[sdiy] OTA to replace variable resistor

Oren Leavitt obl64 at ix.netcom.com
Tue Apr 28 05:55:36 CEST 2020

Hi Kevin,

You may want to look into a voltage controllable envelope generator design.

A good example is Juergen Haible's VC HADSR:


- Oren

On 4/27/20 8:45 PM, Kevin Walsh wrote:
> Hi all,
> I have never posted a question here but I have been subscribed for 
> several years and I have learned a lot from reading the threads. Thank 
> you all for this.
> I am an sdiy hobbyist who enjoys making modules for my own use. I 
> occasionally make music and I have played a few gigs out locally.
> I have recently finished my own version of the Thomas Henry 
> 'Clangora'. I couldn't find NE566's (VCO's) so I just swapped them out 
> for LM13700's and done the same with the NE571 (VCA's) while I was at it.
> I also added the XOR noise source and VCF's from Ken Stone's Cynare 
> circuit.
> It was looking like a huge module so I decided to not add CV control 
> on version one. Now that it is finished I am really happy with how it 
> sounds and I want to add CV control to the next version.
> I know how to stick in a OTA VCA here and there to add CV control to 
> the Sweep and FM etc but I can't get my head around how to use an OTA 
> to replace the variable resistors that are controlling the open and 
> closed decay times.
> I know I could use a vactrol but these are pricey, bulky and slow etc 
> so I'd rather not have to use them.
> My guess after looking at the LM13700 datasheet would be that I need 
> to use the 'floating vc resistor' configuration.
> I am no EE so I am totally patching bits of circuits from my Thomas 
> Henry books together and hoping it sounds good :D
> Is it the 'floating VC resistor' configuration that I need to use in 
> place of the open and closed decay resistors or is it something crazy 
> complicated that's not in the datasheet?
> Any advice on what I need to do here would be greatly appreciated.
> Many thanks in advance for any advice.
> Here's the trigger logic and envelope generator section of the 
> original circuit which I downloaded in a PDF from Nuts&Volts (Nov2003):
> https://i.imgur.com/GXw9KVR.jpg
> My sketchy block diagram:
> https://i.imgur.com/absDij1.png
> A video of my first version of this module:
> https://youtu.be/mq05zgHSYso
> And my (slightly ridiculous) intended layout for the version two front 
> panel:
> https://i.imgur.com/n0LzEoq.png
> My whole synth (last year):
> https://i.imgur.com/lcdscrw.jpg
> Best regards,
> Kevin Walsh.
> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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