Aside from the EDN article quality, I find the filter very interesting, thank you for bringing it!

At a first look it reminded me of the Baxandall tone correction

http://www.tubebooks.org/file_downloads/Baxandall.pdf yet it is different. Tried to find the Bob Pease article referred by EDN, but couldn't find the corresponding Electronic Journal number: "Sallen R.P., Key E.L. “A Practical Method of Designing RC Active Filters”. IRE Transactions on Circuit Theory, 1955, Vol. 2, № 1 (March), pp. 74–85."

Fooled around with the BPF schematic a little (in LTspice for now, my try was with OP07, for no particular reason).

I was looking for the influence of

- stepping the values of R2 (schematic a) while R3 is kept constant, then

- stepping R3 (in schematic b) with R2 constant, then

- stepping both R2 and R3 (in opposite directions) so to keep the frequency constant and vary the quality factor

The response for Both V(out) and V(sum) nodes was plotted for all 3 (a/b/c) types of resistor(s) stepping. As for the practical build, straight to the wish list again.

Would like to use digital potentiometer(s) for R2 and/or R3, and try some audio effects for the fun of it, maybe adding more filters in parallel to make some sort of equalizer. Or, maybe make it full analog, adding a few filters in parallel using a volume potentiometer for R2 and R3, to make some sort of graphic equalizer.

What I would like to try the most, would be to use this BPF filter as a sinusoidal RC oscillator, instead of a Wien osc. Would be easier to adjust the frequency.