Sunday, 3 June 2012

A-143-1 Vactrol Mod

A simple modification to allow voltage control of the attack and decay times of Doepfer's Complex Envelope Generator/ LFO.

Despite its size, I love the A-143-1 for its natural sounding envelopes. The only thing it lacks is voltage control. I considered buying Doepfer's A-101-9 Universal Vactrol module, but reckoned it could be more fun and cheaper to hack one myself. This involved repurposing my A-180/ Quad Slew once again.

The circuit is simplicity itself. The tip of the cable/ jack carries the CV, which travels through the vactrol's LED and back via a resistor to the jack's ground connection. The hard part is gaining access to the potentiometer pins on the A-143-1 to apply the vactrol's resistance. As a lazy hacker, I opted to modify only the lower section of the A-143-1. This meant I didn't have to dismantle the entire unit.

The vactrol is connected to the attack and decay pots via switches. As I only used one vactrol, switching both in effectively connects both pots too. This means the manual pot settings affect the overall resistance of both time constants. It's a kludge, but it works!

See this thread for more A-143-1 mods.

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