Mixing LP and BP from a state variable filter for wah

[Ronan]

I built a breadboarded circuit and liked it, and simulated a similar circuit with LTspice to try to understand it. I'm new to spice, so I hope the results are correct. I put a 10K mix pot on the output to mix between low pass and band pass, and also added a resonance pot. Seems to provide a lot of shaping of the output signal. Much the same, I guess, as Craig Anderton's super tone control. The red line is the mix of LP and BP, the pot is formed by R8 and R9, feeding into R10 as the load. Blue is low pass, green is band pass (edit).





I guess the reason for the post is, besides general interest, that any circuit using a state variable filter could be modified to mix the LP and BP outputs with just a pot, this includes some auto-wahs, envelope filters etc (Mutron). Often these circuits are fixed LP or BP, or at best, switchable between the two.

[Earthscum]

Actually, thanks a bunch for posting this. I thought about trying this some time ago, but put it to the back of my mind. I've been trying to decide on a wah circuit for a project, and have been tossing around parallel circuits, instead. I was looking at a 24dB LP paralleled with a typical BP. Of course, this led to consideration of phase stages, and... you know, over complication of a simpler idea, lol.

I can tack on an additional LP filter to the end and get that little extra cut I'm after and this actually fits the bill perfectly, with a bit more flexibility I think. On the bread board, did you happen to try buffering the normal outputs before mixing? If so, did it change the interaction between the stages? I gotta give this a go tonight. Thanks!

[Ronan]

On the bread board, did you happen to try buffering the normal outputs before mixing? If so, did it change the interaction between the stages?

I think the output impedance of each stage is low, so I don't think buffers are needed. In the sim circuit, a heavy load (20k) lowered the output voltage, but the shape remained the same. I hooked it up with a treadle today and yes, the mix pot is quite effective.