Flight of Harmony Plauge Bearer

The Plague Bearer is called a filter, but that is just a description of the circuit topology, what it can actually do goes way beyond that. It has been described as a filter, a waveshaper, a mangler, a crusher, and – my favorite – as an “FSU module”.
I think calling it a filter may be a bit misleading, but I have no idea what else to call it. It is a filter, but it does more (all at the same time, really) excessive phase shifting, waveforming, ringing, formant generation, and so on. I guess it could be called a waveform modifier or enhancer, but those don’t sound good either. I nameed it the Plague Bearer for a reason.
If you are looking for technical synthophile specs you are out of luck here. All f(h) devices are designed in accordance with how they sound, not to achieve mathematical perfection. The most common question I receive is, “what is the filter slope?”
Honestly, I never measured that. I specifically avoided the conventional approach while I was designing this circuit, so it’s very problematic to try to describe this filter in the normal terms. The slope is directly tied to the gain of the circuit, making it variable, and the Q is affected by all three filter controls. The setup is actually a combined High- and Low-pass. The corner frequencies are adjusted by the controls and can be overlapped completely, which gives a comb-filtering effect.
Most changes in the r4 were for ease of assembly purposes, but two new features were added:
• Input Attenuator: Earlier versions often required use of an external VCA to reduce input signal levels and prevent overdriving the input. This is now handled by the input attenuator.
• Output limiting: The output levels of earlier versions could spike well beyond ±5Vp-p, so an output limiter was added. The limiter has a soft to medium curve and can be disabled by moving the two jumpers as shown on the PCB.