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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
Synthacon VCF - 035 -
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Lorenzo



Joined: Nov 09, 2008
Posts: 375
Location: Trieste - Italy

PostPosted: Thu May 28, 2009 5:30 am    Post subject: Synthacon VCF - 035 -
Subject description: PROBLEMS
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hallo,
I just finished to build my Synthacon VCF.
I'm quite very new of DIY.
This is the first time i built something like this... I was very exited...
but... maybe something is wrong:
1. resonance make crackle noise when used!
2. the filter have not emphasis not even character, it is like e very common EQ setting.
3. there are volume differences for the different inputs, but it is the last problem...

I try the Synthacon with the audio ouput of a Waldorf Pulse
I thought I didn't made mistakes populating the pcb...
there are not any shorts between solderings
I used 2n2 polystyrene cap for these 3 caps
I matched all 2N2222 transistors
but...

I'm very frustrated!!!!!!!!!!!!! Sad

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zthee



Joined: Feb 20, 2008
Posts: 414
Location: Stockholm

PostPosted: Thu May 28, 2009 7:34 am    Post subject: Reply with quote  Mark this post and the followings unread

1, There's some info in the cgs_synth group on yahoo in that matter. The problem seems to be that DC gets into the pot. I can't really remember, but you have to put a cap in parallel with the pot to make it go away. Mine was all scratchy at first as well, but doing the mod fixed it!

3, From the CGS site: If you have inadequate output from the filter, try increasing the value of the feedback resistor of the op-amp.

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slacker



Joined: Nov 18, 2007
Posts: 301
Location: England
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PostPosted: Thu May 28, 2009 10:10 am    Post subject: Reply with quote  Mark this post and the followings unread

I don't know if it's the same mod zthee is talking about but I posted this on the CGS mailing list, it gets rid of the crackle. I think it was my only ever post.

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

I can't remember what value the capacitor was, probably 10uf or so. Ignore the component values they're not part of the mod.
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Lorenzo



Joined: Nov 09, 2008
Posts: 375
Location: Trieste - Italy

PostPosted: Thu May 28, 2009 11:40 am    Post subject: Reply with quote  Mark this post and the followings unread

zthee wrote:
1, There's some info in the cgs_synth group on yahoo in that matter. The problem seems to be that DC gets into the pot. I can't really remember, but you have to put a cap in parallel with the pot to make it go away. Mine was all scratchy at first as well, but doing the mod fixed it!





I found on yahoo group this answer by Ken Stone:
The solution that supposedly works is to wire the 1k pot outer terminals
into the circuit instead of an outer and the wiper. One then adds a
capacitor between the wiper and one end. The result is that the DC passes
through the 1k as required, and the wiper only ever sees AC.

The other solution is to just accept it as a feature of the design.
Ken


Sorry but I don't know what "wiper" is meaning Embarassed
"...outer and the wiper...wiper and one end..." Embarassed
Some one could explaing to me, please? Embarassed

Laurie Biddulph (Elby Designs)
Anyway, at one end of the board is a resistor and capacitor (R210
and C207). Turn the board so that they are nearest to you and the pots are
on your left. You need to desolder the right-hand leg of the resistor and
the left-hand leg of the capacitor and then join these 2 legs together in
mid air. This should resolve the noise problem.


I will tray this...

other answer:
Takada's variant
has a slightly different resonance circuit, with a 68uF in series with the
pot, and a fixed resistor across both to deal with the DC gain requirements.
Perhaps insert a 100uF cap between the existing 1k/1k pot pair (+ve towards
the pnp), then add a 2k2 resistor in parallel with the lot.


Quote:

3, From the CGS site: If you have inadequate output from the filter, try increasing the value of the feedback resistor of the op-amp.


ah... ok... I think I have to had a trim for output were it is wright "level" too!
For increase the ouput level, that seems to be low...


Thank you!
Thank slacker too for the .png!

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slacker



Joined: Nov 18, 2007
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PostPosted: Thu May 28, 2009 12:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

The answer by Ken is the same thing my picture is of, if that helps.

The wiper is the middle connection on a pot. So on this layout from Ken's site http://www.cgs.synth.net/modules/pic/wire_cgs35_syntha_vcf_r1.gif you remove the the wire from the middle connection of the resonance pot and connect it to the left hand side connection instead. Then solder the capacitor between the middle connection and one of the outside ones.
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Lorenzo



Joined: Nov 09, 2008
Posts: 375
Location: Trieste - Italy

PostPosted: Fri May 29, 2009 1:13 am    Post subject: Reply with quote  Mark this post and the followings unread

slacker wrote:
The answer by Ken is the same thing my picture is of, if that helps.

The wiper is the middle connection on a pot. So on this layout from Ken's site http://www.cgs.synth.net/modules/pic/wire_cgs35_syntha_vcf_r1.gif you remove the the wire from the middle connection of the resonance pot and connect it to the left hand side connection instead. Then solder the capacitor between the middle connection and one of the outside ones.


Ok!
thank you for your... "translation"! Wink

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