Equalizers are one of the most powerful tools to change the timbre of an instrument, and this is true also for their pedal incarnations. This almost always implies a graphical eq, not really a diy-friendly solution with the amount of controls needed, be it sliders or rotary pots, and not necessarily the best if chasing a specific frequency or Q. Also, pedal equalizers shouldn't be overestimated when today post-production tools are accessible to anyone: this is just a matter of revising the role of an equalizer when in a compact, player-accessible form: it's best used for either pre-emphasis before distortion or as broad-stroke correction of the general sound before it hits the speakers.
I went looking for an alternative then, one which didn't feel like a downgrade, and a parametric or at least sweepable EQ seemed the best solution.
Parametric EQs aren't new in the guitar world, the Boss SP-1 being an example, and many others through the years. My approach to the bandpass is the Wien bridge eq, in particular a switched Q version through the means of positive feedback very much like in the JHS haunting mids, but which I want to point out I've taken from the book "Small Signal Audio Design" 3rd edition by Douglas Self directly (values and everything point to this being the primary source). Unlike JHS, I haven't just copied the circuit as is, but I've tweaked the values to get the gain, Q and frequency ranges I liked, while using only E6 values because that's how I like it. To do this the Q switch shorts a series resistor instead of adding a parallel one. for example. Shown is the full sweep in both modes at maximum boost:
The gain is +-16dB, the frequency sweep is about 300 to 3.3K, the Q is 2.4 in HI and 1 in LO. The beauty of this circuit is that the Q stays constant across the whole sweep and so does the gain, for both boost and cut.
Before this, there's a low gain input stage which gives a strong input signal, so that the output is loud enough even after cutting, 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.
If the gain provided by the input stage is removed, for example by making it a buffer, and why not reduce the input impedance while you're at it, the circuit will work well also for line-level signals, up to 2.3V peak to peak at that frequency at maximum boost, provided you don't do something silly like boosting the same band with both the mid band and the Baxandall.
Following the band-pass, 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 band-pass. 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 although it's very good practice. 100p is probably generous but it won't affect audible frequencies anyway.
Knowing that EQs tend to struggle with noise, either because the amount of gain they have, especially at high frequencies, or because of the circuit itself, I prioritized noise over headroom, and this is reflected in the choice of a fixed gain input stage and a volume control at the end, which isn't that different from what most EQ pedals do. Headroom is still very good, I've measured input headroom throughout in "worst case" scenarios with max boost, and you have to try hard to make this clip, with an hot input signal, maximum amounts of boost, and maybe overlapping the mid band with the Baxandall. Personally, I didn't have any issues, and it needs to be said that some EQ pedals get away with a single 9v supply and similar amounts of boost, somehow. For line level signals an input pad might work well, and maybe 12V input (check your charge pump specs). The impedances are low throughout to minimize noise, while still avoiding overloading the op amps. For the input stage TL072 is recommended because it's quieter with guitar inputs, while the band EQ uses 5532 which is quieter overall. I think both noise and headroom have been handled well and I don't have issues.
Here's some real measurements with ARTA as demonstration.
I didn't include frequency sweep pictures because at this time I was testing with a 50k pot which results in a narrower range, but I can confirm that it's 300 to 3.3kHz as expected.
This is such a versatile pedal that it's hard to show in a demo everything it can be used for, so I just encourage you to try it for yourself. It's not the end-all tool, but it 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 why the version is 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 from the Self book, 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, although the single gang version remains charming.
Here's a very compact vero layout for this:
And a picture to show off my build:
Hope you enjoyed it and let me know if you try it!