AW: Xpander/Matrix12 filter design

[Haible Juergen]

> I don't think Tom Oberheim had anything to do w/ the Xpander.  If I
> remember correctly, Marcus Reilly did the filter design.  The 3372 
is not
> a 4 pole lowpass vcf/vca chip, it is a chip that has an exponential
> converter linked to four tranconductance stages, and another 
separate
> transconductance stage (or is it two? - my memory on the exact 
features of
> the 3372 is a bit foggy).

> The most obvious application is as a 4 pole lowpass VCF with VCA, 
but I
> suppose there are other things you could use it for besides filters. 

> In the Xpander/M12, CMOS switches are very cleverly used to switch 
around
> the signal path and component values, which changes the filter
> coefficients and thus the filter types.  It's all written up in 
either the
> Xpander or the M12 Service Manual (but not both, I don't think).
>
> FYI "Uncle Tom" is a derogatory racist term in the US.
>
Ø	JDM

Surely it's impressive to build a lot of different filters around some 
building blocks, but what
I always found most impressive was Oberheim's use of a 4-pole chip 
(3320) for a 2-pole
state variable filter (OB-Xa), and even reconfiguring it to 4-pole / 
2-pole with some CMOS switches in
the OB-8. The 3320's stages were intended to be 1/(1+s) blocks. In 
2-pole mode, some are
switched to integrators - 1/s - , and some are used as inverting 
amplifiers (with the capacitor
disconnected).
A very different solution than Roland's 2pole/4pole filters.

JH.




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