The MantisLadder is a diode ladder filter inspired by the EFM diode ladder. The EFM diode ladder uses a pair of OTAs to extract the signal from the diode ladder and to control the resonance, which is also what this filter does. It's got very stable self-oscillation, a controlled sine wave, with a couple fancy extra features.
For the "top" and "bottom" of the diode ladder, I've added stable quality voltage references that aren't effected by noise on the power rails.
The input features a "drive" knob, which overdrives the input transistor of the diode ladder. Overdriving this filter yields warmth and fuzz to your sound.
The output circuit adds saturation when turned past about 1 o'clock. More warmth and fuzz, very pleasant analog distortion.
There is a "Inversion" knob, which feeds a 180 degree out-of-phase signal into the output buffer. This is an old strategy to convert a diode ladder filter into a high-pass filter, and that's not exactly how this filter works, since there's at least two features which introduce distortion into the signal that runs through the filter. It can be massaged to get a high-pass effect if you're careful, but tweaking that knob certainly adds an interesting quality to the tone.
Finally, there's one (1) resistor added to this filter to eliminate the common behavior from ladder filters of the lower frequencies being attenuated when the resonance control is turned up. The one resistor to the OTA that extracts the signal from the ladder was enough to get rid of that amplitude sag.
I encourage you to build this filter. I will release gerbers for the PCB once I'm happy that they work properly. I encourage you to experiment with different capacitor values and types, don't be shy about that. I ran this filter with a 100nF ceramic in one spot, a 10uF MLCC cap in another, and the mismatched capacitors caused the filter to emphasize harmonics and overtones in the signal in a truly magical way. It sounded mystical.