SEM-style controls

[gstopp at fibermux.com]

     Does this mean you don't want any ASM-1's??? :-(
     
     The SEM has many clever tricks - hats off to Tom Oberheim! Those 
     center-tapped pots are quite hard to find in the surplus yards. Garage 
     sale stereo tone controls might be a good source. Also the VCO tuning 
     pots are concentric (I think the MemoryMoog has those too, right?) 
     which saves panel space. The gate and trigger inputs on the envelope 
     generators can be hard-wired internally a bunch of different ways to 
     accomodate any gate voltage standard, inverted or not. The multimode 
     filter output mode is selected with a pot with continuous selection 
     between lowpass and highpass (and notch is an equal mix of those two), 
     plus a switch on the pot for bandpass selection.
     
     All in all there's lots of flexibility in not too many controls.
     
     Can we think of a way to simulate the center-tapped pot, without using 
     VCA's? For those of us who want the same function and still keep our 
     3080's for other things....
     
     - Gene
     gstopp at fibermux.com
     
     p.s. the ASCIImatic did not make it to me, it's garbled....


______________________________ Reply Separator _________________________________
Subject: SEM-style controls
Author:  Haible_Juergen#Tel2743 <HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de>
at ccrelayout
Date:    5/31/96 4:19 PM
     
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")