[sdiy] OTA to replace variable resistor

Kevin Walsh kadub at outlook.com
Thu Apr 30 22:20:32 CEST 2020

Hi Colin,

Very nice.
This looks like it's just what I have been looking for.
Thank you very much.
I'll try it out this evening :D

Best Regards,

Kevin Walsh.


From: ColinMuirDorward <colindorward at gmail.com>
Sent: 30 April 2020 18:07
To: Kevin Walsh <kadub at outlook.com>
Cc: SDIY <synth-diy at synth-diy.org>
Subject: Re: [sdiy] OTA to replace variable resistor

Hi Kevin, not sure if this is helpful but you could check the befaco kickall module which has VC time on a simple decay (release?) envelope.
See schematics here<https://befaco.org/docs/Kickall/Kickall_V1C_Schematic.pdf>.

On Thu, Apr 30, 2020 at 4:01 PM Kevin Walsh <kadub at outlook.com<mailto:kadub at outlook.com>> wrote:
Hi all,

Thanks for the advice.

I have made a  VC envelope generator using the as3310 but I think that's a bit fancy for a drum simulator. I like the open and closed logic of the original Thomas Henry circuit so I will keep all of that for this module too. I just want to be able to modulate the decay times. Nothing too complicated...

Ben, thanks. That was helpful information.
I need to breadboard this yet but I think I have mostly figured it out now.
Simplified Falstad Simulation<http://tinyurl.com/y7pem69d> - (simplified because my browser crashes when I try adding OTA's to the full circuit...) doh!
As you mentioned, the discharge rate is linear so I may need to keep looking for a better solution. I guess I'll give it a listen and decide.
There's always the vactrol option...
Maybe someone might enjoy this simulation of the trigger logic circuit so I'll link it in anyway but it's otherwise irrelevant :p

Thanks again,

Kevin Walsh.


From: Ben Bradley <ben.pi.bradley at gmail.com<mailto:ben.pi.bradley at gmail.com>>
Sent: 28 April 2020 06:05
To: Kevin Walsh <Kadub at outlook.com<mailto:Kadub at outlook.com>>
Subject: Re: [sdiy] OTA to replace variable resistor

I'm looking over your schematic, and it looks like a floating VC
resistor would be overkill, as each one requires two OTAs. Some
simpler circuitry with one OTA per voltage-controlled function, should

The Open Decay and Closed Decay variable resistors control the rate of
discharge of C45, going through each variable resistance and through
the respective diode and then to ground, either through Q1 or through
pin 3 of the 555, depending on whether pin 3 is high or low. Each of
these resistances can be replaced with an OTA, with the control input
current (controlled by an input voltage) determining the discharge
rate.  The Clank Decay pot likewise determines the discharge rate of
C43, and can likewise be replaced with an OTA.

There's a transistor, op-amp, diode and resistors involved with each
OTA to do voltage-to-current conversion and switch things, OTAs will
give linear discharge rates, as opposed to the exponential discharge
rates the pots give, and I don't offhand how much of a sound
difference that will make.. With the voltage drop across the diodes
and Q1, and emitter-follower transistors Q2 and Q4, it appears they're
not perfectly decaying to zero anyway.

On Mon, Apr 27, 2020 at 9:46 PM Kevin Walsh <Kadub at outlook.com<mailto:Kadub at outlook.com>> 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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