[sdiy] tri-sin shaper and THD

[Ian Fritz]

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.

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%).

The circuit and its spectrum are now up at my site:

http://home.earthlink.net/~ijfritz/xfer.htm

These probably won't be up for long, so please grab the schematic now if it 
is of interest!

   Ian