[sdiy] An Improved Sine Shaper Circuit

René Schmitz synth at schmitzbits.de
Fri Apr 17 22:25:07 CEST 2020

Hi Don,

On 17.04.2020 17:53, Donald Tillman wrote:

> I am very familiar with cusp cancellation.  I've used it also.  And it's mentioned in the article.
> This is not cusp cancellation.  While the circuit looks the same, I'm subtracting 1 or 2 orders of magnitude more of the original triangle signal than is necessary to cancel the triangle cusps.

Maybe it would be an addition to do a side by side comparison of your 
circuit with the older method. To emphasize the differences.

> I discovered this using some machine learning tools (Jupyter Notebook, Numpy) and some of my own software to optimize a classic sine shaper circuit... one with emitter resistors and with cusp cancellation.   And it kept pointing me to subtract more and more of the triangle signal.  I thought something had gone wrong, but what the heck, follow the data.  And sure enough, the harmonic spectrum really did get better as the transistor's tanh curve performed a different function.
> Then I realized I was no longer correcting for a little nipple that got through the sine shaper, I had discovered a new way to approximate the sine function.   And that it was crazy accurate.  I couldn't find a reference to this zig-zagging "tanh(x) - beta x" curve mentioned anywhere before.

When I look at the Aries 317 or the Thomas Henry circuit I see that the 
triangle is not just a touch but very substantial in the output. (The TH 
circuit even returns almost all of the OTAs' output current into the 
resistor that connects to the triangle. (Its a high Z node, so if the 
OTA was shut off, it would be just fed through the triangle 100%)
Clearly some form of zig-zagging is happening there too. Whether these 
circuits operate at an/your optimum is another matter.


synth at schmitzbits.de

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