MS20 filter clone, a 12dB lo-pass and a 6dB hi-pass filter connected in series.
The famous original MS20 included two filters: a 12dB lo-pass and a 6dB hi-pass filter connected in series, both with a very special design (the MS20 hi-pass is very often described as 12dB high pass, but this is not true).
Doepfers found a way to use the same circuit simultaneously as lo-pass and hi-pass for 2x different audio signals (a bit similar to the A-101-1 Steiner Vactrol filter that has even different audio inputs available, but with the special MS20 circuit).
Two separate audio inputs for lo-pass (LP) and hi-pass (HP) with separate level controls are available. The sockets are normalled, i.e. the signal applied to the LP input is available for the HP input too provided that no plug is inserted into the HP input socket.
The level control of the HP input is realized as a polarized input. This means that the signal can be added with the same polarity (+ range) or opposite polarity (- range) compared to the LP input. This feature enables notch (+) and bandpass (-) filter functions too.