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 Forum index » DIY Hardware and Software
3 Channel gatewidth Modulator
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synthmonger



Joined: Nov 16, 2006
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Location: flada
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PostPosted: Sun Jun 07, 2009 9:16 pm    Post subject: 3 Channel gatewidth Modulator
Subject description: name changed added schematic
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I redesigned my Lunetta-esque pulsewidth modulator circuit into something a bit beefier, easier to control and a heck of a lot more fun to play with!

I've been troubled for a while now getting good pulsewidth modulation out of the 40106 chip and I've found the solution today by using FETS over NPN/PNPs.

The controls for the PWM are as follows:

CV input and manual frequency control for each channel
Input for each channel
Output and non inverted output for each channel with LED indicator

It's finished now and I'm going to be working on the PCB/schem as well as a new scope/audio demo this week.

To make this thread worthy I've included an audio clip. The input signal is running through a frequency divider whose /2 /4 /8 outputs are running into each channel. I'm using one triangle or noise output from my triple Modulation source (think CGS Pyscho LFO but CVable and insane) to control the PW. The PWM outputs are mixed together at different levels through an external mixer. The levels sound like they're being faded in and out but thats just due to the width of the signal being hardly audible.


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synthmonger



Joined: Nov 16, 2006
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PostPosted: Tue Jun 09, 2009 4:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Added a schematic!

It accepts just about any input signal. Works splendidly for gate signals and low frequencies, not so good with mid-high frequencies.

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fonik



Joined: Jun 07, 2006
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PostPosted: Sun Jun 14, 2009 3:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

so you are using a FET to creat a variable resistance!? great.

thanks for sharing. i almost missed it Shocked

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synthmonger



Joined: Nov 16, 2006
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PostPosted: Sun Jun 14, 2009 3:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
so you are using a FET to creat a variable resistance!? great.

thanks for sharing. i almost missed it Shocked


Yep! It works better than any PNP/NPN combo. I tried it with a few other circuits. One of them was the Resonator. I replaced the vactrols with two Fets and it worked kind of ok. I think my use of 4007's instead of the vactrols worked better.

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funkyfarm



Joined: Jan 21, 2007
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PostPosted: Sun Jun 14, 2009 6:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Great !!

this is kind of digital "voltage control gate length"

"3 channels" ? were is the pcb ? Very Happy
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synthmonger



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PostPosted: Sun Jun 14, 2009 7:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Working on it! Smile
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slo



Joined: Feb 18, 2007
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PostPosted: Wed Jun 17, 2009 7:09 am    Post subject: Reply with quote  Mark this post and the followings unread

Looks like this is what I need, I think.

Questions

Can it operate as a simple gate strecher if I left off the CV circuitry?

Should there be a dot at the junction of R6 - R5 joining them to the FET input?

I'm assuming R8 is the width control

George
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synthmonger



Joined: Nov 16, 2006
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PostPosted: Wed Jun 17, 2009 2:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

No but you can go here for that

http://www.musicfromouterspace.com/analogsynth/mmlogic.html


You can change the polarity of the width modulation by tieing the 1M resistor and fet D to ground instead of V+ which would do this also, but thats what the inverted output is for Wink

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