A Roland System 100m and a Formanta Polivoks both walk into a bar....
Vladimir Kuzmin's 1982-designed Polivoks filter has become one of the great success stories of the last decade (thanks to the sterling efforts of Marc Bareille and The Harvestman), bringing the idea of a capacitorless programmable-power opamp filter to a scene fixated on discrete ladders, vactrols and OTAs as filter building blocks. However it's probably a comment on the conservatism of modern boutique synth design that no-one seems to have questioned how to take that circuit principle into different designs. Instead there seems to be more and more cloners fixated on the authenticity of vintage Soviet silicon.
Ladies and Gentlemen, meet the LM346. This is a much improved quad version of the LM4250 chip that is used in many Polivoks VCF clones (and the UA776 chip which was itself copied by the Soviets!) which has been hiding in plain sight for almost 40 years now. Until now no-one has adapted it to synth use, despite a datasheet promoting its use in filters.
As the Polivoks was an adaptation of the classic OTA-based 2-pole State Variable Filter we thought we would go in the opposite direction and try the 4-pole cascading lowpass OTA design pioneered by Roland and the Elektor Formant in the late 1970s. A schematic quickly sketched in Eagle late one night in a McCafé worked beautifully on breadboard, and the circuit quickly took shape. Difficulty arose because the same mechanism that tunes the filter in a programmable power opamp also progressively power-starves the rest of the chip resulting in some very unpleasant behaviour at the low end of the tuning range if left unaddressed. A bit of detective work and a bit of wisdom from the late Bob Pease helped us fix this and keep the bass nice and creamy!
The result is a rich-sounding lowpass filter which still has plenty of character and sizzle, and won't disappoint anyone looking for something new.