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Class D Switching Power Amplifiers and Power D/A conversion

TDA7498 Ebay amp module
TDA7498 Ebay amp module
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Old 3rd August 2019, 12:37 AM   #1
billbo is offline billbo  Australia
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Default TDA7498 Ebay amp module

Just a basic question in relation to a class D amp module which I purchased from Ebay. The unit works quite well but when I scope the outputs I see that one channel clips a little earlier and asymmetricaly as well. This is at full output power into a PURELY RESISTIVE 8 ohm load. If I introduce a load which resembles a speaker i.e. 8 ohms at D.C. with a little reactance (resistive/inductive) both outputs left and right channel are identical. Clipping is symetrical and output power is the same. The only thing I can think of is that the quality of the filter inductors are not the same hence differences in output character between left and right channel. It doesn't affect anything since it happens at almost full output. Has any one seen this? Also what are your suggestions about this.
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Billy D...
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Old 3rd August 2019, 09:08 AM   #2
FauxFrench is offline FauxFrench  France
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How do you see the asymmetrical clipping? Two oscilliscope probes and subtraction mode?
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Old 3rd August 2019, 10:42 AM   #3
chermann is offline chermann  Austria
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Hi Billy


Here is a thread which i go through before i bought my tda7498 amp. you can inform you here:Ebay cheap TDA7498 boards


there is normally a 2R2 resistor made by these chines boards and these nearly at all boards. show us a pic from yours but i am sure you have the same:
see pic - grey left is the filter at some Chinese boards - right is the datasheet output filter...


this resistor could be the reason - as i remember it should avoid ringing in the filter but expert wrote that it is useless....so that could be that you have a unsymmetrical clipping..= minus of the output - short this resistors.. i put it out and solder a jumper - pic
why are channel L and R not the same? yes maybe the resistor are not the same and yes the filter are not equal.



as FF wrote be carefully use a scope at the output of an amp !- its not a DMM. so a differential probe is needed.


maybe it make sense to go ahead in the other thread...moderator has to decide.


chris
Attached Images
File Type: jpg mods ouput filter short 2R2.jpg (149.6 KB, 194 views)
File Type: jpg wrong filter and resistor should be shorted.JPG (39.0 KB, 194 views)

Last edited by chermann; 3rd August 2019 at 10:46 AM.
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Old 3rd August 2019, 02:41 PM   #4
billbo is offline billbo  Australia
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I have removed the 2R2 resistors already. Yes they were burning up and I was losing output power because of them. They seemed to be in series with the output. Here is a pic of the amp module in question.
Regards
Billy D...
Attached Images
File Type: jpg Screenshot_20190804-003207_eBay.jpg (142.5 KB, 178 views)
File Type: jpg Screenshot_20190804-003231_eBay.jpg (158.3 KB, 175 views)
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Old 4th August 2019, 11:25 AM   #5
chermann is offline chermann  Austria
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and the scope shows other graphs?
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Old 11th August 2019, 09:23 PM   #6
billbo is offline billbo  Australia
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I have managed to look at the cct board to check the component layout and found that the manufacturers installed a 220 ohm resistor instead of a 22 ohm resistor in the first stage RC filter on the speaker outputs. This is an error in the assembly where they have soldered a 221 resistor. I measured it to be 220 ohm. The data sheet shows it as 22 ohm. I have photos below of before and after rectification. Just letting everyone know.
Regards
Billy D...
Attached Images
File Type: jpg 20190812_071729.jpg (222.6 KB, 109 views)
File Type: jpg 20190811_115623.jpg (1,009.3 KB, 44 views)
File Type: jpg 20190804_084919.jpg (955.3 KB, 42 views)
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Old 12th August 2019, 06:34 AM   #7
chermann is offline chermann  Austria
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Thanks billy...good to know/re check

BTW.....its always needed to check the boards against the datahseet requirements
chris
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Old 12th August 2019, 01:38 PM   #8
Turbowatch2 is offline Turbowatch2  Germany
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The cheap TDA7948 chip without the "e" is only useable with more than 6 ohms load.

As most buyers donīt have a clue about such complicated stuff like impedance of speakers, these boards will fail in many cases (most?).

So the Chinese "developer", who does not care about sound quality, as he does not even have a pair of speakers, which could show such details, simply puts a 2.2 ohms resistor in the output. And presto! The amp runīs even with 4 ohm speakers.
Shortcut protection the Chinese way... throwing sound quality in the bin.

Best is to leave the TDA7498 alone and get the "e" version, which is an reliable, powerfull, well sounding amp.
I have the TDA7498"e" version with the small fan, which does not start, as the amp stays cold with normal use . So no cause to fear the turbine
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Old 12th August 2019, 10:35 PM   #9
billbo is offline billbo  Australia
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You right Turbowatch! The 2.2ohm is there for that purpose. With it in it does seem to be more stable. With the resistor bypassed only use 8 ohm speakers and even then with a 36v supply it is borderline functional.
Regards
Billy D...

Last edited by billbo; 12th August 2019 at 10:38 PM.
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