[sdiy] OTA to replace variable resistor

[Tom Wiltshire]

+1 agree. It makes more sense to use a envelope generator design that is intended to take control voltages than to try and tack it onto one that was never designed with that in mind. I’m sure I’ve seen some that use the LM13700 if that’s what you’ve got.

> On 28 Apr 2020, at 04:55, Oren Leavitt <obl64 at ix.netcom.com> wrote:
> 
> Hi Kevin,
> 
> You may want to look into a voltage controllable envelope generator design.
> A good example is Juergen Haible's VC HADSR:
> http://jhaible.com/legacy/tonline_stuff/hj_vc_hadsr.html <http://jhaible.com/legacy/tonline_stuff/hj_vc_hadsr.html>
> 
> - 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 <https://i.imgur.com/GXw9KVR.jpg>
>> 
>> My sketchy block diagram:
>> https://i.imgur.com/absDij1.png <https://i.imgur.com/absDij1.png>
>> 
>> A video of my first version of this module:
>> https://youtu.be/mq05zgHSYso <https://youtu.be/mq05zgHSYso>
>> 
>> And my (slightly ridiculous) intended layout for the version two front panel:
>> https://i.imgur.com/n0LzEoq.png <https://i.imgur.com/n0LzEoq.png>
>> 
>> My whole synth (last year):
>> https://i.imgur.com/lcdscrw.jpg <https://i.imgur.com/lcdscrw.jpg>
>> 
>> Best regards,
>> Kevin Walsh.
>> 
>> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>> 
>> 
>> 
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