Capacitorless filters (was:Re: [sdiy] polivoks filter)

[jhaible at debitel.net]

> There are theories called "Active-R filter" and "Active-R oscillator" 
> which directly use the dominant pole of a opamp to form a filter, 
> though their main applications are RF.  Of course, it's a modern 
> technique and I don't think it was used in Polivoks.


I think it's pretty old (even as old as dominant pole 
compensation?). Farfisa was one example; Sennheiser Vocoder 
is another one. (The latter would more precisely be called 
"capacitorless lossy integrator".) Or think of that Digisound (sp?)
Wavetable oscillator, where a low slew rate opamp interpolates
between samples.

I agree about the HF applications, though. Doesn't work down
to low frequencies because of the low currents and/or high gain
that would be required.

JH.



  
> 
> Sam Hoshuyama
> Kawasaki, JAPAN
> 
> Karl Dalen wrote:
> > 
> >  --- jhaible <jhaible at debitel.net> skrev: > No idea what's inside the
> > russian parts.
> > >
> > > But the Farfisa Synthorchestra had a 4pole VCF that was built
> > > without capacitors. It used a quad opamp with
> > > adjustable internal current sources. And of course there *are*
> > > capacitors:
> > > The silicon-made phase compensation that's on chip.
> > 
> > I had that in mind to but quickly realising that having
> > a production line for 20-25 000 made units relying on
> > the phase compensation capacitance wich would in a FET
> > amp be in the units of pF region i suppose was a bit
> > hair raising at first thought!
> > 
> > Small capacitors and leakages and temperature shifts
> > does not sound healty. Well, i dont know much about
> > silicon process but anyway!
> > 
> > KD
> > 
> > > Basically the same structure as an OTA + cap + buffer, only
> > > scaled down to very small capacitors and very small currents.
> > > Doesn't work too well at low cuttoff frequencies, no surprise.
> > > But it's remarkable that it works at all!
> > >
> > > JH.
> > >
> > > -----Ursprüngliche Nachricht-----
> > > Von: Karl Dalen <karldalen at yahoo.se>
> > > An: <uzs159 at uni-bonn.de>
> > > Cc: <synth-diy at dropmix.xs4all.nl>
> > > Gesendet: Samstag, 11. Oktober 2003 01:52
> > > Betreff: Re: [sdiy] polivoks filter
> > >
> > >
> > > > Exactly, listen here to what designer Vladimir Kuzmin
> > > > says at http://www.muztech.ru/eng/history.php
> > > >
> > > > quoute; VCF doesn't have any capacitor at
> > > >         all and use only two op amps.
> > > >
> > > > Now, that's interesting! I wonder how this is done!
> > > > Another interesting thing is the VCA, it has
> > > > differential outputs!
> > > >
> > > > KD
> > > >
> > > >  --- René_Schmitz <uzs159 at uni-bonn.de> skrev: > I wrote:
> > > > > > The filter caps are not on the filter board, but on the connector
> > > > > board.
> > > > >
> > > > > Oops, no they're not.
> > > > >
> > > > > --
> > > > > uzs159 at uni-bonn.de
> > > > > http://www.uni-bonn.de/~uzs159
> > > >
> > > >
> > > > Höstrusk och grå moln - köp en resa till solen på Yahoo! Resor på
> 
> > > adressen
> > > http://se.docs.yahoo.com/travel/index.html
> > > >
> > >
> > >
> > 
> > Höstrusk och grå moln - köp en resa till solen på Yahoo! Resor på adressen
> http://se.docs.yahoo.com/travel/index.html
> 
> 




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