/diy/ - Do It Yourself

I wanna make a voltage controlled filter where I can control the cutoff frequency with some DC, ramp, or other periodic control voltage. Be nice to control Q too but not absolutely necessary, cutoff is more important.

I'm using a Sallen-key low pass filter. I figure varying the frequency will involve varying the resistors, capacitors, or both. Now I can do resistance with FETs in their Ohmic region or with digital pots but FETs in their Ohmic region are only truly linear over a vary narrow range as the curve tapers off as it's coming up to the linear region so that's a reject and digital pots would be great except I have none on hand and I want to breadboard this thing now. For varying capacitence there are varactor diodes but their capacitence is too small for low frequency filters necessitating very large resistors plus again I have none on hand and I want to build this now plus any I've ever seen for sale are all surface mount and I can't breadboard that.

I'm figuring my other options are current sources either pumping current into or out of some node in the circuit or the feedback path. Like if I have an RC filter block that when a signal is applied I develop a voltage across it then if I have a current source pulling some current away from that output it will change the voltage and the filter properties I think? I'm not really sure. Anyway a VCCS shouldn't be hard to build so it seems like the way to go but I tried a few current source/sink configurations with the filter and I can't really get it to do what I want.

Is it possible to make an active 2-way loudspeaker crossover controllable with CMOS logic? I'd like to have some control over the crossover frequency. Can I somehow use a CD4017 counter to connect one of several different resistance values to the filters?