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 Forum index » DIY Hardware and Software » Developers' Corner
LAZY BEGGAR'S VCA
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beep



Joined: May 05, 2013
Posts: 105
Location: Germany

PostPosted: Mon Jul 25, 2016 2:37 pm    Post subject: LAZY BEGGAR'S VCA
Subject description: super simple CMOS VCA with less distortion than you may think
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Hey,

I was trying out that 4007 dual VCA circuit by Scott Gravenhorst:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

I did not get this thing working. Only had a very quiet distorted
always-on signal at the output which then got attenuated by
inverse cv-response. OK, that's not what I wanted.
Then I swapped CV and AUDIO inputs and tied v+ of the chip
and WOW!
Then I tied v+ of the chip to the CV input and another pin
to ground to get rid of some disturbing hum + a smoother CV response.

After adding an output cap, the whole thing got fucked up
again. I got around that by adding a 13k load resistor
BEFORE the output cap. (got that value by testing with a pot)
maybe this might vary and in some cases another value is necessary,
so I replaced the load resistor with a 20k trim pot. Don't know. Very Happy

With the right input level and load resistor value, the sawtooth still sounded
like a sawtooth, no pwmish distortion and stuff at low voltages.

It doesn't even need a power supply.

Tell me, what you think.

edit: a video of it in action:
https://youtu.be/ESe2kvdqpls


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donpachi



Joined: Jul 16, 2009
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PostPosted: Mon Jul 25, 2016 10:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Great stuff, thank you for sharing! I will try this.

Just for reference, which brand of 4007 did you use?
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beep



Joined: May 05, 2013
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PostPosted: Tue Jul 26, 2016 9:54 am    Post subject: Reply with quote  Mark this post and the followings unread

tried with NXP HEF4007UB and ST HCF4007UBE
looks like brand does not matter

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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Wed Jul 27, 2016 2:03 am    Post subject: Reply with quote  Mark this post and the followings unread

oh that's a nice odd VCA, I like it! Very Happy
I might give this a test and see how it looks on my scope.
what are the levels of the CV and the input signal that you are using ?
have you tried controlling 2 signals with 1 CV ?

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beep



Joined: May 05, 2013
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Location: Germany

PostPosted: Wed Jul 27, 2016 9:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
have you tried controlling 2 signals with 1 CV ?


wat?

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PHOBoS



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PostPosted: Wed Jul 27, 2016 10:07 am    Post subject: Reply with quote  Mark this post and the followings unread

using the other complementary pair, so audio 2 in connected to pin 10, audio 2 out from pin 12, pin 9 to GND and then connect pin 11 to pin 2.

hmm actually now that I look at the chip that probably isn't going to work. I only noticed now that scott has pin 1 and 5 connected and I wrongly
assumed that it was an internal connection. But you have pin 5 floating and you can't do that with the other pair because that one does have an
internal connection.

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beep



Joined: May 05, 2013
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Location: Germany

PostPosted: Thu Jul 28, 2016 12:50 am    Post subject: Reply with quote  Mark this post and the followings unread

I did this by trial and error. Later I recognized that my VCA seems to use one of the P channel mosfets rather than the N channel ones.
At least it works. No waste of an OTA, those 4000 series cmos ICs are
pretty available for cheap. Going to buy shitloads of them at the beginning
of the month.

Also the 40106 is pretty cool for making a single supply saw core dual vco
with exponential control.

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