A voltage controlled filter on a chip, produced by Curtis Electromusic. It was widely used in early-to-mid-'80s polyphonic synths, including the Memorymoog and the Rev 3 versions of the Prophet-5. The 3320 implemented a four-pole filter, with voltage control over cutoff frequency and resonance. Each of the four poles was wired to its own I/O pins, and implementing multimode and complex responses was possible, but sadly this capability was rarely exploited; most of the synths that used the chip used it strictly as a low pass filter. The major exception was the Oberheim Matrix-12 and Xpander design, which used the 3320 to implement a variety of complex filter responses, and showed what the 3320 was capable of.
The 3320 used OTA cells to perform the filtering; an external capacitor was required for each cell. Additionally, external resistor networks were required to mix the cell outputs for the desired filter response. The cutoff frequency control input contained an exponential converter, so the input could accept a volts/octave input directly, and built-in temperature compensation kept the filter in tune. The onboard VCA that implemented the voltage controlled resonance had a linear response. A control voltage biasing circuit prevented control voltage feedthrough.
Like nearly all Curtis ICs, the 3320 is long out of production and now extremely rare. Remaining units should be reserved to repair vintage synths, rather than being used in new designs. The Doepfer A-121 24db/octave high pass filter, which used this chip, was retired from production due to lack of availability.