[sdiy] OTA to replace variable resistor

[Kevin Walsh]

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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