2040/3320 Differences? was Re: also... Re: [sdiy] SSM2044 Filter schematic??

[JH.]

As long as you compare the linear part (transfer function) of a 3320 and a
2040,
they are very similar. (And not much different from a ladder filter either,
btw.)

It's the nonlinear behaviour that makes a difference, where the "filter"
isn't
just attenuating and amplifying harmonics of the input signal, but is adding
harmonics of its own, and consequently filters theses harmonics with the
following filter stages. Add to that a special dynamic behaviour that slowly
moves the bias point (and thus the type and amount of distortion) as
a reaction to large input transients, not unlike a tube stage with "soft"
power
supply.

This all comes from an interaction of gm cells with limited output
voltage swing, and the darlington buffers. I'm pretty sure that part of this
was a "happy accident", as Dave Rossum certainly was aiming for a
"high fidelity" 4-pole filter (just as he did with the 2044, and as Curtis
did with the 3320). It was a certain economy on transistor count that
led to the 2040 structure (two current mirrors less per gm cell,
compared to a 3080-based filter), and which created a true classic
in filter design.

I think that's the main thing that makes the 2040 unique. (This being my
favorite VCF of all time, I have made tons of Spice simulations of the
large signal behaviour, including the "tube like" transient response.)
Also, the transistors used in the 2040 are very low noise, compared
to, say, a CA3080. This helps to give the desired powerfull sound,
but it's not enough to explain it, as the 3320 is very quiet as well,
so we're back to the nonlinear stuff.

As an interesting side note, the MOTM-440 (my discrete version
of a 2040-based filter) is still using SSM transistors: AFAIK, SSM
was bought by PMI, and PMI was bought by Analog Devices. The
transistors used in the MOTM-440 are still labelled "SSM".

JH.



----- Original Message ----- 
From: "Scott Stites" <scottnoanh at peoplepc.com>
To: "Synth-Diy at Dropmix. Xs4all. Nl" <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, May 22, 2004 9:50 PM
Subject: 2040/3320 Differences? was Re: also... Re: [sdiy] SSM2044 Filter
schematic??


> I'm confused now - what exactly was the reason the production was ceased
on
> the 2040?  As I understand it, there was a charge of infringement or
> something like that with the 2040 design.  I had thought that it was
because
> of similiarities of the 2040 to transistor ladder filters, but now I know
> better - the 2040 is four VC gain cells, like the CEM3320.  Was it the
> similiarity to the CEM3320 that contributed to the 2040's downfall?
>
> Also, from what I gather, the 2040 was one sweet chip - I have never read
> any disparaging remarks about it, though it seems rather common for the
> 3320 - for example, in remarks about differences between rev's 2 and 3 of
> the Prophet 5.  What differences were there between the two chips that
cause
> such a disparity of perception?  I'm quite curious about that.
>
> BTW, I've made a module out of Rene Schmitz's 2040 clone, which is based
on
> JH's 2040 clone, and it is definitely very, very sweet.  If the original
> 2040 was like that, I can see why it is such a coveted little chip.
>
> Cheers,
> Scott
>
>
> > The similarity between CEM 3320 and SSM 2040 is striking. The SSM 2044
is
> a
> > totally different breed altogether.
>
>