log converter for the lm13700 data sheet vco

Werebear · Joined: Nov 23, 2009 Posts: 63 Location: UK

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

Mongo1 · Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC

Werebear · Joined: Nov 23, 2009 Posts: 63 Location: UK

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

Mongo1 · Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC

Werebear · Joined: Nov 23, 2009 Posts: 63 Location: UK

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