I think that part of the distortion may stem from envelope ripple produced by the added low-frequency input into the envelope extractor portion. While it is true that you want it to be sensitive to low notes, it is also true that beats and things "live" in the low end, and these can add a ripple or "burr" to the sweep that many people describe as "distortion".
The advice to double the caps use to set the filter range is normally sound, but in this case you may not need to do so. Try .015 and see if that helps a bit.
The other thing is that the filter used is actually a state-variable filter. This means that you can get a bandpass function if you tap in one place (pin 8 ), but you can also get a lowpass function if you tap the signal at pin 9 instead. I've done it and it works. This will ket you keep the top end but always keep the bottom, since you are only sweeping the upper end of the filter, not the upper and lower as you would with a bandpass.
Just a word of warning, though. The lowpass output is noticeably louder than bandpass, and also noticeably louder than bypass. So, I would suggest omitting R22, and feed R21 to a 100k log or linear pot to adjust output level so that you get the level you want when you kick the effect in.
So, to recap (pardon the unintended pun):
- reduce C9/C10 down to .015,
- change C4 to something like .0022 or .0033, but not much higher,
- connect C11 to pin 9 instead of pin 8 (this means cutting a trace)
- replace R22 with a pot instead of a fixed resistor
If you wish, you can install a toggle to select between bandpass and lowpass. If you do, you'll be glad you installed the output level pot. I didn't and I wish I had.
