[sdiy] op amp based filter

[Tim Stinchcombe]

As Dave has already pointed out, this is exactly what the Polivoks VCF does,
using a 'programmable gain' op amp.

Marc Bareille has a nice page of info about here:

http://m.bareille.free.fr/modular1/vcf_polivoks/vcf_polivoks.htm

including the original schematic:

http://m.bareille.free.fr/modular1/vcf_polivoks/vcf_polivoks_sh0.gif

and his clone, using a uA776:

http://m.bareille.free.fr/modular1/vcf_polivoks/polivoks_schem.pdf

Hell, you can even buy a euro-format module from Harvestman, which uses the
genuine Russian chips:

http://www.theharvestman.org/r1982.php

I have also seen the same idea in an old filter textbook - I could probably
dig out the reference if anyone is interested...

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk


> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> cheater cheater
> Sent: 24 July 2010 05:08
> To: synth-diy
> Subject: [sdiy] op amp based filter
> 
> 
> 
> Hi guys,
> if I took an op amp with a very low GBW, and set the gain so 
> that the bandwidth becomes the cutoff frequency I need, could 
> I make a low pass filter out of that? I would need to make up 
> for the gain before and after that op amp (with op amps too, 
> but ones that are much higher bandwidth of course), and I 
> would probably have to chain several such modules (say 4-8) 
> one after another to get a strong effect, but is the basic 
> idea correct?
> 
> The only 741 datasheets I could find mentioned a GBW of 4MHz, 
> so that's a bit too much - to have a cutoff of 4 Hz you'd 
> need to have gain of 92 dB, which could be a bit too much to 
> ask of the system without drowning in noise. I've noticed 
> Digikey has a lot of op amps with a very low GBW, even down 
> to 2.7 kHz.
> 
> Any ideas are welcome!
> 
> Cheers,
> D.
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