[sdiy] understanding the CAP arrangement on the PT2399 (filters)

[Tom Wiltshire]

Hi Dan,

I think most of those filters are multiple-feedback lowpass filters.  
Drawn out like that, they're hard to recognize. Look at LPF2 for  
example. If you draw it out on a a bit of paper, you'll see it's the  
same as this (below), with an extra RC stage stuck on the end to  
increase the rolloff. LPF1 looks to be the same, but without the  
extra RC.

http://en.wikipedia.org/wiki/Multiple_feedback_topology_(electronics)

Hope this helps,

T.


On 6 Oct 2009, at 03:22, Dan Snazelle wrote:

>
> looking at
>
> http://sites.google.com/site/hearnmorley/synth-diy-resources/ 
> GHSYN2bWeb.pdf?attredirects=0
>
> or even at the PT2399 datasheet, i have never understood the signal  
> flow in the "internal opamps"
>
> to me it looks like the input is connecting VIA r5 to the first  
> opamp which has a 1n in its feedback loop. but is r6 connecting  
> into the next opamp or into the feedback loop of the first one? and  
> there is similar confusion on following the path (and determining  
> the cutoffs) on the additional internal opamps
>
> thanks for any help
>
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