The Basics
The Transistor Ladder Filter
Diode Ladder Filter
Sallen-Key Filter
Operational Transconductance Amplifiers (OTA Filters)
Further Filter Fundamentals
Marc Doty and the Moog Foundation go deep into Filters and Synthesis in this excellent course in the AskAudio Academy.
Watch Marc Doty and the Moog Foundation go deep into Filters and Synthesis in our excellent video course in the AskAudio Academy.
Discussion
According to this thread, originally it had a 4-pole filter using 4 CA3080 OpAmps then later switched to a 2040. Both were cascaded OTA filters to avoid the Moog transistor ladder patent.
https://www.gearslutz.com/board/electronic-music-instruments-and-electronic-music-production/1084564-octave-cat-vs-cat-rev-vs-srm-1-a.html
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