The CGS Roland VP300 vocal filter was one of the first new modules I built after discovering the synth-diy mailing list some years ago.
The module is based on the fixed vocal filter section of the Roland VP300, the schematic of the relevant section mailed to me by a list member.
Some elements were left out, as I didn't need the keyboard split, and a mixer added, with normalized connections allowing easy wiring of the mixer to give feedback/resonance.
The module has recently been reskinned, the old plain aluminium with black pen markings being replaced by the unicorn picture.
The circuit is simply a bank of seven filters of a common configuration, the details of which are often available in linear data books, with their center frequencies set to give vocal formants. Each of the three inputs Alto, Tenor and Bass use only some of the filters each.
Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work.
Vocal Filter by Ken Stone, 1999, with permission of the author - archived