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 Forum index » DIY Hardware and Software » Thomas Henry designs
VCF and VCA Analogue Synth for the 21st Century
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Sam Doug



Joined: Nov 05, 2012
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Location: Truro, England

PostPosted: Tue May 07, 2013 8:04 am    Post subject: VCF and VCA Analogue Synth for the 21st Century Reply with quote  Mark this post and the followings unread

Hello, I was just wondering if there is any sort circuit explanation for the VCF & VCA in An Analogue Synth For The 21st Century?

Also could anyone point me in the direction for some maths of the VCO-1?

Anything is a great help.

Thanks, Sam
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elmegil



Joined: Mar 20, 2012
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PostPosted: Tue May 07, 2013 9:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Not sure I understand what you mean by "some maths of the VCO-1" ?

I don't know of any commentary for the Analog Synth... VCF/VCA that is similar to the stuff on birthofasynth.com, but if you could maybe look through it and ask some questions I'm sure we could help Smile
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Sam Doug



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Location: Truro, England

PostPosted: Wed May 08, 2013 4:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the reply. I was wondering about the function of the SSM2164 VCA. The VCF-1 and VCA-1 use the dual transistor exp current converter, however, the circuits in an analogue synth dont. I was wondering where the current conversion comes from. Im also pondering on the function or placement of components for the filter cut off points. Do you know of any good articles on VCFs?

And by some maths of the VCO-1, I mean some sort of mathematical breakdown/explanation of the circuit.
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elmegil



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PostPosted: Wed May 08, 2013 5:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, I see. Yes, it does have to do with the 2164. I know that David Dixon (DrEtchNSketch over at Muffwiggler's, and maybe here too) talks about doing expo conversion with it somewhat regularly. Unfortunately, I do not have the chops to try and explain it. I'm guessing searching for something like 2164 expo converter here or on Muff's might turn up some explanation though.
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Tim Servo



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PostPosted: Thu May 09, 2013 1:16 pm    Post subject: VCF and VCA Analogue Synth for the 21st Century Reply with quote  Mark this post and the followings unread

A bit of an explanation: The VCF in the AS21C book doesn't need an exponential converter because the 2164 chip has built-in exponential response. The VCF uses two sections of the 2164 to create a 12dB multi-mode VCF.

The VCA is actually a linear response circuit. It also uses two sections of the 2164, but it runs them in a special configuration to get a linear CV response.

Circuits that use linear OTAs (e.g. 3080, 3280, 13600, 13700) need an exponential converter to get an exponential CV response. This is what you see on the LM VCO, VCO-1, VCF-1 and others.

Hope this helps.

Tim (control curves in da house) Servo
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Sam Doug



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PostPosted: Mon May 13, 2013 6:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes this helps, thanks very much for the reply
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