Re(2): 2nd generation VCFs (was: Xpander VCF)

[tomg]

This actually works! Well the ladder part anyway....;-) I'll get 
to the otas and such later. I'm swamped right now doing other stuff
but I had to stop and try it. Damm! What about that? That was a lot 
easier than I thought it would be. Thanks guys.

Tom




> I just come from reading, 3:43 AM - you're still awake ?
> 
> > > > Right. I would strongly advice you to read the patent behind the SSM
> > > > 2044. It is US Pat. 4,404,529. That patent expired in 1995 BTW, so
> > > > it's content is out for grabs!
> [...]
> > The bleedthru may not be covered, but there are many other aspects...
> 
> No it isn't. Nor is it in the preceding patent 3.969.682.
> Dave R. talks about the need of buffering the final stage, but for reasons
> of driving low Z stages, not to prevent bleedthru. For driving low
> Z stages, voltage amplification (divider resistors) is not needed. But
> as you need extra gain for resonance/oscillation anyway, it's clear that
> gain and buffering would be combined in the same stage.
> 
> Now the 2044 is rather close to the Moog ladder, where chaining
> stages together without intermediate boosting and attenuating is quite
> "natural". The same thing built with OTAs is covered in the Rossum /
> Oberheim patent 3.969.682, Figure 9. That's what I was refering to
> in my initial posting.
> 
> I found an interesting sentence in one of these patents about the
> possibility of the last stage of several to have a lower cutoff
> frequency than the preceding ones, providing the benefit of low
> noise by filtering noise from previous stages. But - was this
> really used in synth filters ? I'm sceptical, because normally
> you try to start with a quadruple pole at no resonance and the
> "X"-shaped pole migration with increasing resonance. (And
> 4-pole filters which don't use that scheme, as EMS or 303,
> have a different sound.) And there was some mention about
> *that* in the patents as well. So I guess the cap is just different
> because of the different gain of the 4th stage, thus resulting
> in the *same* pole location as the preceding 3 stages.
> I may be wrong here - somebody has to calculate it.
> 
> JH.
> 
>