[sdiy] LFO with variable waveshaping

[Tony Allgood]

> http://www.till.com/articles/VariableSaw/index.html

I tried this, and even gave it its name as the 'tillman modulator'. I
was very impressed with the design topology but the implementation I
chose had some shortcomings. The list archives will tell you more. I
used the LM13600 as my OTA... bad choice really, the offsets will add
unwanted discontiuites at the waveform edges. Also with any system like
this its very diffcult to get rise/fall ratios better than 10%. Pretty
good for most people though.

My circuit needed more work but I abandoned it when I found out that
Paul had the MOTM-320 coming out. Paul, actually I think its a Juergen
Haible design, does his a different way and is probably similar to the
EMS/Transcendant 2000 waveshaper methodology. This creates a near
perfect triangle at the centre and saws at the ends, but allows for
discontinuities at every other point. But it is a good effect and sounds
good at audio too.

I also tried using a pair OTAs in an integrator/schmitt VCO to create
imbalances in the triangle charge/discharge times. Although, some
success was had with this method, the frequency stability was poor.

Come to think of it, some sucess may be had by feeding a CV and the
inverse CV into a MOTM-820 with a OMS-820 companion too. I'll try this
sometime.

If you are looking for a non VC option then the Korg MS-20 is not bad,
and my own Little-LFO improves on this further using a DG403 analogue
switch.

All the best,

Tony

www.oakleysound.com/projects.htm