[sdiy] SSM2040 vs CEM3320

[Andrew Simper]

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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