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 Forum index » DIY Hardware and Software
Ray Wilson VCO trimming
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211



Joined: Jul 18, 2006
Posts: 50
Location: Marseille - France

PostPosted: Tue Jul 18, 2006 2:41 am    Post subject: Ray Wilson VCO trimming Reply with quote  Mark this post and the followings unread

First message for me here, so, Hi all Very Happy

I got a growing up DIY modular with several common modules (Korg late MS 20 vcf , bergfotron minimoog vcf, ken stone synthacon, papareil ADSR and some EFM modules.. )

I've built 2 Ray Wilson VCO (REV 1 -> http://www.musicfromouterspace.com/analogsynth/VCO200503REV01/VCO200503REV01.html ). I made all the suggested enhancements ( OPA2134 opamp, 2N4391 FET.. ). V/oct trimming is really good, but I experienced some Offsets problems.

On one VCO, I noticed (with my sound card, as I don't have got a scope) when the Tuning pot is turned to produce Low frequencies, the waves square/tri peaks are more in the "negative domain", and raise up as long as I tune the VCO Up ..

The waves peaks are not well centered on all the range.. I suspect my vca don't like too much that, and makes it clipping at high amplitude..

Does a builder experienced that ? Is it a bad "R7" trimmer adjustement ?

thanks for your help !
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zipzap



Joined: Nov 22, 2005
Posts: 559
Location: germany
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PostPosted: Tue Jul 18, 2006 2:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Hello
I got a growing up DIY modular with several common modules (Korg late MS 20 vcf , bergfotron minimoog vcf, ken stone synthacon, papareil ADSR and some EFM modules.. )

That´s nice.
You´r soundcard is propably ac-coupled, so it´s not really able to display any offset.
I think you can do two things: Get a scope or use your ears. If those trimmers are just very a little out of the region, you might not even notice by ear, if there is clipping inside the vco you definately will. EG, if you trim r5 till you see and hear a triangle (as few overtones as you can get) you can be sure that your saw out is centered exactly around ground. So it´s ok to set them the way you like to hear them, and of course you can check the actual waveform with your soundcard, just not the offset.
If the offset worries you you can add cabs to the vco outs.
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211



Joined: Jul 18, 2006
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Location: Marseille - France

PostPosted: Tue Jul 18, 2006 3:40 am    Post subject: Reply with quote  Mark this post and the followings unread

hi zipzap,


Yes this setup begins too be very nice, unfortunatly, I pay today errors of my first modules, when I wasn't experienced Smile

I'm ok with you, about the soundcard, but here is what I see with a soft-scope:

Here is a quick drawing, It's very exaggerated, but it show well what I see..

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



Joined: Dec 10, 2005
Posts: 766
Location: oakland, ca
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PostPosted: Tue Jul 18, 2006 3:47 am    Post subject: Reply with quote  Mark this post and the followings unread

looks like an ac-coupled square wave
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211



Joined: Jul 18, 2006
Posts: 50
Location: Marseille - France

PostPosted: Tue Jul 18, 2006 3:51 am    Post subject: Reply with quote  Mark this post and the followings unread

yes exactly, It's an AC coupled square, but as shown on the second drawing, the spikes move as long as the frequency goes up.
Shoudn't it be as the first drawing ?
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toppobrillo



Joined: Dec 10, 2005
Posts: 766
Location: oakland, ca
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PostPosted: Tue Jul 18, 2006 11:00 am    Post subject: Reply with quote  Mark this post and the followings unread

well, yes, but this isn't because of your vco, its your scope i guess. like zipzap said, in ac mode, it will not show dc offsets and any ac signal will be centered around 0.
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mosc
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PostPosted: Tue Jul 18, 2006 11:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Notice what's happening in those drawings. In the bottom drawing, the signal is bocoming distorted - the wave form is changing to have higher and higher peaks.
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