[sdiy] Non-linear properties of SSM2040

[René Schmitz]

On 08.09.2024 17:01, Rutger Vlek wrote:
> @Richie Burnett <mailto:rburnett at richieburnett.co.uk> thanks! I have 
> found Spice to be cumbersome for more advanced data analysis, so 
> resorted to Python for most of it. There I am studying non linearities 
> using saw, ramp (inverted saw) and sine test signals. The latter I use 
> mostly for spectral analysis, while the first are also informative 
> about time domain effects. I also created the option to put out wat 
> files so that I can audition them. Some effects that were impressive 
> in the time domain where irrelevant when listening, and (more 
> importantly!) vice versa. I had to make a bad prototype to remind me 
> of this... What resembled the time domain of a Moog filter did not 
> sound like one. As soon as I studied the spectral domain data I 
> understood why.
>
> The spectral effects on a sawtooth pushed through a non linear filter, 
> without any resonance are impressive by themselves by the way. I never 
> knew the effect of an analogue filter could be this substantial.
>
> @René Schmitz <mailto:synth at schmitzbits.de> Thanks for the 
> explanation! It confirms what I already suspected. The non linear 
> effects are a combination of TanH and something else. I have the 
> feeling your answer also implies that the latter effect is important 
> for the character of the ssm2040.


The tanh works on the input voltage to current translation, and the 
clipping is on the cap side, so that's after the current was integrated 
back to a voltage .

Hence it's not something you can lump together into one non linearity. 
You only get so far if you look at it statically.

How fast the cap is charged to -0.6V depends of course on the control 
current, the size of the cap, and how long it is applied. Plus the 
initial conditions.

The input attenuation at the bases of the long tail pair together with 
the input amplitude determine to what degree the tanh is actually driven.

If it's below 25mV you don't have a significant contribution from it, 
pretty linear.

(With the 10k/200 divider common to JHs and mine the 2040 cells are 
driven quite beefy, for +/-2.5V input, resulting in +/-50mV after the 
attenuation.)

And there is also a feedback loop that works to get things back into 
balance at the bases, i.e. to reduce deltaVbe.

Then we could ask what effect does the darlington emitter follower with 
it's two Vbes have?

Spoiler alert, it shifts the whole waveform voltage at the cap upwards, 
away from the clipping.

Theoretically you could add more Vbe drops and steer it clear of the 
clipping zone (but who wants that...).


There surely is a lot going on in that simple circuit.


Best,
  René



--
synth at schmitzbits.de
http://schmitzbits.de
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