[sdiy] OTA to replace variable resistor

ColinMuirDorward colindorward at gmail.com
Thu Apr 30 19:07:04 CEST 2020

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

On Thu, Apr 30, 2020 at 4:01 PM Kevin Walsh <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
> http://tinyurl.com/ycvp5gv8
> Thanks again,
> Kevin Walsh.
> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ------------------------------
> *From:* Ben Bradley <ben.pi.bradley at gmail.com>
> *Sent:* 28 April 2020 06:05
> *To:* Kevin Walsh <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
> work.
> 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> 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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