[sdiy] SSM2040 vs CEM3320

Andrew Simper andy at cytomic.com
Thu Nov 2 06:04:13 CET 2023


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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