AW: SEM Style knobbies

[Haible_Juergen#Tel2743]

> 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.

Hey, *this* one is easy!
While I was speaking about such knobs on the SEM that do *different*
things in both halves (think of FM and PWM), *you* will only need an 
ordinary
linear potentiometer.
Just connect the filter's LP output to the CCW side, the HP output to CW
side, and the output is at the tap.
With the knob in middle position, you get the Notch response automatically,
just like in the SEM.
With the BP it's a differt thing, however: in the SEM, you get the BP with a 
switch
that is integrated into the potentiometer. If you know these volume pots on
old car stereos (or cheap stuff like old portable radios, TB303s ...), you 
got it.
But take care that you get a *linear* one; most of these pots are 
logarithmic.

JH.