[sdiy] Non-linear properties of SSM2040

[Rutger Vlek]

By the way, forgot to mention that I dug through our list's history and it
seems Jurgen Haible observed something similar, questioning if the SSM2040
is symmetric after all....
https://synth-diy.org/pipermail/synth-diy/1996-April/035864.html

Also, the clipping effect that I see at higher levels seems much different
between the implementation of Jurgen Haible and that of Rene Schmitz:
https://www.schmitzbits.de/rs2040.html. If it is crucial for the character
of the SSM2040, these two implementations should sound very differently,
unless I'm mistaken?

Finally, I'm hoping that Andrew Simper will chime in, as I love his digital
implementation of the SSM2040! Of course I'm also understanding in case you
cannot share too many details, Andrew :).

Regards,

Rutger

Op zo 8 sep 2024 om 12:34 schreef Rutger Vlek <rutgervlek at gmail.com>:

> Dear all,
>
> In the sparse moments between day time job and fatherhood I'm trying to
> push forward with an idea I've had for a long time. I'm trying to capture
> the variation in favourable non-linear characteristics from well-known
> filters (read SSM2040 and Moog Ladder) and implement them in a more modern
> topology (read SSI2164). Would also be great to be able to select between
> them, while using the same filter core. I'm not necessarily aiming for
> perfectly cloning the response of vintage filters, but rather hope to take
> inspiration from them and perhaps to discover other pleasant
> non-linearities.
>
> I've been approaching this with Spice as well as with mathematical
> modelling in Python, using a multi-dimensional Newton-Raphson solver with
> the system of equations needed to describe the various filters.
>
> Right now I'm trying to understand the character of the SSM2040, and am a
> bit puzzled. I am familiar with the inverting cascaded topology of this
> filter. And with the typical math that describes the non-linearities of an
> OTA-based filter:
> Vout = g * tanh(Vin-Vout).
>
> I have been reading across the internet about the asymmetrical saturation
> of this filter, most notably in the application notes for making filters
> with the SSI2164: https://www.soundsemiconductor.com/downloads/AN701.pdf.
> Based on this information, I had assumed that the tanh only operates in one
> direction, something like this:
> y = tanh(x)    if x < 0
> y =x               if x > 0
>
> However, in Spice, when simulating the internals of the SSM2040 using
> Jurgen Haible's schematic (
> http://jhaible.com/legacy/tonline_stuff/jh2040.gif), I don't see that
> happening. Instead, I see tanh distortion in both directions.
>
> Only when I push the input harder (beyond 1Vpp), I see one side of the
> output clipping much sooner than the other. This seems in line with the
> effect described in AN701, but is the SSM2040 really driven that hard in
> real world applications, and is it really *this* additional effect, on top
> of the already present tanh distortion, that explains the SSM2040's
> character? And if so... how would one model it mathematically?
>
> Rutger
>
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