[sdiy] Another new module: 4 pole multimode filter

[The Old Crow]

  I am reminded of the Oberheim Xpander/Matrix12 filter, which used the
cells of a CEM3372 along with a string of transfer function coefficients
as long as a train (translation: a couple inverting opamps and a pile of
precision resistors along with analog switches) to provide 15 selectable
filter models.

  Hm, I have the xpvoice schematic and the blurb about deriving the 
S-transform archived on oldcrows.net as an unlisted link:

  http://www.oldcrows.net/~oldcrow/synth/xp/

  To answer your question about resonance for BP, I think you need
negative feedback of the resonance, such as into the inverting input of
the first opamp.  This also implies a reverse action of the res.  control
(CW becomes CCW).

Crow
/**/


On Sat, 4 Oct 2003, Scott Bernardi wrote:

> Just what we need - another filter circuit. Anyway, I've always liked 
> the CEM3320 filter chips, which has 4 transconductance stages which can 
> yield any of the standard filter response (HP, LP, BP, and All Pass) in 
> 4th order, depending on how you interconnect the stages.  I hit on the 
> idea of using analog switches to do the interconnecting and build a 
> switchable (not simultaneous) multimode filter. I previously built one 
> with a CEM3320 that yielded BP and LP functions.  The  new circuit 
> contains no CEMs but is built with LM13700 OTA's and MAX413 analog switches.
> The HP and LP functions work perfectly, just as I expected. The BP 
> however doesn't seem to respond to the resonance settings as well as it 
> should (and certainly not as well as the CEM3320 BP).
> Maybe some of you geniuses can take a look a make suggestions (make 
> suggestions even if you don't consider yourself a genius!).
> I've got to break down and build me another case. I've got a box full of 
> loose modules without a home.
> 
> http://home.comcast.net/~sbernardi/elec/og2/og3_4pmultimode.html
> 
>