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 Forum index » DIY Hardware and Software » Developers' Corner
Exponential Converter Circuits
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Stream Operator


Joined: Oct 13, 2007
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PostPosted: Sun Sep 13, 2009 6:22 am    Post subject: Exponential Converter Circuits
Subject description: How do you get 1V/Octave?
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Hi, Jan pointed out to me in the chatroom that my Karplus-Strong circuit board has a linear voltage to frequency response, when it is preferred to have a 1V/Octave or 1.2V/Octave exponential converter.

I don't see why that cannot be done with just a diode and maybe a resistor. Just have a summing amp and put the diode and resistor in the feedback loop. Should work as a compact but imprecise way to do it, yes or no?

Also, how is this normally done?

Thanks,

Les

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wmonk



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PostPosted: Sun Sep 13, 2009 11:27 am    Post subject: Reply with quote  Mark this post and the followings unread

I like the expo converters on the Thomas Henry VCOs. He uses a matched NPN and a temperature compensated resistor.

In the XR-VCO schematics on Scott Stites website:
http://mypeoplepc.com/members/scottnoanh/sitebuildercontent/sitebuilderfiles/xr_schem1.pdf

Almost the same for the SN-Voice.

(Please remove if Scott or Thomas don't want a direct link)

Note: this is an exponential current source.

EDIT: Ray Wilson has a similar approach in his 1V/Oct VCO.
http://www.musicfromouterspace.com/analogsynth/VCO200604REV01/VCO200604REV01.html
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Stream Operator


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PostPosted: Sun Sep 13, 2009 11:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks TP, I'm reading the schematic now. Don't quite understand it though, and I think I can do it simpler albeit less accurately.

Les

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Blue Hell
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PostPosted: Sun Sep 13, 2009 11:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Les, it is important that it is accurate, the human ear is very sensitive for frequency deviations.
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wmonk



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PostPosted: Sun Sep 13, 2009 12:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

The MFOS VCO has an accurate tracking of 5 to 6 octaves, which is really nice. With only a resistor and a diode, I don't think your accurate tracking will exceed 2 octaves.
One of the good old 1V/Oct examples is the ASM-1 synthesizer. Ken Stone refers to this site on his VCO webpage.
http://home.att.net/~synthsource/ens76/vco1.htm
It's a nice description of a voltage to exponential current circuit.
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