Doepfer A-101-6 Opto FET VCF

Like any module of the 101-X series also Opto VCF resorts to opto-electronic parts but in this case so-called Opto-FETs are used instead Vactrols which have a avery different behaviour. The module consists of six 6dB filter stages arranged in series. Each stage can act individually as lowpass, highpass or as one of two all-pass filters. Plenty of options to experiment with, from usual filtering to phase shifting effects due to all-pass filters.

The components used in the A-101-6 are Opto-FETs, or optical field effect transistors. They are way faster than Vactrols and can be fed with fast CVs and allow for FM modualtions but on the other hand they have a stronger distortion behaviour at higher levels.

The filter type is selected by jumpers on the pc board (factory setting: low pass). The type of filter determined by the jumpers positions can be marked by means of a water-resistant felt pen at the front panel.
The resonance is controlled by the Feedback control up to self oscillation. By means of a trimming potentiometer the maximal feedback can be adjusted. High feedback values can be used mainly in the allpass mode to obtain very extreme self oscillation sounds. Even an external feedback signal can be used instead of the internal feedback connection (FB In socket).
The Mix control is used to pan between the original signal (CCW position) and the effect signal (CW position). In filter mode (LP/HP) this control is usually set fully CW. In the allpass modes one obtains phasing sounds at center position or "pure" allpass sound in fully CW position.