Doepfer A-101-1 Vactrol Steiner Filter

Vactrol-based multimode filter based on a schematic by Nyle Steiner. Separate inputs for lowpass, bandpass and highpass.

This filter's unique feature is that different filter types are realized by injecting the audio signal at different points of the circuit. For this reason the A-101-1 has three inputs with level control: one lowpasss input which is normalized to the bandpass input, which itself is normalized to the highpass input. The level controls make it possible to mix the filters and to generate new filter types. e.g. with all three filter levels turned to maximum level you get a notch filter.
Furthermore the cutoff frequency influences which filter type is "monitored". This way you can blend from LP over BP to HP with increasing cutoff, known as "frequency scanning". Interesting effects happen when three different audio signals are used because they are both morphed and filtered at the same time.