Putting both a couple of Boowy/Hotei videos and the schematic together, I get the sense that the unit is a kind of alternate approach to a bright compressed sound, different from compressors that include a wet/dry blend, but aiming for the same outcome.
The compressor portion seems like a ringer for the Dynacomp/Ross, and the output of it goes to a gain-recovery stage, followed by a Big Muff style tone control (Color) and buffered output. The circuit is a bit simpler if one uses true bypass switching, rather than the FET switching shown.
What differentiates it is the inclusion of an exciter-type circuit to supplement the compressed signal. IC2C provides 90 degrees of phase shift, beginning around 480hz. IC2D provides a 2-pole highpass filter, to isolate the higher-frequency content of the compressed signal. IC2A provides a simple gain of just over 3x to make the HF content more evident. IC3A provides a unity-gain phase-flipping stage, such that when the Exciter control pans between the outputs of IC2A and IC3A, it is either favouring the non-inverted or inverted version of the highpass signal, or allowing them to cancel each other out (middle position). If a builder finds they would like to have more sizzle to their excitement, simply make R30 larger (e.g., 27k or 33k) or make R29 smaller (e.g., 8k2 or 6k8).
Unlike the Big Muff, whose lowpass and highpass sections form the tone control, with corner frequencies chosen to provide a midscoop, the lowpass (R36/C17) and highpass (C18/R37) sections in the PS-021 overlap, such that there is still a fair amount of treble at the full bass setting and still a fair amount of lower mids at the full treble setting.
What I think is fairly evident from the videos and the circuit diagram, as well as from my experience with exciters in general, is that the guitar itself has to have some HF content, and the amp has to be fairly bright, for what is distinctive about this pedal to stand out. In other words, dull-sounding HBs or P90s are not going to realize any magic from this pedal. In the absence of those factors, it becomes simply a Dynacomp with a tone control. That's certainly not a bad thing - I imagine many a player would love to have a tone control on their Dynacomp/Ross/Keeley/Retrosonic compressor. But what sets it apart is the ability to add the upper frequency content, to give more articulation. And if you don't feed it what it needs to do that, you won't hear the difference. Again, that is true of pretty much ALL exciter pedals; which I feel is largely responsible for them not selling very well, historically, despite enjoying considerable usage in studios. Bottom line: they need a single-coil-equipped guitar.
In view of the considerable similarity to the Dynacomp, I draw your attention to R19, which sets gain-recovery time. This is often replaced with a pot in series with a 10k resistor, and labelled "Attack". Shortening the gain recovery time, combined with the exciter function, should bring one squarely into Nile Rogers territory...which isn't a terrible place to be.
Finally, I will draw a parallel here with the Woody acoustic simulator I whipped up a decade back. Where the PS-021 uses a 2-pole highpass and boosts that, I used a 3-pole highpass, and boosted/clipped that output before blending back in with the straight signal. I stole the idea from Jules Ryckebusch's
Harmonic Sweetener, that had originally appeared in
Electronic Musician. Jules used 4 poles of highpass and LEDs for clipping, in contrast to my 3 poles and asymmetrical silicon-diode clipping. He added an inverting unity-gain stage to bring everything into phase prior to blending. Like the PS-021, I also use a single phase-shift stage, but where the PS-021 shifts phase for a smaller part of the spectrum, and does so ahead of the highpass and boost, I do it
after everything (before the mix control), and set max phase-shift for 160hz and up. No two ways about it, the Woody can be hissy, and could probably benefit from a built-in noise-gate. BUt the basic principle of blending in addentuated high-end with a straight signal, can be a desirable thing.
