This is one of Doepfer’s older filter designs, nonetheless it is quite an interesting one. It offers four filter modes: Notch – Highpass – Bandpass – Lowpass, each with a separate output (so you may use it together with a switch or a voltage controlled mixer).

Soundwise, it is a bit rough and would be a good companion for e.g. an A-108 or an A-106-6. It can self-resonante in all four modes and its 1V/Oct-scaling is among the most exact ones I know (so this may be used as an oscillator as well). Unfortunately, the special circuit CEM3320 (Curtis) is no longer available, so this filter will run out of stock. Technical details can be found on Doepfer’s Homepage:

http://www.doepfer.de/a121.htm

I have used three A-110 VCOs (sawtooh, one A-110 is 1 octave below), driven by an A-155 sequencer and put into the filter with high audio level. Filter out is then sent into an A-132-3 VCA (controlled by an A-140 ADSR).

We start with Notch mode (0:00-1:22), then Highpass mode (1:22-2:53), Bandpass mode (2:53-4:30) and Lowpass mode (4:30-6:17). Within each mode, I start with no resonance and filter frequency only being modulated by a very slow LFO. Then I slowly increase resonance up to maximum, add ADSR modulation of filter frequency, decrease resonance back to zero and finally fade out.

Here we go:

http://www.andreaskrebs.de/assets/media/A-121.mp3

Have fun,

Andreas