I modified the DABP for parametric use.
Here's the simulation.
I added two amps, a range-limited linear pot for positive feedback, and a triple gang rev log pot (25K/10K/10K) to maintain constant maximum Q so the feedback pot can do its job (.40 CCW, 1.2 CTR, 2.7 CW).
More feedback
widens the bandwidth but leaves output the same gain as input. Maximum Q is supposed to be that first pot section divided by either of the next pot sections, but in sim it comes out a little higher. Obviously a stepped 3-deck frequency control of 12- to 24 steps would be better tracking and less obscure than the pot. In low quantities, a 3-deck Grayhill 71 is not that much more costly than a custom 3-gang pot from State Electronics.
It is also possible to use a 4-gang 10K rev log pot with two sections in series (20K/10K/10K), though accuracy will degrade and Q range will be a bit limited. With a bit of a noise tradeoff, a 3-gang 50K/20K/20K would net the same Q range as drawn, with a 20:1 frequency range. This might be a good solution for general purpose tracking. A three-band 500 series EQ might look like 40Hz-800Hz / 400Hz-8kHz / 800Hz-16kHz. Low shelf is possible using this filter as a source, and for high shelf one could add a diff amp on the back end that subtracts the LP output from the original signal.
This filter's main advantage over the State Variable is that it has a lower noise floor
(I think?), particularly when set up for a low Q range. Very low Q SVF sections require a fair amount of attenuation-then-amplification. It can be set up for very high Q very easily, with Johnson noise of the input resistor being the only additional penalty. The main disadvantage (aside from the spendy control method) is that you can't come in to the filter at full level; it requires 6dB headroom. Pretty similar to an SVF in that way, where you have to lose some signal on the way in if you want to avoid overloading the mix amp and LP integrator at sub-1.0 Qs.
I could not figure out a way to use slugged linear pots in this architecture — maybe someone can
point out a method? Alternatively, if there is a way to make a resistor seem larger than it is, in series with a non-inverting input, this would be an ideal application - specifically to be able to get a healthy Q range out of a more-readily-available three-gang 10k pot. I looked at negative resistance but that’s negating a load — different problem. I also contemplated bootstrapping to make the input amp see a larger impedance (say, x5), but i’m a noob at calculating such things.
Any ideas?