[sdiy] SSM2040 vs CEM3320

Andrew Simper andy at cytomic.com
Thu Nov 2 07:33:50 CET 2023

And a few patents referenced in the articles:





On Thu, 2 Nov 2023 at 13:04, Andrew Simper <andy at cytomic.com> wrote:

> For anyone interested I found what looks like a great 4 part set of
> articles "VCAs Investigated" by BenDuncan in Studio Sound magazine. Here
> are the links:
> https://www.worldradiohistory.com/Archive-All-Audio/Archive-Studio-Sound/80s/Studio-Sound-1989-06.pdf
> https://www.worldradiohistory.com/Archive-All-Audio/Archive-Studio-Sound/80s/Studio-Sound-1989-07.pdf
> https://www.worldradiohistory.com/Archive-All-Audio/Archive-Studio-Sound/80s/Studio-Sound-1989-08.pdf
> https://www.worldradiohistory.com/Archive-All-Audio/Archive-Studio-Sound/80s/Studio-Sound-1989-09.pdf
> I love that in the first link above is an article entitled "DSP - The
> Future Of Audio Creativity", and giving it a quick read they got it bang on.
> Andy
> On Thu, 2 Nov 2023 at 12:13, Andrew Simper <andy at cytomic.com> wrote:
>> 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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