[sdiy] Non-linear properties of SSM2040

[Tom Bugs]

I have little technical to offer, but wondered whether you were aware 
that SSI have the SSI2040 with a pretty detailed datasheet (updated Mar 
'23) - https://www.soundsemiconductor.com/downloads/ssi2140datasheet.pdf

Curious what might be gained from trying to implement it with the 2164 
instead?

Tom

On 08/09/2024 11:34, Rutger Vlek via Synth-diy wrote:
> 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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