TDA7498 Ebay amp module

[billbo]

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.
Regards
Billy D...

 

[FauxFrench]

How do you see the asymmetrical clipping? Two oscilliscope probes and subtraction mode?

 

[chermann]

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

 

[billbo]

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...

 

[chermann]

and the scope shows other graphs?

 

[billbo]

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...

 

[chermann]

Thanks billy...good to know/re check

BTW.....its always needed to check the boards against the datahseet requirements
chris

 

[Turbowatch2]

The cheap TDA7948 chip without the "e" is only useable with more than 6 ohms load.

As most buyers don´t 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 run´s 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

 

[billbo]

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...