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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Low Pass Resonant Filter Question
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fishfude



Joined: Jan 06, 2014
Posts: 3
Location: Ireland

PostPosted: Mon Jan 06, 2014 1:24 pm    Post subject: Low Pass Resonant Filter Question
Subject description: Why the 470k resistor across the 500k resonance pot
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Hiya folks,

So I was building the resonant low pass filter section from the weird sound generator and I was deeply curious as to the effect of having the 470k (R35) resistor across legs 1 & 3 of the 500k resonant filter (R32), as opposed to just using a pot of half the value? (See image)

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Also, I decided to just go with a 220k pot instead and it didnt seem to work, it just introduced noise when opened.

Finally, I decided not to connect leg 2 to leg 3 on any of the pots (they are acting as series resistors so I dont have to right?) and very interestingly, when I touch my finger between legs 2 and 3 of the pots I get oscillations and sonic textures. Anybody able to explain this?

Cheers,

ff
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Mongo1



Joined: Aug 11, 2011
Posts: 411
Location: Raleigh NC

PostPosted: Wed Jan 08, 2014 8:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
as opposed to just using a pot of half the value?


If it were two pots paralleled together, using a pot of half the value would work. When you put a fixed resistor in parallel with a pot, it gives it a different response curve.

I'd suggest using a simple spreadsheet, and put 10 different settings of the pot in it. You'll be able to see the difference...

Gary
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fishfude



Joined: Jan 06, 2014
Posts: 3
Location: Ireland

PostPosted: Thu Jan 09, 2014 4:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Gary,

Thanks for that. I managed to find out some more information on it from GEOFEX and other sources.

So, yeah, it's creating a reverse log curve for the pot with a max at ~50% the pot value.
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