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Old 25th November 2007, 04:05 AM   #1
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Unhappy Carrier frequency remains after the output filter

Hi !

I built a D amp, it sounds very nice, much better than I was expecting.

Well, althought it works fine, I can measure 1,7V peak to peak of switching waveform on the speaker output. The frequency is 325Khz.

Inductor is 22uH and capacitor is 680nF.

I don't know if it's related, but it heats considerably when IDLE, and just a little bit more when playing at full power. I was expecting it to be cold during idle period and slightly warm when playing.

Is it fine, or is my output filter faultly ?


Thank you very much !

Best Regards
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Old 25th November 2007, 05:32 AM   #2
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Default Re: Carrier frequency remains after the output filter

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Originally posted by -_nando-_
Well, althought it works fine, I can measure 1,7V peak to peak of switching waveform on the speaker output. The frequency is 325Khz.

Inductor is 22uH and capacitor is 680nF.

I don't know if it's related, but it heats considerably when IDLE, and just a little bit more when playing at full power. I was expecting it to be cold during idle period and slightly warm when playing.

Is it fine, or is my output filter faultly ?


Thank you very much !

Best Regards
You cannot eliminate carrier waveform completely, only thing you can do is attenuate it using filter.......so the 1.7pk residual is just fine.....nothing to worry.....

regarding heating at idle i think its a more shoot through or low deadtime problem......post some waveforms.....of output
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Old 25th November 2007, 06:05 AM   #3
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If you make selfoscilating UCD style, this residual is needed to keep the amp working. If you eliminate this, the amp won't work at all. But if you make classD with feedback before LC, its a different story.
I've tried to eliminate this residual by using another LC (after feedback point, that makes it 2 sets of LC), the residual is almost 0.
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Old 25th November 2007, 01:00 PM   #4
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The amp is a TDA8920BJ.
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File Type: png waveform_d1.png (72.4 KB, 328 views)
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Old 25th November 2007, 01:03 PM   #5
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Another picture.

This amp features "Zero dead time switching".
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Old 25th November 2007, 02:19 PM   #6
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Hi, Nando,

I've never built this IC, so I don't know about it.
From page 6 of the datasheet, it seems you better try to get about 500khz frequency (by adjusting Rosc?)
From page 10 schematic, the LC used is 100uH+390nF. You use 22uH+680nF. Could the problem is because of these 2 factors?
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Old 25th November 2007, 06:51 PM   #7
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Default Re: Re: Carrier frequency remains after the output filter

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Originally posted by Workhorse
You cannot eliminate carrier waveform completely, only thing you can do is attenuate it using filter
This is a little off topic, but there is a way to all but eliminate the residual without filtering. Instead of connecting the speaker to ground you can connect it to the output of a noninverting amplifier/buffer whose input signal is the residual. Since the residual voltage is relatively low a class ab amp with low rail voltages can be used so there is minimal power dissipation from this extra output stage. I believe I saw this method shown in a patent that was linked to or described somewhere in this forum. It was a while ago but it is an interesting way to solve the problem (even though it really isn't a problem).
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Old 25th November 2007, 09:15 PM   #8
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The residual is not a problem at all for speakers. Furthermore, residual cancellation methods (also class BD modulation) actually change the differential-mode residual into a more harmful common-mode thus resulting higher EMI.

In my opinion, the ideal approach would produce equal residuals in both output speaker wires, but with inverse polarity (for zero common mode component). A straight full bridge (no BD modulation) achieves this.


BTW: Nando, I think that your waveforms actually contain substantial RF ringing after each switching transient that is almost hidden because it seems to be outside the frequency response of your oscilloscope or measurement. Note how the carrier residual becomes noisy for a long time after each zero crossing (when switching takes place). If this RF is actually there, you will have still a lot of work to do to get a good amplifier...
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Old 25th November 2007, 09:59 PM   #9
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Hi Eva !

This is a very experimental through hole approach, the layout isn't that good and... Maybe is this RF is making my amp oscillate and get warmer than it should ?

There's absolutelly nothing special with the power supply, probably the noise is coming from there...

All the 100nF bypass caps are placed as close as possible of the pins, and connected directly to the massive ground. Actually its out of the layout and it is placed manually bellow the chipamp. They are polyester types, 63v epcos (the blue package). Would ceramic caps be better?
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Old 25th November 2007, 10:01 PM   #10
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Quote:
Originally posted by lumanauw
Hi, Nando,

I've never built this IC, so I don't know about it.
From page 6 of the datasheet, it seems you better try to get about 500khz frequency (by adjusting Rosc?)
From page 10 schematic, the LC used is 100uH+390nF. You use 22uH+680nF. Could the problem is because of these 2 factors?
You are reading a very old datasheet. Look at this:

http://www.nxp.com/acrobat/datasheets/TDA8920B_2.pdf
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