SEM-style controls

[Haible_Juergen#Tel2743]

Hi!

Here's a little thing that might be useful for somebody.

As you might know, I am develloping something like
an Obie n-voice at the moment. While I am momentarily
working on the keyboard scanner, I also think about how the
voice modules should look like. I don't know if I will make
a clone of the actual SEM modules, but one thing I have found
was very clever with the SEM: That's these potentiometers that
have a zero position in the middle, but do some very different
things at cw and ccw ends. But how could I do this ??

This is easy if you have a potentiometer that has a 4th connection
in its middle - just ground this one. But it is not as easy if you don't
have these special pots and even more if you want to have
voltage control. (And guess what I want on my voice modules !!)

You might say: just take two seperate controls, but that's really a
problem if you want to build a polyphonic multitimbral synth
with immediate acess to every parameter (like the Obie n-voice was).
I also could use a CPU and some software, to separate the two halves
of the pot's range before it controls the VCAs, but then I could not
use the module without the pot scanning/storage option.
So I looked for a very simple circuit that could control 2 OTAs in
the desired way. I started with a VC crossfader (as in the Prophet5's
modulation source mixer), tried this and that, and finally ended up
with the following very simple circuit with 2 PNPs and 3 resistors:
(I'd call it "biased darlington")


     CV in  o----------+---------------------------------------!
                                ! 
                                                !
                                ! 
                                                !
                               R1           VCC                           !
                                !                ! 
                               !
                                 \    !         R2 
                            !
                                _\!  !          ! 
                               !
                                      !--------+-----------! 
               !
                                   /  !          !                \     ! 
        !
                                 /    !         R3            _ \!  ! 
        !
                                !                ! 
                     !-------+
                                !               GND          /    !
                                V                               /     !
                              Iabc2                         !
                                                                 !
                                                                V
                                                               Iabc1


Asuming your CV has a 0...10V range and VCC is +15V,
R2 should be two times R3.
R1 = 5V/Iabc1max, and R2!!R3 = 5V/Iabc2max.

So with CV at 5V you have both OTAs off. OTA 1 gets on linearly
above 5.5V, OTA 2 below 4.5V, which would be ideal for the
desired purpose.

I hope you found this useful.

JH.