[sdiy] Variable slope with a SSM2164

[Aaron Lanterman]

One of my students (Sid) is trying to built a variable-slope filter for 
his project. I read up on some variable-slope ideas in electronotes, and 
considered his project worthy enough to sacrifice one of my SSM2164s to 
the cause so he could avoid having to build four separate converters.

He started with Osamu's design as a base:

http://userdisk.webry.biglobe.ne.jp/000/024/65/1/VcfSSM0602a.GIF

He got it working quite nicely. The input turns 1 V into 0.2 V, since the 
circuit, as set up, seems to work on a 0.2 V/octave sort of thing at the 
control pins of the 2164 (assuming we calculated things right in the lab 
the other day).

Two questions:

1) For a variable slope, based on my electronote recollections, I 
suggested he drive the first stage as shown; but then for each successive 
stage, add some multiple of a "slope" control voltage, like this:

mainCV + multiple1 * slopeCV
mainCV + multiple2 * slopeCV
mainCV + multiple3 * slopeCV

This is simply implemented with a few summing nodes and op amp buffers.

Realizing that, at that stage, things are 0.2 V per octave. Thinking that 
a 0-5 voltage would be a good slopeCV range... any thoughts on what 
multiple1, multiple2, and multiple3 should be to get the poles spreading 
out in an interesting way as the slopeCV goes up?

2) There's a note that "clipping is needed for smooth sine wave 
oscillation." No kidding! If he cranks the resonance past a certain point, 
the scope goes insane. In the audio range, it makes everyone in the lab 
rapidly pissed, and it will also seem to oscillate in the above-audio 
range - you can't hear it, but you can see it on the scope.

So, I suggested he try Rene Schmitz's MS-20 clipping circuit, with the 
LEDs. I haven't heard what it sounds like myself, but my student said he 
didn't like the way it sounded. (He hasn't spent a lot of time with a real 
MS-20 though, but here I was hoping to go for something more smooth than 
an MS-20 anyway).

Any other ideas re: the clipping? Maybe try something more like in the 
original MS-20, with the three diodes?

As before, I'm going to post all the student project reports, so everyone 
can benefit from our efforts and the combined feedback.

- Aaron

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Dr. Aaron Lanterman, Asst. Prof.
and Demetrius T. Paris Junior Prof.    Voice:  404-385-2548
College of Electrical and Comp. Eng.   Fax:    404-894-8363
Georgia Institute of Technology        E-mail: lanterma at ece.gatech.edu
Mail Code 0250                         Web:    users.ece.gatech.edu/~lanterma
Atlanta, GA 30332                      Office: Centergy 5212