[sdiy] tri-sin shaper and THD

[ryan williams]

hi Ian,

Ian Fritz wrote:
> Hello Ryan and all --
> 
> Several months ago I built a Sin shaper for the aux VCO in my chaos 
> box.  It incorporated the suggestion by JH to add negative feedback to 
> suppress the points at the peaks of the usual OTA shaper design.  I 
> adjusted both the amount of input drive and feedback empirically to 
> obtain the best looking and sounding waveform.

your results look pretty good. Thanks for posting that. I wonder if you 
tried using more trimmers (feeback and offset)? Or was this just not needed?

> After Ryan's nice analysis of this method, I had another look at my 
> circuit, measuring the direct and feedback inputs to the OTA.  I found 
> that these inputs were almost exactly the same as Ryan found in his 
> analysis: 0.26 V from the direct input and 0.26 * 0.47 = 0.12 V from the 
> feedback.  Note that this is fairly heavy feedback (~ 32%).

wow, that's really close to what I came up with. Closer than I would 
have expected.

-ryan williams