How hard would it be to hack a state variable filter wich is pretty diy open to have a summed post crossfader output instead or additionaly to lp,bp,hp outs? Like a blend between these filter modes.
Easiest way would be to feed the outputs to a vc mixer. Try it first with a non-vc mixer to see if it is actually interesting.
Sure a passive 3-1 attenuator would also work but would add a blend/crossfading knob
I think the Nonlinear Circuits Noiro-ze has something like the cross-fading you’re describing, though how it accomplishes it is a bit mysterious to me just glancing at the schematic.
It’s kind of the opposite topology of what you’re taking about though, since is has individual LP, BP, and HP inputs and 1 output
Cool concept. From the single demo it doesnt really sound very nice to me i suppose.
Was thinking of hacking a cinnamon. But maybe a wet/dry or simply 3-1 attenuator does the thing i think of. But the idea was more to travel through filter behaviour with 1 knob
There’s a somewhat easier to listen to demo of how the filter sounds here:
I haven’t tried it myself, but it’s on the list for future diy builds for me for sure.
The only way I can think of to accomplish the one knob thing off the top of my head otherwise is a Frames, which is a bit of an expensive cross fader.
Maybe a clever Cold Mac patch could work too
crossfader between 2 inputs is very simple, single opamp IC will do it
here’s an example
3 inputs is more complicated. i think it would be best to treat it as 3x VCAs, run voltage through potentiometer, then create 3x deadzones/ramps from that voltage. this means more opamps than can fit in a single package and is no longer suitable a simple point-to-point hack. (and in any case, that set of functionality seems more useful as its own module.)
so yeah, i’d consider other schemes. ask yourself whether it’s really always useful to go LP <-> BP <-> HP (since you would be hardwiring this relationship in a rather complex circuit.) maybe LP->HP is already useful, with BP blended as a separate level. or maybe see it as two layers of xfade: (LP <-> HP) <-> BP.
Worth noting the Oberheim SEM filter, which has a blend between LP and HP where Notch mode is in between.
Also the Fritz Threeler, though it’s switched and not crossfaded, but has a wealth of combinations of single stages producing multiple outputs and modes.
Phase relationships between the outputs will be relevant in what you end up getting, worth experimentation with an external module before hacking on a PCB
This design from Craog Anderton might be of interest… http://www.synthdiy.com/files/2002/MIF.pdf.