Friday 10 April 2015

A-171-2 VCS Expander

Adding gated hold & burst functions and an End of Rise output to Doepfer’s Serge VCS. An update to this post.

A recent forum thread about the Serge 1973 envelope got me thinking about whether it would be possible to add its hold function to the VCS. Once I’d understood what was needed, a burst and EOR pulse were obvious additions.

Both Burst and Hold can be activated manually or by an external signal. Hold freezes the envelope in its tracks. It’s different to patching via a S&H as the envelope continues where it left off. Burst simply cycles the envelope at will.

Audio examples: Hold (cycling VCS FMs a VCO, button pressed to hold), Burst (first manual, then activated by 2nd row of sequencer).

The EOR is needed for quadrature functions with two envelopes (the other fires at the End of Cycle). Conditioning the pulse required some creative thinking, a case of patching with ICs! Its width can be varied, so it can be used to ping filters or delays.

These modifications are simple to implement with switches and logic inversion (schematic to follow). The additions borrow from my favourite envelopes and make the A-171-2 close to ideal. Yes, I could have just bought a Function but it wouldn’t have been as fun or educational!