The A-101-6 from Doepfer is a flexible 6-stage filter module that uses opto-FETs—light-controlled field effect transistors—instead of traditional vactrols, resulting in a much faster response and a different tonal character. Each of the six stages can be configured as a lowpass, highpass, or one of two types of allpass filters, allowing for a wide range of filtering and phasing effects.

Unlike vactrol-based designs, opto-FETs handle faster modulation signals, enabling audio-rate frequency modulation and more dynamic control. However, this speed comes with a slightly more aggressive and characterful distortion at higher levels.

The module’s behaviour is shaped by internal jumpers (factory-set to lowpass), and you can label the selected modes directly on the panel if needed. The resonance is voltage controllable and goes all the way to self-oscillation, particularly wild when used with allpass modes. External feedback input is available for creative patching beyond the internal loop.

A mix control blends between the dry signal and processed sound, useful for dialling in phasing effects or fully wet filter tones.

• 6-stage filter with configurable modes per stage (lowpass, highpass, allpass)

• Uses opto-FETs for fast CV response and audio-rate modulation

• CV-controllable resonance with optional external feedback

• Mix control for dry/wet blending

• Flexible tool for filtering, phasing, and experimental sound shaping