TDA7498 ebay strange output levels

[TroelsM]

TDA7498 ebay strange output levels [Problem solved]

Hi

I've been building an active speaker with a TDA7498 amp from ebay:

http://www.diyaudio.com/forums/multi-way/271951-el-cheapo-active-loudspeaker-5.html

The amp-module is the cheapest one from ebay:
100W High Power 100W Class D Amplifier Board DC20V to DC36V TDA7498 | eBay

The amp works, but measuring the output levels of the bridges I got some odd results.

PSU was approx 32V (lab psu, stable). One half of each bridge did approx 25Vpp into 8Ohm without visible distortion on a scope, but the other half does only approx 15Vpp at the same time.

This goes for both channels on the board and for two boards purchased 6weeks apart.

Is this normal operation for a TDA7498?

EDIT: Problem is solved. See page 2.

 

[Dinithm]

Hi

I've been building an active speaker with a TDA7498 amp from ebay:

http://www.diyaudio.com/forums/multi-way/271951-el-cheapo-active-loudspeaker-5.html

The amp-module is the cheapest one from ebay:
100W High Power 100W Class D Amplifier Board DC20V to DC36V TDA7498 | eBay

The amp works, but measuring the output levels of the bridges I got some odd results.

PSU was approx 32V (lab psu, stable). One half of each bridge did approx 25Vpp into 8Ohm without visible distortion on a scope, but the other half does only approx 15Vpp at the same time.

This goes for both channels on the board and for two boards purchased 6weeks apart.

Is this normal operation for a TDA7498?

yes it is,,,,
just google full bridge configuration(full bridge amplifier,H bridge)

 

[JonSnell Electronic]

A lot of Bridged class D amplifiers use virtual ground. That will confuse most readings!

 

[TroelsM]

Hi.

Thanks for the answers.

I understand how a bridge-amp works and the amp-board in question do indeed use a virtual GND. Input is referred to Vss.

But that stilll dosnt explain my findings. No matter what reference-point we use, the two halves of a bridge should output about the same level PP. The two signal are 180 deg out of phase, but approx the same level pp. Otherwise one half will clip before the other and output-level becomes lower than expected.

 

[doctormord]

You may check for the bootstrap voltages.

 

[Michael Bean]

I once saw a sign that said..."Our work is Good, Fast and Cheap, you may choose any two; Good and Fast won't be Cheap, Good and Cheap won't be Fast, and Cheap and Fast won't be Good." Aparently you chose Fast and Cheap.

Mike

 

[nigelwright7557]

Might be worth measuring virtual ground to each rail 0v and 35v.
If that is not the same you will get same error on output.

 

[TroelsM]

@Doctormord: If the bootstrap-voltages was sagging, then both parts of the bridge should be equally effected yes?

@Mike: Cheap; yes. Fast? Not so much. I'm not blaming the seller. Just being curious and trying to improve. Part of the Ebay-thing is getting the parts to actually work .
Do you have any input on the OT?

@Nigel: If the virtual GND was not at the correct potential, then both parts of the bridge should be affected?

Kind regards TroelsM

 

[doctormord]

@Doctormord: If the bootstrap-voltages was sagging, then both parts of the bridge should be equally effected yes?

Would guess, theres something wrong with the caps at volume level. Have a measure for the caps in the working brigde compared to the not working one and see if they capacitance differs.

Quick and easy, change all bs-caps to known working ones with the right capacitance.

 

[Michael Bean]

@Doctormord: If the bootstrap-voltages was sagging, then both parts of the bridge should be equally effected yes?

@Mike: Cheap; yes. Fast? Not so much. I'm not blaming the seller. Just being curious and trying to improve. Part of the Ebay-thing is getting the parts to actually work .
Do you have any input on the OT?

@Nigel: If the virtual GND was not at the correct potential, then both parts of the bridge should be affected?

Kind regards TroelsM

OK, My point is that if you buy something that is TOO inexpensive for what it claims to be, either it is of inferior quality, design or was derived from slave labor. I dislike all three possibilities.

Mike

 

[radiosmuck]

OK, My point is that if you buy something that is TOO inexpensive for what it claims to be, either it is of inferior quality, design or was derived from slave labor. I dislike all three possibilities.

Mike

Would the minimum wage in the US qualify as slave labour?

 

[Lo_Tse]

OK, My point is that if you buy something that is TOO inexpensive for what it claims to be, either it is of inferior quality, design or was derived from slave labor. I dislike all three possibilities.

Mike

It is a personal choice. No one should force you to change and you should not impress your opinion on others, either directly or indirectly.

Let's stay on the topic.

Regards,

 

[doctormord]

Any news on this?

 

[TroelsM]

I found the error. It was more stupid and obvious than I had expected.

In this thread there is some discussions about these modules:
http://www.diyaudio.com/forums/class-d/265387-ebay-cheap-tda7498-boards-9.html

It turns out that the manufacturer have implemented the RC-damper in the output very poorly. Wrong actually. - or at least in a way I dont understand.
The 2.2Ohm resistor is NOT part of the RC, but in series with the output....

This explains the asymmetric output voltages and the lack of bass that some people have reported.

So 2 solutions: 1)Short out the 2.2 resistor. 2) pull out the capacitor and the resistor and implement the the RC correct. Both versions work. Approx the same residual noise on the output.

TroelsM

 

[Mark2727]

Input and ground hookup

Hello,

I just got this very amp written about here.

When facing the front/volume pot there is a jack for the inputs.

Are the lugs (from left to right).... Left RCA, ground of the RCA's, Right RCA?

I searched around all the eBay sellers, and don't see a diagram.

Thanks in advance whoever helps me,

Mark

 

[Jerms]

On my TPA3116 it's RED/BLACK/WHITE.... Middle pin is ground
Some of the eBay pictures seem to show GND being the middle terminal.

The connector lead was supplied with the TPA3116 dual chip.

If you need leads or connectors, plenty of options on eBay. They are JST XH connectors. "Extension" cables all over eBay for "2S" RC battery packs.

 

[Mark2727]

Thanks Jerms,

I'm fiddling with the tpa7498 right now. I took the plunge and decided to try it. And the leads on that jack would indeed be as I guessed. When looking at the front/volume pot the jack leads or lugs from left to right will connect as follows:

Left RCA, middle is the ground, then Right RCA

Thanks for the tip on what those connector wires are known as. I could use some more of them. Now I know what to punch in the search bar on eBay.

Your help is appreciated!
Happy Sunday,
Mark

 

[Mark2727]

I got one of the tda7498 amps, am listening to it now in fact. And I really like it.

I'm using a 30vdc Cincon Electronics brand brick type supply, 3.3 amps.

Question: Could anyone point me towards a good, reasonably priced similar supply just in case this one should ever fail? I've searched Google, e-Bay, etc. and they seem difficult to locate.

Thanks in advance,
Mark

 

[majorwedgy]

take a look at printer power supplies, have just ordered an old HP PSU rated at 32v 2A, kodak did one at 36v 1.8A, sure there are plenty of others and they are CHEAP and possibly even free if you can find someone binning an old inkjet