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richardc64
Joined: Jun 01, 2006 Posts: 679 Location: NYC
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Posted: Sun Aug 31, 2008 8:14 am Post subject:
Schmitt VCO w/Up-Down sweep |
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The circuit I suggested and posted here http://electro-music.com/forum/post-203237.html#203237 gave me some trouble. Not only was upward sweep much greater than downward, but no matter how I tinkered with the values, with the Sweep potentiometer centered (zero sweep, equal amounts of "up" and "down",) I couldn't get the frequency to stay the same with maximum CV and zero CV.
To deal with down sweep depth being not as great as up, I thought instead of having the bottom transistor affect a single capacitor, why not have it add a varying amount of a second cap? It turned out that the second, "down" cap had to be about 4 times the "up" cap. This sort of worked, but "down" still didn't go as low as I wanted. I eliminated the lower transistor and instead used a diode. Surprisingly, this worked much better than the transistor. How it works is, as the voltage rises and the diode conducts more heavily, its "dynamic impedance" decreases, connecting "more" capacitance to ground. (Or something like that. Think of it as similar to the variable resistor of a cheesey 1960s phonograph's "Tone" control.)
All that remained was to get the centered position of SWEEP to give the same frequency with or without a CV applied. A resistor to ground at the "up" end of the pot, forming a voltage divider, took care of that. (A trimpot here to get zero sweep precisely in the center of the pot's rotation might be a good idea.) Values shown are for a 9V supply. It's highly probable that values will need adjusting for a lower or higher supply voltage.
If the modulating signal is an LFO's triangle or square wave, there isn't much difference between up sweep and down. But if the modulation is a ramp (sawtooth) or, better yet, an envelope, then adding the extra few components becomes worthwhile.
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_________________ Revenge is a dish best served with a fork... to the eye |
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synthmonger
Joined: Nov 16, 2006 Posts: 578 Location: flada
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richardc64
Joined: Jun 01, 2006 Posts: 679 Location: NYC
Audio files: 26
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Posted: Tue Sep 02, 2008 10:26 am Post subject:
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synthmonger wrote: | My 40106 vco has expo control (could be modded for linear) and produces a ramp and super narrow pulse waveform. |
Care to provide more details?
Quote: | I thought the lunetta didn't use transistors though? |
Perhaps it's time Mosc restated the Way of Lunetta _________________ Revenge is a dish best served with a fork... to the eye |
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synthmonger
Joined: Nov 16, 2006 Posts: 578 Location: flada
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Posted: Tue Sep 02, 2008 3:28 pm Post subject:
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Pin 1 = ramp pin 2 = super narrow pulse, similar to the bugcrusher vco.
I used a ghetto NPN/PNP expo arrangement and it worked well. I didn't test tracking or anything, but the range is huge. Obviously some sort of buffers will be needed.
You can get a linear response and other goodies by using any typical cv source found on cgs, mfos, google, etc.
I added a sloppy sync using a transistor as a switch between pins 1 and two but it cuts out at times and is just too sloppy. I need to fix that.
My lunetta type vco is a bit whacky. I really wanted a phat square wave and was only able to get it by using a frequency divider. I ran the narrow pulse into a 4024 and it's first two sub octaves each through a pair of 40106 gates to get the old fashion PWM trick. To get use out of the other 5 sub-octaves I mixed them together seperately and XOR'd them with a 4030 for staircasing and neat harmonic effects. |
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loss1234
Joined: Jul 24, 2007 Posts: 1536 Location: nyc
Audio files: 41
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loss1234
Joined: Jul 24, 2007 Posts: 1536 Location: nyc
Audio files: 41
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synthmonger
Joined: Nov 16, 2006 Posts: 578 Location: flada
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richardc64
Joined: Jun 01, 2006 Posts: 679 Location: NYC
Audio files: 26
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Posted: Thu Sep 04, 2008 4:46 am Post subject:
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Thanks, loss. I wasn't too interested in expo-control for my purposes, but that circuit certainly looks simple enough to try. But it's bipolar. Can it be made single-supply?
As for having up/down sweep, I wanted to keep an already simple circuit simple, and not resort to an op amp attenuverter, which I didn't know how to do in single-supply anyway -- until recently.
Yes 'monger, I knew there's a triangle at the gate input, but I thought it had to be buffered by a high-impedance op amp. I assumed a garden-variety transistor wouldn't be good enough. I'll have to try that, too.
Thanks.
(This is all straying far from Lunetta principles, I think.) _________________ Revenge is a dish best served with a fork... to the eye |
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loss1234
Joined: Jul 24, 2007 Posts: 1536 Location: nyc
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mike page
Joined: Sep 26, 2016 Posts: 134 Location: norwich, uk
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Posted: Wed Aug 01, 2018 5:35 am Post subject:
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A little modification I need to test but I think it should work
looking at it now tho, the CV in bit looks a bit weird |
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Richarius
Joined: Feb 22, 2014 Posts: 81 Location: Kitchener, Ontario, Canada
Audio files: 1
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Posted: Sat Aug 18, 2018 4:37 am Post subject:
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mike page wrote: | A little modification I need to test but I think it should work
looking at it now tho, the CV in bit looks a bit weird |
Just read this one, now.
The CV input, has 2 separate inputs. 1 is from external to the VCO. The other, is a voltage divider between + voltage and Ground. It's the VCO's 'Frequency' / 'Pitch' control. _________________ https://tsol.space/crafting/composing-music/albums/ - my punk, ambient and experimental tunes |
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mike page
Joined: Sep 26, 2016 Posts: 134 Location: norwich, uk
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Posted: Sun Aug 19, 2018 5:10 am Post subject:
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[quote="Richarius"] mike page wrote: |
The CV input, has 2 separate inputs. 1 is from external to the VCO. The other, is a voltage divider between + voltage and Ground. It's the VCO's 'Frequency' / 'Pitch' control. |
yeah its my drawing ha ha |
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richardc64
Joined: Jun 01, 2006 Posts: 679 Location: NYC
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Mon Sep 17, 2018 4:08 am Post subject:
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synthmonger wrote: | My 40106 vco has expo control (could be modded for linear) and produces a ramp and super narrow pulse waveform.
I thought the lunetta didn't use transistors though? |
In my view, Lunetta can be whatever you want it to be using whatever parts you have or like. The most important part of the Lunetta paradigm, in my opinion, is FUN. If you're getting that, you're doing it right. _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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haebbmaster
Joined: Aug 27, 2016 Posts: 40 Location: Windenreute, Germany
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Posted: Wed Oct 24, 2018 1:37 pm Post subject:
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JovianPyx wrote: | In my view, Lunetta can be whatever you want it to be using whatever parts you have or like. The most important part of the Lunetta paradigm, in my opinion, is FUN. If you're getting that, you're doing it right. |
+1 _________________ Make modular synthesizers great again!
Modular Synthesizer DIY blog: http://modular.haebbmaster.de |
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