A-121s

The clever A-121s filter from Doepfer with special attention to stereo and dual filter applications and filter type morphing.

Dieter Doepfer brings with the A-121s a great new filter among us nerds. Like all modules from Doepfer, this one is incredibly versatile due to normalizations, jumpers and clever patching. You can use it as a filter for one stereo signal, as two filters for two different mono signals, or serially for one mono signal. Via two knobs (one per filter) or CV (!) you can morph between the filter types lowpass, notch, highpass and bandpass. This allows for incredibly great sounding results!

However, to focus the operation on stereo and dual matters, there is a common frequency control and a Delta F control that lets you adjust the difference between the two filter frequencies (filter spread). In addition, the Link to 1 toggle switch lets you choose whether the Filter Type 1 pot also controls that of Filter 2.

If you’re wondering now: where is this great Filter Type CV input, then don’t worry here’s the explanation:

The two jacks at the bottom right of the module are named QM/TM1 and QM/TM2. This stands for Q-Modulation-CV i.e. Resonance-CV and Type-Modulation-CV i.e. Filter-Type-CV. Which of the two is controlled here can be set via jumpers on the back.