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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
high frequency oscillations on cgs04 d.c. mixer
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numbertalk



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PostPosted: Sun Mar 01, 2009 8:19 pm    Post subject: high frequency oscillations on cgs04 d.c. mixer Reply with quote  Mark this post and the followings unread

I think I'm getting high frequency oscillations with a every CGS04 mixer board I try to build - http://www.cgs.synth.net/modules/cgs04_mix.html - I think so because I'm getting a "band" in the scope signal for the waveforms I pass though the circuit (fyi I'm passing in a single waveform, using the boards as individual waveform amplifiers). Oddly enough the band clears up into a nice solid waveform when I am touching the tip of the output jack with my finger. I noticed that though there are power decoupling caps there wasn't a place on the board for ferrite beads or resistors so I don't have these in place for either the positive or the negative rails - would running the power through ferrite beads take care of this?
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varice



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PostPosted: Sun Mar 01, 2009 9:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

You should add the caps as suggested by the first note at the bottom of the page that you linked. That will probably solve the problem.
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numbertalk



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PostPosted: Mon Mar 02, 2009 8:38 am    Post subject: Reply with quote  Mark this post and the followings unread

varice wrote:
You should add the caps as suggested by the first note at the bottom of the page that you linked. That will probably solve the problem.


*slaps forehead*

I got stuck on the part earlier where Ken mentions putting a cap between pins 1 & 2 and he provides pads for this but not pads for one between 6 & 7. But yeah, there is that 2nd op amp and I even had seen that's where the signal was starting to get unstable. This is definitey the fix - I touch a cap between these pins on the IC and I get a nice stable waveform. Guess I have to solder it on top of the board to the feedback resistor since there aren't pads provided.

Thanks for pointing this out to me!
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andrewF



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PostPosted: Mon Mar 02, 2009 3:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

If you are putting a cap on the 2nd opamp stage, don't do it across the feedback resistor, as that connects pin6 to the output. Solder it across pins 6&7. You could do one pin of the cap to 100k and the other to 1k - the ends closest to pin8.
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numbertalk



Joined: May 05, 2008
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PostPosted: Mon Mar 02, 2009 4:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

andrewF wrote:
If you are putting a cap on the 2nd opamp stage, don't do it across the feedback resistor, as that connects pin6 to the output. Solder it across pins 6&7. You could do one pin of the cap to 100k and the other to 1k - the ends closest to pin8.


Ah, yes, I see what you mean. I already soldered caps across the 100K resistors on my boards and the waveforms are coming out looking nice & stable but maybe there is still some HFO activity (or the risk of it) and I could very easily snip the one lead from the 100k and bend it over and solder to the 1K instead.

Thanks!
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LetterBeacon



Joined: Mar 18, 2008
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PostPosted: Sat Aug 14, 2010 5:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've just finished two of these wired in the same way Fonik has done his here.

I'm not getting high frequency oscillations but the audio I put through it suddenly cuts out when I start moving the pots.

I've replaced the resistor between A and B with a pot so I soldered in a 47pf cap between there too, although I missed the note saying it should be done between pins 6 and 7 too. Do you think adding in the cap between 6 and 7 might be the answer? I would try it now but it's two in the morning here!

Also, just wanted to double check the wiring of the pot between pads A and B. If you're looking at the pot from behind, I have the left lug going to ground, middle to point A and right to point B, does that sound right to you?

Thanks a lot!
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andrewF



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PostPosted: Sat Aug 14, 2010 10:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:


Also, just wanted to double check the wiring of the pot between pads A and B. If you're looking at the pot from behind, I have the left lug going to ground, middle to point A and right to point B, does that sound right to you?



No! Smile
remove the connection to ground!
This pot is connected as a rheostsat (variable resistor)
when you have one pin to ground it becomes a voltage divider.
wikipedia explains it well enough - http://en.wikipedia.org/wiki/Potentiometer
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LetterBeacon



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PostPosted: Sun Aug 15, 2010 5:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Haha! Of course! That'll teach me to partake in late night soldering!
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