[sdiy] SSM2040 vs CEM3320

[Andrew Simper]

Yeah, I can't find anything on the CEM3320 either. I'm guessing it may be
just a flipped OTA, where you put the input as a current to the long tail
pair, then provide the possibly differential control voltages to the bases
of the pair.

Another option is for a low transistor count biased log -> add gain voltage
-> exp type circuit. This second one would be consistent with primarily 2nd
order harmonics, since the differential Blackmer version gives mainly 3rd
order.

Andy

On Thu, 2 Nov 2023 at 15:12, brianw <brianw at audiobanshee.com> wrote:

> 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
>
>
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