The A-121d from Doepfer is a dual Multimode filter based on a new filter design that has never been seen before in the Doepfer universe. Filter 1 offers a combination of bandpass and highpass, the second filter generates bandpass and lowpass. The two filters can be used in parallel, in series or as a dual-mono filter. Stereo can also be used with some minor compromises. The control elements and control voltage inputs have been designed in such a way that the application as a dual filter has the greatest utility value. Cutoff both filters are set simultaneously, for filter 2 there is the difference control "Delta", with which the distance between the frequencies of the filters is set manually. Resonance, the CV intensity on the cutoff parameter and the level of the audio input can be set individually for each filter. Some of the audio and CV outputs and inputs are normalized, which means that, depending on Patch , less cabling is required, useful routing is possible without having to use a Multiple and filter combinations are practical. High-pass 1, Lowpass 2, Bandpass 2 and the output of the 2-channel mixer are connected to the inverted labeled outputs.