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 Forum index » DIY Hardware and Software » YuSynth
some VCA questions
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isak



Joined: Dec 13, 2009
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PostPosted: Tue Jun 18, 2013 12:45 pm    Post subject: some VCA questions Reply with quote  Mark this post and the followings unread

hi Yves

i built the VCA, it works great.
i have few questions tho...

question 1:
is it normal that in high freq the sine wave will be like tri or some strange wave form?
example..

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

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

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

question 2:
why with AC mode i need to use log pots?
now i use lin pots and it seems to work ok.
any special reason for that?

question 3:
it seems that the vca is a bit "distorted", is it normal?
It's distorted not in aggressive way, only a bit, like a very gentle overdrive.
thanks in advance.
Isak E.

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yusynth



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PostPosted: Wed Jun 19, 2013 4:22 am    Post subject: Reply with quote  Mark this post and the followings unread

It is normal that a VCA with such a simple design add some distortion but this kind of distortion is weird indeed.
What is the input level ?

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isak



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PostPosted: Wed Jun 19, 2013 8:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for replying.

I'll check layer on and let you know.

Cheers.

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yusynth



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PostPosted: Wed Jun 19, 2013 8:54 am    Post subject: Reply with quote  Mark this post and the followings unread

I assume that the upper trace on the scope is the output and the lower trace the input ?
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isak



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PostPosted: Wed Jun 19, 2013 9:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Indeed.
The lower is the source the upper is the output.
As you will see the freq counter is on the upper left of the scope screen.
As I get the freq higher the wave changes the shape.

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isak



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PostPosted: Thu Jun 20, 2013 2:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

yusynth wrote:
It is normal that a VCA with such a simple design add some distortion but this kind of distortion is weird indeed.
What is the input level ?


ok, ive checked the input V, its 5V when in 20khz.
its less when lowering the freq.
BTW... something strange, with my multimeter i didnt manage to measure on DC, only on AC mode, is that reasonable?

i use MFOS SVF self osc mode for sine wave.

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yusynth



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PostPosted: Sat Jun 22, 2013 12:35 am    Post subject: Reply with quote  Mark this post and the followings unread

You can measure the peak to peak voltage with your scope. Since you set the voltage range of the inputs (number of volts per division)
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yusynth



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PostPosted: Sat Jun 22, 2013 1:00 am    Post subject: Reply with quote  Mark this post and the followings unread

I checked on a yusynth VCA with a 19.3kHz input sinewave 20Vpp and I obtain (with the GAIN pot at maximum) a slightly rounded sinewave (normal distortion) and a perfect sinewave if I slightly reduce the GAIN pot level.

I think that may be you have a problem with your scope probe (not correctly set for high frequency). Check that there a small screw on your probe and turn it and check if that changes the shape on the scope.
Read the manual of your scope and go to the chapter dedicated to calibration of probes.

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isak



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PostPosted: Sat Jun 22, 2013 7:52 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks Yves.

i answered in the Arp post, i think the problem is related with both.

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