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Class D Switching Power Amplifiers and Power D/A conversion

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Old 4th September 2009, 08:28 PM   #11
Pafi is offline Pafi  Hungary
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Do you mean the output stage interferes with its own triangle wave? It's impossible, since they have the same freq, and their phases are locked together.

Whatever happen, if it is periodic with the same freq as the triangle wave, it can cause DC error only. But what you hear is not DC error! You have to search for some error source that hasn't the same freq!
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Old 5th September 2009, 12:17 PM   #12
nitrate is offline nitrate  United Kingdom
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Searching....
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Old 5th September 2009, 02:32 PM   #13
Pafi is offline Pafi  Hungary
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Have you tried turning off your PC?
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Old 5th September 2009, 05:05 PM   #14
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Lol, The problem is definately on board and not an external source of interference. Looking at the cd4093 wavforms on the scope it seems the timing circuit has a much better square coming out of it than the buffer does. Also the timing caps waveform is an excellent tri-wave compared to the one being purposly generated due to the lack of drive from the buffer. I think i may be forced to redesign the tri-wave, or go to TTL and add some extra supplies. Either that or i could feed the timing caps waveform into the comparator, maybe buffer it first with an op-amp... Hmmm..

I've spent all afternoon trying all sorts of power rail filtering, nothing seems to effect the whining noises. The only thing that makes a real difference is putting ones finger near the cd4093 or touching the FET bridge. Thats why i'm convinced it's the poor way i've generated my triangle, thus this circuit must go!

Leigh
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Old 6th September 2009, 11:42 AM   #15
Pafi is offline Pafi  Hungary
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You didn't tell anything convincing. I don't know why are you ingoring me.
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Old 6th September 2009, 11:51 AM   #16
nitrate is offline nitrate  United Kingdom
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Pafi,

I'm not ignoring you. I value all suggestions and input in trying to get this problem sorted. The most benign or obvious suggestions may just be the key to this as it really does seem impossible for this to be happening. I've took all your comments into consideration and really do apreciate them and hope they keep coming as you or anyone else may just think of somthing obvious that i havn't thought of or tried yet. So keep those ideas coming

In the mean time i'm gonna try an external clock source to see if that helps. If not i must be overlooking some other means of intermodulation.

Leigh
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Old 6th September 2009, 12:27 PM   #17
nitrate is offline nitrate  United Kingdom
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Hmm, interesting...
Using an external clean clock, the amp seems to 'Tune' at regular frequencys. Around 150KHz, 200KHz, 250KHz ( the target freq ) 300KHz, 350KHz, you get the picture. Must be the carrier clashing with harmonics of anougher carrier, but what carrier?? There is no other switching aparatus in the room. It must be generated on board. The switching fets are well snubbed and there is hardly any ringing on the scope so it cant be that. Anyone any ideas?? This is cooking my noodles now.

Dunno if anybodys experienced this before?

Leigh
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Old 6th September 2009, 06:17 PM   #18
Pafi is offline Pafi  Hungary
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Leigh!

Thank you for being open minded, but please answer my question (PC on/off)! Please attach an exact schematic (including the possibly unconnected gates)! If you use energy saving lamp, or halogen with SMPS, then switch them off! What is the power supply of the amp?

Attach a loop antenna to your scope to discover any EMI source!
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Old 7th September 2009, 08:56 AM   #19
nitrate is offline nitrate  United Kingdom
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There is no computer in the room i do my work in. The power supply of the amp is linear, simple transformer - bridge - C R C, then onto the amp PCB. The lighting in the room is natural daylight. My scope and all other test equipment is also linear in its supplies and operation. I'll post a schematic in a few days. I need time to lay it out but time is not on my side as i work 15 hour days.

Loop antenna shows no signs of emi at all. Just the large fet bridge waveform.

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Last edited by nitrate; 7th September 2009 at 08:57 AM. Reason: Typo..
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Old 7th September 2009, 03:48 PM   #20
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Quote:
Loop antenna shows no signs of emi at all. Just the large fet bridge waveform.
If you see your switching signal with a loop antenna then this is definitely EMI as well ! It just depends on what and how much damage it makes !

Regards

Charles
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