If they use an oscilloscope to verify the voltage and waveform, it can't be overstated.
Ah, of course, you are absolutely correct.
But they can be
unstated.
In other words, at what frequency, or frequency range, is the amplifier capable of delivering power, and at what THD limit will the amplifier never exceed
at any time during the frequency test? Is this limit just THD, or THD + Noise?
And how many channels are driven during the test? Is the full voltage from the power supply available to just the channel under test? That's not a realistic real-world operating condition, but it's a common test condition.
By the way, what is the power supply used for the test capable of delivering? If a product does not come with an integrated power supply (and a wall-wart is not an integrated power supply), you can use whatever you want for the test.
And so on.
We don't know for sure, but we do have some clues if we dig a bit.
The typical power rating for a Tripath TA-2024 in product literature, web advertising, or eBay auctions, is 15 watts per channel into 4 ohms. I realize that the product you linked to is not a TA-2024 based unit, but bear with me a bit.
For the most part, we have to take that for what it is. And it's not an inaccurate specification, if you are just a tiny bit generous with the THD.
Unless you get out the Distortion Analyzer and 'Scope hiding in your closet, you are pretty much stuck with that as the only info to go on.
But, Sonic Impact did briefly sell an "audiophile" version of it's T-Amp, the "Super T", which sold for $160 as a plug-and-play amplifier.
Sonic Impact rates their version at 11 watts/4 ohms @ 0.1% THD + N; 6 watts into 8 ohms, and 98 dB S/N. That is their entire specification; nothing more is available.
The beauty of this amp, is that Stereophile took the trouble to do a full review, including lab tests, on the Super T. I don't really care what your opinion of the magazine is, all I can say is I don't know of anyone else who has ever put one to a comprehensive set of tests, so we don't really have much choice here.
When under test by John Atkinson, this amplifier's 1 kHz power into 8 ohms begins to clip at just under 5 watts into 8 ohms; 7 watts into 4 ohms; and goes into thermal protection without clipping at about 12 watts into 2 ohms. If you ignore the knee of the THD+N graph and just use 1% THD+N as your limit, you can expect about 5.5 watts into 8 ohms and 9 watts into 4 ohms. *
With a 2.83V output (=1 watt into 8 ohms; 2 watts into 4 ohms) THD + N at 1 Khz, 8 ohms, is a respectable "just over" 0.1%. At 20 Khz, though, it's almost 1.5%. Like all Class D amps, supersonic distortion products continue to rise above 20 Khz. Meanwhile, at 100 Hz, it's only 0.06%. (All of these had to be read from a graph, so no arguing about 0.01%, please).
So, we have an chip amplifier that the Chinese (and Parts Express) say is 15 watts, but can really only put out half that cleanly, with the supplied 3A power supply that comes with the Super T. And no-one has tested the power bandwidth (Atkinson usually does, so it seems he just gave up or else decided to just keep quiet) so we don't know how much power falls at 20 or 20KHz.
A weak specification set can be accurate, but still tell us very little.
I'm not suggesting the D-class amps are somehow not worthwhile, I'm just saying that these boards are not, and don't have to be, sold as consumer items, they are just parts. There is no obligation to provide a complete specification to you. You are taking on the task of deciding the suitability, and the testing if necessary, upon yourself.
* The 3A power supply of the Super T as supplied may be a significant limiting factor, or it may not (in this case I think it is; I personally would suggest a 5A supply with a Tripath TA-2024). Perhaps even a 3.5A supply would have resulted in the Super T meeting it's published specification. I think the point, however, is that even though people do measure 15 W at 4 ohms with a TA-2024, without further defining the spec it's not particularly useful; you would still have to measure yourself under the conditions you intend to use the unit to get a more complete picture.
