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

[tomg]

Another note or two...

> 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

Well...It's true it works and it works several different ways. The patent
calls for n,p,n,p current mirrors. n,n,n,n also works but has about
1/2 the gain. That's not a problem and it removes the bias resistors for
each stage not to mention the pnps. It's just not quite as dramatic. There 
is a ton of gain BTW or should I say minimal loss.

> > 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.

Coupling is just like the Moog version. Not because of gain though
...see above.

> > Now the 2044 is rather close to the Moog ladder, where chaining
> > stages together without intermediate boosting and attenuating is quite
> > "natural". 

It's very much like it! I'm surprised they got a patent. I can't argue that
it's not a improved version but it's very close to the original.

> > 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. 
> > I may be wrong here - somebody has to calculate it.
> > 
> > JH.

You are not wrong. To my ears about twice that of an intermediate 
stage. I had to cut the cap value in half to get it out of the storage trunk!
It's very strong. 

Tom