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 Forum index » DIY Hardware and Software » Thomas Henry designs
3080 VCO current sources
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Conboy



Joined: May 01, 2009
Posts: 25
Location: London

PostPosted: Tue May 14, 2013 2:42 am    Post subject: 3080 VCO current sources
Subject description: a few issues with current sources from Making Music with 3080
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I have been having varying degrees of success building a simple VCO from the designs in Thomas's book Making Music with the 3080 OTA.

The VCO section is working nicely now on my veroboard design. But my problems are with getting the current source to work correctly.

I have tried building the "Easy Exponential Current Source" which ought to suit my needs. However the pots range is unusable.
All the range is within the 1/3rd of central position on the pot - at either end (upper 1/3rd and lower 1/3rd) the VCO bottoms or tops out with the frequency remaining the same for the rest of the range.
I can't work out how this is adjusted? Or why it should be so.

I actually get much better results just using the "simple manual arrangement" from the simple linear current source chapter! From which I am able to get the full range of the VCO from maybe 5Hz to 20kHz +

Seems wrong that the super simple solution works better! Smile

Also - with both designs I get a stepping effect. Almost digital in quality (ie not welcome!) - is this an inherent part of the design? Can it be smoothed out in any way

any advice much appreciated

Paul
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forbin



Joined: Jan 29, 2009
Posts: 118
Location: Fremantle, Australia

PostPosted: Tue May 14, 2013 5:30 am    Post subject: Reply with quote  Mark this post and the followings unread

It does sound a little bit like you may have one of the transistors the wrong way around... i had a similar problem with a vco where i had accidentally used the wrong polarity transistors NPN <=> PNP....
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widdly



Joined: Jun 25, 2007
Posts: 265
Location: singapore
G2 patch files: 2

PostPosted: Wed May 15, 2013 2:51 am    Post subject: Reply with quote  Mark this post and the followings unread

If only the middle third of the pot is useful, add a 100k resistor between the 15v and the top of the pot and another 100k between the bottom of the pot and the -15v (assuming you are using a 100k pot).


I've built that vco using the precision expo source and there is no stepping.
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oculus



Joined: Oct 30, 2011
Posts: 30
Location: Iceland, Reykjavik

PostPosted: Thu Jul 11, 2013 9:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

i don´t have this stepping issue but i have same problem with the range on the VCO-1 i just put a extra 100k on the middle pin of the pitch knob that seemed to fix things for me,
so basicly i just shortened the range of the pitch knob
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