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 Forum index » DIY Hardware and Software » Developers' Corner
Question about fm implementation for ota filter
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L´Andratté



Joined: Sep 23, 2012
Posts: 52
Location: Hamburg, Germany
Audio files: 5

PostPosted: Sat Jan 26, 2013 11:57 am    Post subject: Question about fm implementation for ota filter Reply with quote  Mark this post and the followings unread

I´m building a vcf of the ms 20 variety, in fact using exactly the ms 20 expo converter and want to implement linear modulation of the filter frequency.
http://www.uni-bonn.de/~uzs159/index.html

Some 30 min. searching didn´t bring up anything useful, so hopefully someone around here has some more knowledge to share (or a link for a thread i overlooked)!

I´m thinking of just building a linear voltage to current converter and passivly mixing the output with that of the expo.
Am i totally wrong? Is there a simpler way (like ray wilson does in this one with npn? Injecting the audio into the inv. input of the op-amp)
http://www.musicfromouterspace.com/forums.html?MAINTAB=SYNTHDIY&PROJARG=VCO20120618REV0/VCO20120618REV0.html&VPW=1321&VPH=500

I would really appreciate a little bit of insight on this (and will do some experimenting right now)!
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L´Andratté



Joined: Sep 23, 2012
Posts: 52
Location: Hamburg, Germany
Audio files: 5

PostPosted: Sat Jan 26, 2013 12:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just found this on, where the linear cv is fed (after some biasing) directly in the Iabc of the ota...? this is really simple!
http://www.musicfromouterspace.com/forums.html?MAINTAB=SYNTHDIY&PROJARG=DUALVCA/DLLVCA001.html&VPW=1321&VPH=500

Hmmm...
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L´Andratté



Joined: Sep 23, 2012
Posts: 52
Location: Hamburg, Germany
Audio files: 5

PostPosted: Tue Jan 29, 2013 12:24 pm    Post subject:   Reply with quote  Mark this post and the followings unread

Now being confronted with what I can only interpret as pedagogical silence, Wink I continued searching and found quite some stuff...

Simple injection of cv via resistor into the ota control input seems to be generally a bad idea, and it is better to use a single transistor as voltage to current converter for linear response.

Good basic explanation can be found here (jump to 58 min.)
http://blip.tv/abovenyquist/ems-session-6-operational-transcondance-amplifiers-ota-voltage-controlled-amplifiers-vca-and-linear-current-sources-647878

So I´ll certainly be back soon with some annoying questions, cheers all!
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elmegil



Joined: Mar 20, 2012
Posts: 1345
Location: Chicago
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PostPosted: Tue Jan 29, 2013 6:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'd reply, if I knew anything about ota design. Smile
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L´Andratté



Joined: Sep 23, 2012
Posts: 52
Location: Hamburg, Germany
Audio files: 5

PostPosted: Tue Jan 29, 2013 7:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

hey everything´s cool with me! it´s about 3 am in germany & and i should really get some sleep but are currently reading this thread, which is exactly corresponding to my puzzling aroundhttp://electro-music.com/forum/topic-22235-25.html so please listen to this it just made me HAPPY
http://electro-music.com/forum/phpbb-files/berserklee2_153.mp3

bye
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