Yup, there it is.
State-variable biquad, using a similar single-gang 'Q' solution to the one which Graham Langley favoured.
Center-tapped cut/boost pots.
U2a sees about an 800Ω load, since it's looking at 6x5kΩ in parallel (R13 and R2 both go to vitrual grounds, and the clockwise halves of the four cut/boost controls all show 5kΩ each to their grounded center-taps. It might make U2 work a little hard at high levels, but it's fine for lower level stuff. Personally I'd have thought of generating one or two more inverted signal feeds to share the load of driving the total number of cut/boost pots just to make sure that the op-amps don't get asked to do hard labour when the signal gets big...
I have a couple of thoughts about this design; it's essentially partly what I had in mind when I suggested the "Ne-Mek" EQ a while ago. Not that I thought it was particularly original on my part, but that this design really IS very DIY parts-sourcing friendly. The "Neve" part of the Nemek was the grounded-end linear frequency pots part, as used in the Neve V-series. The "Amek" part of Nemek was from Graham Langley's approach to controlling a single shunt leg of a feedback loop *AND* have the same resistor be the shunt leg of the input signal attenuation. That way, as the gain of the filter rises with 'Q', the input signal attenuates in a precisely-matched manner (down to the accuracy of the series leg resistors, which is bound to be a LOT better than inter-gang pot tracking!)
This particular design uses a much quieter way of avoiding the noise from unused bands. The Neve and Amek designs both permanently sum the outputs of the various filters in with the signal. The problem with this is that any bands that are being neither boosted or cut still contribute regenerated noise to the signal. As an additional curse, the approach of grounding one end of a linear frequency pot tends to be noisier than using reverse log pots, particularly at the high-frequency end of the range. With both Neve and Amek designs both, you can hear a faint filtered-noise sweep as you move the frequency controls of the neither-cut-nor-boosted bands, particularly in the high mid area. -This may not be too bad on a single channel, but on a console when you have several dozen EQs switched in, many of them left in the exact same "normalled" position for unused bands, the noise can become an issue, believe it or not. I've certainly heard things get quieter with all the EQs switched in & out with no boost or cut applied before now. This design does things rather better; when a band is at the center position, its output is attenuated to ground and the filter contributes nothing to the signal.
-However, unless I'm missing something, with all pots in full boost, looks like the signal might also see 4 x 2k2s in parallel, which would under an extreme circumstance bring the total load impedance to U2 well below 400Ω, unless I'm misreading it? (mind you, if you really do have all 4 bands at full boost, you're probably long-since abandoned all worship for sonic fidelity! :wink: )
Keith