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numbertalk
Joined: May 05, 2008
Posts: 992
Location: Austin, TX
Audio files: 5
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 Posted: Sun Mar 01, 2009 8:19 pm Post subject:
high frequency oscillations on cgs04 d.c. mixer |
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| 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
Joined: Dec 29, 2004
Posts: 961
Location: Northeastern shore of Toledo Bend
Audio files: 29
G2 patch files: 54
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| Posted: Sun Mar 01, 2009 9:38 pm Post subject:
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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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varice |
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numbertalk
Joined: May 05, 2008
Posts: 992
Location: Austin, TX
Audio files: 5
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| Posted: Mon Mar 02, 2009 8:38 am Post subject:
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| 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
Joined: Dec 29, 2006
Posts: 1176
Location: australia
Audio files: 4
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| Posted: Mon Mar 02, 2009 3:33 pm Post subject:
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| 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
Posts: 992
Location: Austin, TX
Audio files: 5
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| Posted: Mon Mar 02, 2009 4:21 pm Post subject:
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| 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
Posts: 454
Location: London, UK
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| Posted: Sat Aug 14, 2010 5:58 pm Post subject:
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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
Joined: Dec 29, 2006
Posts: 1176
Location: australia
Audio files: 4
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| Posted: Sat Aug 14, 2010 10:20 pm Post subject:
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| 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?
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No! 
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
Joined: Mar 18, 2008
Posts: 454
Location: London, UK
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| Posted: Sun Aug 15, 2010 5:29 am Post subject:
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| Haha! Of course! That'll teach me to partake in late night soldering! |
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