SSM2040 Secret unveiled !

[Joachim Verghese]

Hi,

Juergen and I have been discussing the SSM2040 filter lately,
its distortion characteristics, and how to modify other filters
to make them behave like the 2040. We now thought that we'd like
to share the information with the others on the DIY list.

-joachim

(forwarded:)

>> Maybe by simply changing these bias resistors we might push
>> a 3320 filter a good deal into the 2040 direction - imagine what
>> this would mean to the sound of the Rev. 3 Prophets!
>
> Yes, unless you agree with Graham Hinton's statements on the subject. :)
> (http://www.hollis.co.uk/hinton/myths.html)

Hey, Graham is right !!!
I have taken another close look into the Rev. 2 schematics, and I was
surprised. I mean, all that I said about the SSM2040's distortion
characteristic is still true (hopefully ....) in standalone applications 
like a modular synth, it is not the main distortion factor in the Rev. 2
Prophets. I really was surprised to see a 200k resistor from the A/B
mixer OTAs (2020's) to the filter input.

With this 200k resistor, the 2040 could handle 40V peak-to-peak
input signals without distortion worth mentioning (With resonance applied
it's a different story, of course.) If fact, this 200k resistor makes the
2020's outputs clipping near the supplies when the gain is too large.

Now, the good news is: *This* should easily be implemented in the Rev. 3
models. Just add a resistor from the the 3280's to the 3320. Should be in 
the 400k range to achieve similar results, but tests have yet to be done.

Let me make some wild guess now: I believe that the P5 designer has put in
the 200k resistor to *prevent* the 2040 from clipping. (After all, the 2040
specifications say "do *not* exceed 1Vpp", but it doesn't say why (;->) ).
Now in the later design (rev.3) they  obviously prevented the 3320 from
overload by limiting the Mixer OTA gain: The amplitude in the filter
is *way* on the save side, far from clipping.
BTW, shifting a 3320 into distortion with an additional bias resistor at the
first stage works fine. I tried this on the OB-8, where I have far better 
access to the boards than with the Prophet.

JH.