Schematics / Farfisa Synthorchestra Filter Section
Farfisa Synthorchestra Filter section ,brief analysis of these by Aaron D. Lanterman http://users.ece.gatech.edu/~lanterma/ems11/projectideas_sp11.html
"There seem to be two VCF circuits in the schematics (you'll have to rotate them clockwise by 90 degrees in your PDF viewer for them to make sense), one in the "Poli" section (upper right corner of page 6) and one in the "Mono" section (upper right corner of page Cool. They're quite strange. It looks like they're using JFETs as variable resistors; this is usually easy to do in a predicable way if one side of the "resistor" is grounded, but here they're "floating," which is likely to induce all sorts of odd behavior. The Poli one seems to be an cascade of two RC stages, but without buffering between them, so the poles of the filter will be spread out a bit, and I don't seem to see any feedback. The Mono version seems to have a feedback loop from the output back to the input, but that feedback loop itself has some unbuffered RC stages in it, and there's a swtich that you can use to switch this feedback loop in and out. I'd love for someone to rig this up and hear how it sounds! (This would probably make a good one person project.) "