[sdiy] SSM2040 vs CEM3320

[Andrew Simper]

Yeah, scrap that. Reading the datasheet it says the gain cell is a current
in current out design with predominantly 2nd order distortion. Ok, time to
look at some basic current in current out designs with the fewest
transistors!

Cheers,

Andy



On Thu, 2 Nov 2023 at 11:12, Andrew Simper <andy at cytomic.com> wrote:

> Does anyone know if the OTA cores in the CEM3320 are very similar to those
> in the SSM2040?
>
> Thanks to JH and his “all discrete” filter schematic the internals of the
> SSM2040 are shown. I’ve recently done some analysis and modelling of the
> SSM2040 / SSI2040 based off the JH discrete schematic, and can see the
> major difference between using it and a regular CA3080 / LM13700 OTA is
> just the extra two negative rail current mirrors, so in the 2040 the
> negative input transistor attached to the capacitor clips the signal in an
> asymmetric way depending on the dc offset introduced by the buffer of that
> stage that is fed back to it.
>
> Looking at some SSM3320 designs I see the filters are careful to manually
> introduce a lot of negative dc offset by pulling the input to stages 2, 3,
> 4 down towards the negative rail, which would be consistent with wanting to
> manipulate the asymmetric clipping point of the OTA core.
>
> For anyone interested I did a video of my model being matched to the LA67
> Maca filter which is based off the SSI2040 datasheet Low pass, using a moog
> matriarch attenuverter knob hooked up to the filter cutoff of both the Maca
> and digitised via an ES8 to bring that CV into my model:
>
> https://youtu.be/wVYYDvAXEjY
>
> Cheers,
>
> Andy
>
>
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