Is there any standard about test condition of amp's output power?

[JamesWang]

Hello,I'm a novice of class D amplifier.
I have been DIYing a class D amp board based on TDA7498.
From the datasheet,I learned that it outputs 100W power on the condition that
6 ohm,THD+N 10%,36V DC power supply.I'm wondering more these conditions.
First, is there any less complicated methods to discern the THD+N 10% situation, just from the oscilloscope screen?
Second, is there any standard about test condition of amp's nominal output power? I mean the amp board.As we know, audio amp can't always output the nominal max power,or it will be damaged by overheat.

I'm looking forward to the skilled practitioner's help.
Thanks

[bear]

What do you want to learn about your amp?

You can view the distortion waveform by using the scope in "ADD" and invert one channel. One channel is the input sinewave and the other channel is the output waveform.

You adjust the gain of the vertical amps until you get the "best null" and that corresponds more or less to the waveform of the distortion. If it is 10% of the full output,that is quite a bit of signal...

There are a variety of tests that can be used to determine how well an amp is working, including looking at the current & voltage vs. load, and one could include a reactive load if you are fearless... but you might not like what you see...

I'd not worry about it, unless there is a reason to verify the performance. Just crank it up to full power into a load (6 or 8 ohm) and see that it makes the requisite waveform up to clipping.

_-_-bear

[JamesWang]

Thanks for your help.
In fact, I'm facing up with a problem about output power now.
The current test phenomenon is that individual channel is capable of outputing 83W RMS or so.I use a 40V DC SWPS and 2 7.5 ohm resistant loads so that 83W is derived by 25*25/7.5.
While the max out power of each channel reduced down to 38W RMS when both of channels worked.
When approaching that boundary state, the coil sounds continuously and the output waveform turns back to a straight line intermittently.
The deficiency of power supply loop layout has been ruled out by adding additional power wire experiment.
And I've choose the coil with 6A saturation current.
Have you meet with the similar problem before?
Hoping for some suggestions sincerely .

[digits]

Salespeople have no standards

[Eva]

Distortion rises very quickly when an amplifier clips, so the difference in power between 0.1% THD and 10% THD is not substantial in any good amplifier, maybe just 1dB or 2dB. Also, THD involves testing with continuous tones, which usually results in more power supply sag than when playing music.

However, a bad class D amplifier may as well exhibit 1% THD or more well below clipping. Giving the power output at 10% THD hides this phenomena.

[phase_accurate]

Are you sure your SMPS can deliver enough power to feed two of your amps continuously ?

Regards

Charles

[JamesWang]

Yes.The rating output power of the SWPS is 600W.
I'm doubting one of the self-protection functions has always been activated.
I'll collect more experimental data including pins' voltage and output waveform.
I guess there may be solutions inside many clues above.

[JamesWang]

Hello,it's been several weeks before I posted the threads lastly.
And my TDA7498 amp board still can't reach its max power as the datasheet described.
It always was activated by its self-protection since the diagnostic pin output the positive pluses.
I've fed the board with 40V and mounted a heatsink with a fan on the slug of the chip.
So it is not the UVP (under voltage protection) and OTP(Over temperature protection) that lead to the intermittent output power.
Besides that,OVP(Over Voltage Protection) has also been excluded because the waveform from the PVCC pins haven't reach the absolute maximum ratings 44V.
At last,it seems that some unaccountable reason aroused the OCP(Over Current Protection)
The typical Iovp from the datasheet is 6A.
While the real current never reached more than 5A(20V/6.7ohm=2.9A RMS).
In fact,I am doubting there's something unknown that limited the maximum output current.
Another potential reason is the saturation current of the coil.I've already replace it with
an alternative one whose Isat is nearly 9A.
Looking forward to help.

[bear]

How are you measuring the 44v? Or as close as you come?

You need a scope - since that pin likely looks at *peak* not RMS or average level!



_-_-bear

[authlxl]

We usually meausure rated power for one amp with sinewave signal(1Khz for full range amp),sustained signal added until over tempreture protection or over 5 minutes.
We usualy make amp burn-in one by one channel , full power 20minutes or half power 24 hours.
IEEE suggest EIAJ standard.we call it burst mode.