Puzzle: one ON-OFF-ON switch and some caps

[quote author="bcarso"][quote author="JohnRoberts"]
One helpful approach when using them in something like a state variable is to configure them so their end points of adjustment are independent of bulk resistance. So instead of a 10K pot with a 1k end limit resistor to deliver a 10:1 adjustment range use no end limit resistor so wiper goes all the way to ground and parallel the wiper resistor with something like a 9X R from the top of the pot. Now the endpoints are the 5% resistor tolerance or better not 20%.

JR[/quote]

I can't quite visualize that. Are you talking about a Kerwin-Huelsman SVF with two inverting integrators?

For that topology I used this arrangement: have a fixed series R, call it R1, to the inverting opamp input with feedback C. Hang the pot from the inverting amp input to common/noninverting opamp input. Have resistor R2 from the pot wiper to the driven end of R1.

Now with pot at max ccw you have exactly R1 as your integrator R. With pot at max cw you have the parallel value of R1 and R2.

Slight disadvantages: the noise gain is higher a bit because the pot end-to-end R is hanging there all the time. Also your previous stage is driving a heavier load at the low frequency endpoint than it would with a simple series R plus variable R.[/quote]

Sorry don't recognise those names...

If I follow your approach it may give an interesting taper or law, but you are using the pot element resistance in the path so it's 20% tolerance can bite you for accuracy. I prefer to use the pot's as potentiometers so you are dealing more with ratios.

Pot goes from preceding output to ground. resistor from wiper, and resistor from top of pot to integrator - input.

When wiper is to ground wiper resistor is out of circuit and LF is defined by resistor in parallel.

Sorry if not clear...

JR

I see now what you meant.

Although there will always be some influence, I see that as you make the potentiometer's nominal end-to-end (or "bulk") R smaller you do achieve more of a potentiometric effect, with the ultimate limitation based on the loading of the driving stage output.

However, on reflection, the two approaches are equivalent, in terms of potentiometry, with the cost function in one case the loading, in the other the noise gain. In mine the stiffer the pot (lower bulk R) the more potentiometric, acting as a current divider between the opamp summing node and actual common. But the noise gain gets high.

I guess in your case you could insert a buffer off the wiper, if the additional phase shifts were tolerable. I think I like your approach more :grin: .

The Kerwin-Huelsman topology, which for historical reasons is the way Bruce Hofer of AP et al. refer to it, is just the pretty standard two ideal integrators and a summing amp, with the three simultaneous LP, BP, and HP outputs. EDIT: There are others with positive-gain ("Deboo") integrators that have only two outputs and two opamps.

[quote author="bcarso"]

The Kerwin-Huelsman topology, which for historical reasons is the way Bruce Hofer of AP et al. refer to it, is just the pretty standard two ideal integrators and a summing amp, with the three simultaneous LP, BP, and HP outputs. EDIT: There are others with positive-gain ("Deboo") integrators that have only two outputs and two opamps.[/quote]

I'd always heard that referred to a state-variable but realize that name is for the more general case of cascaded integrators.

I did a 4 pole (actually switchable between 3 pole butterworth, 4 pole L-R ) crossover by cascading more integrator sections... took a little experimentation with gains and feedback connections to get it stable. There's probably equations that would have told me that but I was a "figure it out by doing" kind of designer.

Also used the 2 integrator state variable for sundry parametric EQ implementations over the years.

JR