I think equalizers are very very powerful effects, altough they shouldn't be overestimated when post production is an option. Usually diy-ing a graphic eq is hindered by the amount of controls needed for a decent amount of bands, being them pots or worse, sliders. So I wanted to look for an alternative, and one which didn't feel like a downgrade, and a parametric or at least sweepable eq seemed the best solution.
Before this, there's a low gain input stage which gives a strong input signal and buffers the input while avoiding clipping at guitar levels throughout the circuit. The charge pump helps with the headroom, providing a +-9V supply so the boost levels can be reached without problems.
Following the bandpass, there's a very standard Baxandall 2 band EQ to take care of low and high frequency and add quite a lot of flexibility to the circuit when combined with the bandpass. The cutoffs seem to work well in my experience but they can easily be switched by switching caps if more controls want to be added. I chose the one-capacitor version for both bass and treble networks because that's the least amount of components and I'm cheap
but also because I like the cutoff being more constant and the loading on U1B seems to be good. C8 limits the gain at ultrasonic frequencies and is not always found altough it's very good practice. 100p is probably generous but that's not an issue for guitar especially.
Here's some real measurements with ARTA as demonstration.
Flatness vs bypassed signal:
No demo of this because it's such a versatile pedal it's hard to do it justice. It's not the end all tool, but works surprisingly well in shaping the sound of clean and distortion alike, and that was the goal. For finer tasks post production can take over for me, even a graphic eq won't be enough.
Finally, if you're wondering about the version 2.5, here's the short story: V1 was a gyrator based three band eq inspired by an ESP project. It works well in theory and has the advantage of being able to add more bands easily. Sadly, I wanted to have one or maybe two sweepable bands, and while gyrators can be sweeped across a decent range, the Q variation is too much, I calculated it to be equal to the frequency ratio.
V2 used another kind of Wien bridge bandpass, a simple one that required a single op amp, and the input stage was a darlington buffer to keep the op amp number even. For V2.5 I also considered even another version of Wien bridge, one that required two op amps but managed to keep Q roughly constant with a single gang pot. After finding available 100KC dual pots I went with this one with a very constant Q and the switchable Q, altough the single gang is charming.
Here's a not pretty but very compact vero for it:
