Entferner B is a stereo, resonant, and self-oscillating 12/24dB band-pass filter.
Entferner filters, in general, are based on a cascade of very versatile 6dB filters. In the Entferner B filter, two or four of the filters are used and can be modulated either using the internal signal or any external CV.
Since the filter is using internal saturation, the sound is highly dependent on the input voltages. Red LED lights beside the ports indicate any voltage overdrive. If the LED is blinking from time to time, the filter is operating in its sweet spot. If the LED is continuously red, the filter will still work but might result in a harsh sound or unwanted behavior.