[sdiy] SSM2040 vs CEM3320

[brianw]

Very interesting!

It's always nice to see someone digging in to these classic circuits.

I wanted to respond to your first message, but I have no information about the CEM3320. Looks like you found some answers already.

Brian


On Nov 1, 2023, at 11:33 PM, Andrew Simper wrote:
> And a few patents referenced in the articles:
> 
> Embley:
> https://patents.google.com/patent/US3532868A/en
> 
> Buff:
> https://patents.google.com/patent/US4341962A/en
> 
> Blackmer:
> https://patents.google.com/patent/US3714462A/en
> https://patents.google.com/patent/US4403199A/en
> 
> Andy
> 
> On Thu, 2 Nov 2023 at 13:04, Andrew Simper 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 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 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