SEM Style knobbies

[Christopher List]

Hi List - 

A week ago "SEM style inputs" were discussed, and it was mentioned that the SEM 
filters have a knob to x-fade from HP through notch to lowpass.

I do this kind of thing now all time using a dedicated x-fader module and 
realize what a great idea it is to have it as part of the filter itself. 

I don't want to be able to voltage control this x-fade, so I don't want to do 
it with VCA's. It seems like it would be easy to do this x-fade with a 
dual-ganged pot, but I was wondering if there's an easy "trick" circuit for 
doing this using only a normal pot and op-amps. I'm not looking for "true" 
exponential x-fade, BTW.

- Thanks,
Chris