geofex para eq question etc

[Eddododo]

in the schematic at http://www.geofex.com/article_folders/eqs/paramet.htm for the parametric eq circuit;
is there a reason the gyrator's effective inductance could not be varied with the 470ohm resistor, rather than using the series resistance to change the L/freq, as well as affecting the Q?

Has anyone had success with a similar application of a different gyrator topology?

Thanks, as always

Ed

[PRR]

The 10K series pot does not affect L.

(If you are looking at gyrator formulas, you want the 470 ohm and the 51K+1Meg resistor.)

[Eddododo]

I actually was referring to the 1M 'freq' pot.
After looking at it again, i realized my mistake i think; I was assuming the 1M pot and the 51k would affect the Q, thinking that they were the contributors to the apparent series resistance of the inductor itself (as opposed to the 'external' resistance of the 10k 'resonance' pot) . I now realize, or i think i do, that the 470ohm feedback resistor is the 'apparent' series resistance, which would actually affect freq and Q simultaneously if varied. I had the relationship backwards, so it appears that the Q would stay pretty constant as the 1M freq is swept.. correct?

[MrStab]

by the sounds of it you have a better grasp over all this than i do, but i found this link to be very useful in working out the (ideal/theoretical) effects of each component in the gyrator: http://www.muzique.com/lab/gyrator.htm

slightly off-topic, but a varying Q seems to be a problem on this relatively-simple approach to a parametric EQ. it's far from scientific, but i ran a white noise generator through my own Geofex EQ attempt and then into a USB interface a while ago, and here's an illustration of the Q thing: