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log converter for the lm13700 data sheet vco
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Werebear



Joined: Nov 23, 2009
Posts: 33
Location: UK

PostPosted: Mon Sep 19, 2011 9:52 am    Post subject: log converter for the lm13700 data sheet vco
Subject description: why do we need to use pnp trannies for this?
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Hi

I've breadborded the LM13700 vco from the datasheet once or twice and am quite fond of it and it's simplicity - something we share in common, ho,ho,ho.

I would like to use it as is with a log converter to give me something that approximates 1v/oct as a basis for an ondes type ribbon instrument. (Old hat I know, since Dana Countryman and the chap on youtube with his Ginette. Those are my comments about QTC by the way, something I intend to experiment with when the current project is finished with.)

I've seen Harmony Central's ideas for the Daleks Handbag synth vco, which is basically what I'm after, but is the log converter he shows there enough for me to equally scale a ribbon based based on a 100K precision multi turn pot and a current source based around what Mr. Wilson uses for his keyboard controllers?

I know the answer is work it out for yourself, but has anyone any thoughts in that direction?

Da Bear

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Mongo1



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

PostPosted: Mon Sep 19, 2011 11:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I've seen Harmony Central's ideas for the Daleks Handbag synth vco, which is basically what I'm after, but is the log converter he shows there enough for me to equally scale a ribbon based based on a 100K precision multi turn pot and a current source based around what Mr. Wilson uses for his keyboard controllers?


Hmmm - I googled around for the Dalek's Handbag, and that led me to that old Roland guitar synth. Beyond that I don't know what circuit in particular you're talking about. Do you have a link to it?

The actual log converter circuitry used in most VCOs is very similar, and pretty simple. If you're just trying to make a single VCO it seems that there's not much reason to reinvent the wheel.

Just my two cents
Gary
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Werebear



Joined: Nov 23, 2009
Posts: 33
Location: UK

PostPosted: Wed Sep 21, 2011 7:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Mongo

Doh! It's Frequency Central - http://www.diystompboxes.com/smfforum/forums.html?topic=70222.0

Hope this works.

He is using the lm13700 datasheet vco with a log converter made up of a BC547 and a BC557 with a trim presumably to give volt/octave scaling though I don't think that that was an important consideration to the synth he built.

What I wants to do is use a 100K precision pot driven by a constant current source to produce a CV for the VCO. It doesn't have to be volt/octave as this will be a stand alone instrument without cv ins and outs but I want it to be as linear as possible considering I'm having no luck with finding PTC thermistors in the 1K/2K range.

I have breadboarded similar log converters before with a modicum of success so maybe I should just go for it and see what I get.

Thanks for the input Mongo!

Da Bear

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Mongo1



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PostPosted: Wed Sep 21, 2011 7:42 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
He is using the lm13700 datasheet vco with a log converter made up of a BC547 and a BC557 with a trim presumably to give volt/octave scaling though I don't think that that was an important consideration to the synth he built.

What I wants to do is use a 100K precision pot driven by a constant current source to produce a CV for the VCO. It doesn't have to be volt/octave as this will be a stand alone instrument without cv ins and outs but I want it to be as linear as possible considering I'm having no luck with finding PTC thermistors in the 1K/2K range.


Ah - thanks for the link! I found the circuit you mean. I think that should work fine for your needs. You won't have the snazzy temperature stability, but I doubt that that will matter too much for a ribbon controller. I'd say give it a try - what can you lose?

Gary
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Werebear



Joined: Nov 23, 2009
Posts: 33
Location: UK

PostPosted: Thu Sep 22, 2011 7:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Gary.

Actually I do want the snazy temperature stability as this is an instrument I want to be able to play "Happy Birthday" on as it where.

Without re-inventing the wheel I think I'm just going to have to experiment from scratch. I've got the notes on log converters from Electronotes and I've played around with some of the math. I will probably build a triangle core from op amps and use some of the THAT NPN trannie arrays I bought a while ago. If I'd been brighter I'd have bought the dual NPN/PNP so I could use the LM13700 and use the NPN pair as a sign converter - ala long tail pair amp. However, learning as I go, I make this kind of mistake and end up with components that aren't quite what I wanted! Oh well...

Thanks for the input Gary

Jim

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