It is not one single signal to include all frequencies, it is a misunderstanding. Again, direct quotation says
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Rated power shall be obtainable at all frequencies means when driven at individual frequencies. Make a representative set like 20Hz, 100Hz, 1kHz, 5kHz, 10kHz, 20kHz and test on single frequencies from this set and on each of them you have to keep the rated power for 5 minutes. Good thermal and circuit design will make it. If not, derate the advertised rated power. We are in circles. Pink noise is fun for the amp under test. White noise is easy to pass as well, provided there is no clipping. More later, maybe.
Okay, that's reasonable, but that's not what it says.
As a person who occasionally writes government policy, this statement would never have made it through my agency's good guidance review.
Your selection of test frequencies is a sampling, not a comprehensive test. What if an amp goes into oscillation at, say, 17,432 Hz? That would make the amp not meet the requirement, but the test you outlined would have missed it. (John's chirps might have found it.) Is that likely? No. But it's also not likely that anyone can describe any remotely realistic use case that would require a 20 KHz sine wave at full rated power for five minutes, when five seconds might be enough to fry any tweeters on the market.
I get the commitment to truth in advertising, but what I've learned is that the whole concept of power output (in watts) depends so much on surrounding operating conditions that it's just about meaningless for making a purchase decision. And making reasonable purchase decisions is the ONLY reason the FTC has any standing at all. The U.S. has an intentionally weak federal government, limited by authorities specifically authorized by Congress or specifically enumerated in the Constitution. Everyone agrees with that except when they want the federal government to be the enforcer on the specific topic of interest to them.
I spent quite a lot of time trying to determine if the NC502MP (8-ohm power claimed to be "time-limited because of thermal properties" 350 wpc) was actually more powerful than my B&K Reference 125.2 amps (125 wpc into 8 ohms claimed). Would an FTC number have simplified that comparison? I don't think so, because the rule is so unrelated to my actual use case. There is no question that the NC502MP can produce significantly higher voltages without clipping caused by insufficient current to fill out the waveform at frequencies humans (especially this human) can hear, and least for the periods needed to demonstrate that and for periods relevant to playing music, but it took a lot of graph study to be sure . Thus, it will play music noticeably louder, and do so indefinitely, with lower distortion--not that the distortion of the B&K was ever a problem--because the peaks in music (even brickwalled music) are not indefinitely long. But suppose it can't produce 20 KHz (or 20 Hz) sine waves above 100 wpc for five minutes without shutting down, while the B&K can. And in response to that, they downrate the advertised power of the NC502MP to, say, the 100 watts they currently rate it at for continuous operation. And supposing the MOSFETs in the B&K can produce 125W at 20K (or 20) for five minutes, but has only 1.8 dB of headroom, so that they can't fill out the same tall waveforms for shorter periods. By that one measure, the B&K would be rated as more powerful than the 502, but--and here's the point Amir is been trying to make throughout--it would not play music louder and thus would not actually provide more power of the type that people actually want. The 502 can fill out peaks up to a 350 (ish) wpc envelope for a useful length of time, while the B&K can't do that higher than 185 wpc for any length of time. The only reason people care about power output is the ability to play real content louder, so in this example the FTC number would undermine an appropriate market choice, not enhance it.
Back in 1974, the enthusiast market simply lacked the understanding that playing a sine wave for five minutes is a different dimension than the ability to fill out waveforms that are far shorter but still relevant to actual use cases. And that ambiguity made it possible for manufacturers to BS their customers, because nobody outside of (a few) magazines--which covered a miniscule sample of the marketplace--really tested stuff and even they didn't test this. Also, the amps most people could afford weren't really powerful enough for their rock-music-college-dorm-party use cases. So, the FTC stepped in with an arbitrary measure just to nail something down, but even then it I would bet a medium-sized Chinese feast that it was driven by U.S. manufacturers trying to protect their design conventions as the industry was transitioning from vacuum tubes to solid state, but once it was there everyone designed to it (rather than to the use cases of interest to users). Now that most amps are already loud enough for most consumers, I just don't understand why the FTC has a compelling interest to regulate the market, particularly using a dimension only approximately related to the one actually required by consumer use cases.
Rick "yes, the 502MP makes music
noticeably louder than the B&K" Denney
sound-au.com
