Try the SWTC (Stupidly Wonderful Tone Control) or one of its variants — basically it'll help clean up the high fizz generated...
If true, It would do it regardless of your understanding. -Bill's Law.If I understand It correctly. I believe the guitar signal is triggering the transistor to complete the circuit to the LEDs
Where did you study it, if you don't mind me asking? It's such a cool circuit, and I'm glad they've documented the history of it as it seems to have influenced a lot of other designs.Wow! Cool. I remember studying that filter years ago.
Transistor diff amps are easily overdriven. Anything over 60mV at the input to your filter will start distorting. Bottom line: you have way too much gain in the opamp input stage. Try increasing R9. Or remove R9 & C10, making the input stage a unity gain follower. There are also tricks you can use to "pre-distort" the signal going in. The LM13700 has that option built-in. In fact, you can mimic the freq response of this circuit with cascaded LM13700s. The advantage with the LM13700 is the transistors inside are matched.
Yeah I've seen those arrays in quite a few transistor ladder schematic, I'd give them a go if I could find a reasonably priced equivalent. I'm leaning more towards the BCM847 NPN matched pairs as they're cheap (~$0.50ea) & easy to drop in to other differential pairs like VCAs & oscillators. Most of the stuff I've read on ladder filter circuits points to the top & bottom pairs as being the most critical to match closely, but I haven't seen much on how matching between pairs effects the sound.The advantage of a transistor array is they are all matched and run at the same temperature.
The way they're implementing the DRIVE control makes good sense. Is that Audio IN jack intended for guitar level signals or line level signals?