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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
MFOS VCF caps
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bunker



Joined: Mar 19, 2009
Posts: 57
Location: Manchester, UK

PostPosted: Thu Nov 28, 2013 12:24 pm    Post subject: MFOS VCF caps Reply with quote  Mark this post and the followings unread

Hi all.

I'm having some problems sourcing the 100pF polystyrene caps that are spec'd in the VCA schematic. They connect between the output of the LM13700 OTA, non inverting inputs of the TL082 op amps that make up each filter cell and ground. I can get hold of 150pF polystyrene's no problem so would I NEED to start re-scaling the resistors in the filter core too? Can anyone hazard a guess at what the result would be if I didn't re-scale resistors given the 50pF difference? It shouldn't be a big deal to rescale things to suit the bigger caps should it? Its not like I'm subbing the 100pF caps unto, say 1000pF.

Thanks
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bunker



Joined: Mar 19, 2009
Posts: 57
Location: Manchester, UK

PostPosted: Thu Nov 28, 2013 12:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thinking about it if the OTA is acting as the frequency setting resistor I'm assuming the 100pF cap is the C in the CR formula? In which case re-scaling the OTA shouldn't be required as its already the variable in the equation? So changing the value of the cap will just shift the cutoff frequency? Or am I barking up the wrong tree entirely? Rolling Eyes
Cheers!
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mrmrshoes



Joined: Feb 19, 2011
Posts: 73
Location: Newcastle Upon Tyne
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PostPosted: Thu Nov 28, 2013 2:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

i saw a link to a company over at muffs, it was in this thread.
(Polystyrene Capacitors: who stocks these??)
http://www.muffwiggler.com/forum/topic-98122-0-10.html


uk company and decent prices and stock.
https://www.rush-on-line.co.uk/cap_forums.html

haven't ordered from them but others have.
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bunker



Joined: Mar 19, 2009
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PostPosted: Thu Nov 28, 2013 6:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

Most excellent links!! Thank you very muchly Very Happy
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elmegil



Joined: Mar 20, 2012
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Location: Chicago
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PostPosted: Thu Nov 28, 2013 6:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

bunker wrote:
Thinking about it if the OTA is acting as the frequency setting resistor I'm assuming the 100pF cap is the C in the CR formula? In which case re-scaling the OTA shouldn't be required as its already the variable in the equation? So changing the value of the cap will just shift the cutoff frequency? Or am I barking up the wrong tree entirely? Rolling Eyes
Cheers!


That's how I would look at it. You're just changing the range.
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