog mono synth which was released in 1979.

EuroRack Module:   10HP wide
User Manual:            Download PDF

The VCA is based on the BA662 OTA chip (using an AS662 clone) with two audio inputs (INA and INB) with a MIX potentiometer to balance the two signals. There is an audio output (VCA OUT) at Eurorack level and a rotary VOLUME control, to adjust the level of the output.

The VCA is controlled by the on-board ADSR, with no external CV required. 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.

The original ADSR circuit in the SH-2 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 4s
• Decay: 2ms to 10s
• Sustain: 0 to 100%
• Release: 5ms to 10s

The AMSynths circuit replicates the original ADSR, with the attack maximum increased to 5 seconds. The slide potentiometers have soft yellow LED's set at a low lighting level.

The ADSR can be triggered just by the GATE external input but using both GATE and TRIGGER signals is highly recommeneded as this provides a retriggering of the envelope on every key change. A keyboard with both GATE and TRIGGER outputs is recommended (a Roland 181) or the AM8210 utility module can be used to convert a pitch CV into a trigger signal.

There is a manual (red) momentary push button which triggers the ADSR, and a red front panel LED indicates a gate signal is present. The ADSR output (0 to +10V) is available on the ENV OUT jack socket, and the envelope CV signal can be patched into a filter, such as the AMSynths AM8102.

The volume control is usually set at 3 o'clock when using 10V audio signals, but it can be increased to maximum to boost 5V audio signals. Soft clipping occurs at maximum volume and 10V signals. 

There are six 3.5 mm jack sockets on the front panel:

• INA, INB - VCA audio inputs
• GATE - traditional +5V gate signal for VCA and ADSR
• TRIG - +5V short trigger signal
• ENV OUT - Output of the ADSR
• VCA OUT - Ouput of the VCA

The module comes with a set of M3 mounting screws and washers, and a standard Doepfer power cable.

PRODUCT STATUS: IN PRODUCTION