The A-121s brings a sophisticated stereo architecture to the classic Doepfer filter lineup. It consists of two identical multimode filters that can be used as a matched stereo pair, two independent mono processors, or a serial filter chain for steep slopes and complex resonances. Unlike standard fixed-mode filters, the A-121s allows you to morph seamlessly between Lowpass, Notch, Highpass, and Bandpass modes. This morphing can be controlled manually via dedicated pots or automated through CV, leading to evolving spectral textures that shift fluidly between frequency bands.

The module is built for ease of use in stereo patches. It features a common frequency control for global sweeps, while the Delta F control introduces a frequency offset (filter spread) between the two sides, instantly adding width and movement to your sound. For streamlined performance, the Link to 1 toggle allows you to lock the filter type of both units to a single control. Clever internal routing and user-configurable jumpers on the back of the module let you decide whether the bottom CV inputs control resonance (Q) or filter type, giving you the flexibility to prioritize the modulation that best fits your workflow.