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Hi all,</div>
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Thanks for the advice.</div>
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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...</div>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;">Ben, thanks. That was helpful information.</span></div>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;">I need to breadboard this yet but I think I have mostly figured it out now.</span></div>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;"><span style="font-family: "Times New Roman"; font-size: medium; display: inline !important"><a href="http://tinyurl.com/y7pem69d" title="http://tinyurl.com/y7pem69d">Simplified
Falstad Simulation</a></span> - (simplified because my browser crashes when I try adding OTA's to the full circuit...) doh!</span><br>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;">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.</span></div>
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There's always the vactrol option...</div>
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Maybe someone might enjoy this simulation of the trigger logic circuit so I'll link it in anyway but it's otherwise irrelevant :p </div>
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<span style="font-family: "Times New Roman"; font-size: medium; display: inline !important"><a href="http://tinyurl.com/ycvp5gv8" id="LPlnk378842">http://tinyurl.com/ycvp5gv8</a></span><br>
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Thanks again,</div>
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<div>Kevin Walsh.</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Ben Bradley <ben.pi.bradley@gmail.com><br>
<b>Sent:</b> 28 April 2020 06:05<br>
<b>To:</b> Kevin Walsh <Kadub@outlook.com><br>
<b>Subject:</b> Re: [sdiy] OTA to replace variable resistor</font>
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<div class="PlainText">I'm looking over your schematic, and it looks like a floating VC<br>
resistor would be overkill, as each one requires two OTAs. Some<br>
simpler circuitry with one OTA per voltage-controlled function, should<br>
work.<br>
<br>
The Open Decay and Closed Decay variable resistors control the rate of<br>
discharge of C45, going through each variable resistance and through<br>
the respective diode and then to ground, either through Q1 or through<br>
pin 3 of the 555, depending on whether pin 3 is high or low. Each of<br>
these resistances can be replaced with an OTA, with the control input<br>
current (controlled by an input voltage) determining the discharge<br>
rate. The Clank Decay pot likewise determines the discharge rate of<br>
C43, and can likewise be replaced with an OTA.<br>
<br>
There's a transistor, op-amp, diode and resistors involved with each<br>
OTA to do voltage-to-current conversion and switch things, OTAs will<br>
give linear discharge rates, as opposed to the exponential discharge<br>
rates the pots give, and I don't offhand how much of a sound<br>
difference that will make.. With the voltage drop across the diodes<br>
and Q1, and emitter-follower transistors Q2 and Q4, it appears they're<br>
not perfectly decaying to zero anyway.<br>
<br>
On Mon, Apr 27, 2020 at 9:46 PM Kevin Walsh <Kadub@outlook.com> wrote:<br>
><br>
> Hi all,<br>
><br>
> 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.<br>
><br>
> 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.<br>
><br>
> 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.<br>
><br>
> I also added the XOR noise source and VCF's from Ken Stone's Cynare circuit.<br>
><br>
> 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.<br>
><br>
> 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.<br>
><br>
> I know I could use a vactrol but these are pricey, bulky and slow etc so I'd rather not have to use them.<br>
><br>
> My guess after looking at the LM13700 datasheet would be that I need to use the 'floating vc resistor' configuration.<br>
><br>
> I am no EE so I am totally patching bits of circuits from my Thomas Henry books together and hoping it sounds good :D<br>
><br>
> 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?<br>
><br>
> Any advice on what I need to do here would be greatly appreciated.<br>
><br>
> Many thanks in advance for any advice.<br>
><br>
> Here's the trigger logic and envelope generator section of the original circuit which I downloaded in a PDF from Nuts&Volts (Nov2003):<br>
> <a href="https://i.imgur.com/GXw9KVR.jpg">https://i.imgur.com/GXw9KVR.jpg</a><br>
><br>
> My sketchy block diagram:<br>
> <a href="https://i.imgur.com/absDij1.png">https://i.imgur.com/absDij1.png</a><br>
><br>
> A video of my first version of this module:<br>
> <a href="https://youtu.be/mq05zgHSYso">https://youtu.be/mq05zgHSYso</a><br>
><br>
> And my (slightly ridiculous) intended layout for the version two front panel:<br>
> <a href="https://i.imgur.com/n0LzEoq.png">https://i.imgur.com/n0LzEoq.png</a><br>
><br>
> My whole synth (last year):<br>
> <a href="https://i.imgur.com/lcdscrw.jpg">https://i.imgur.com/lcdscrw.jpg</a><br>
><br>
> Best regards,<br>
> Kevin Walsh.<br>
><br>
> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------<br>
><br>
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