The Sallen-Key filter topology or design (first created in 1955) creates a “second order” or two-pole low, high, or bandpass filter and is capable of high resonance or Q, although with poor(?) component selection it can have odd behavior above its cutoff frequency. Among module designers, it is prized for its simplicity, near-infinite input impedance, and near-zero output impedance. This is the design used in the Korg MS-20 filter and its clones, among others.
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