mobanmarketWhen I joined Hi-Fi News in the mid-1970s, one of that magazine’s stable mates reviewed cars. An automotive writer appeared in the pub one lunchtime—”I rolled another one,” he said, as he joined us at the bar. It turned out that one of his tasks was to take a car he was testing to the skid pad to see how many lateral G’s the car could handle. Of course, the chances of a consumer turning that car over were minimal, but the reviewer was investigating the edges of the performance envelope.
As I became familiar with audio measurements, it struck me that the equivalent of the skid pan test was the thermal preconditioning we perform when we get an amplifier on the test bench. Even if an end-user doesn’t drive his amplifier into thermal meltdown, the edges of the envelope need to be explored.
This preconditioning is based on the FTC’s original recommendation in what came to be called the “Amplifier Rule (Title 16 Part 432),” which, at the end of 2020, the FTC proposed eliminating. The FTC regularly examines and revises its rules, and according to FTC commissioner Christine S. Wilson: “Freeing businesses from unnecessarily prescriptive requirements benefits consumers.” (Wilson’s comment can be downloaded here.)
“Unnecessarily prescriptive requirements?” Some history is in order. (My thanks to Jim Austin for the on-line research.)
In the hi-fi boom that began in the 1960s, the Institute of High Fidelity became alarmed by amplifier manufacturers exaggerating their products’ output power. Such mystical numbers as “Peak Power” and “Music Power” were used willy-nilly to produce sales-oriented ratings that had nothing to do with reality. The IHF’s proposed standard was examined in a paper presented at a 1967 Audio Engineering Society convention by Daniel von Recklinghausen, then with manufacturer H.H .Scott, and reprinted in the Journal of the Audio Engineering Society in July 1968.
The IHF proposal resulted in much discussion in audio engineering circles and in 1974, following open hearings, the FTC published its first Amplifier Rule. The purpose was to impose uniform, conservative standards on how domestic amplifier powers were rated and advertised.
The Amplifier Rule was revised in 2000. It imposed the following conditions on manufacturers:
• Power output is to be expressed as “average continuous power in watts.”
• Load impedance shall be 8 ohms or cited if otherwise.
• All associated channels must be driven at rated power. (This was later dropped for multichannel home-theater amplifiers, where only two channels needed to be driven.)
• Power bandwidth or power frequency response must be cited.
• Maximum THD from 0.25W to rated power must be cited.
• The amplifier must be preconditioned at 1/8 rated power for 60 minutes.
The latter is a modification of the original IHF requirement, which involved operating both channels of the amplifier at 1/3 power into 8 ohms for 60 minutes. With an amplifier that biases its output devices into class-AB, this results in the maximum dissipation in those devices, hence imposing the maximum thermal stress on the amplifier. The Consumer Electronics Association (CEA) was concerned about the impact of the original rule on multichannel home-theater amplifiers and in its own standard (CEA-490-A), changed the preconditioning operating time from one hour to 30 minutes.
Why was the CEA concerned? For an amplifier to pass the original 1/3 power preconditioning, its heatsinks need adequate thermal capacity. After an amplifier’s chassis and power transformer, the heatsink is the most expensive part, so underspecifying the heatsink is a way of reducing both the manufacturer’s bill of materials and the retail price. It is also fair to argue that in actual use, with music recordings that have a normal crest factor (the ratio between the peak and average levels) and loudspeakers with a sensitivity around 88dB/2.83V/m (the average of all the loudspeakers reviewed in Stereophile) an amplifier will never get close to being operated at 1/3 power for an hour. The consumer will therefore be paying for a capability that he will never need.
Even so, I have always felt the preconditioning information will be useful to the magazine’s readers, which is why we have been doing it since we started accompanying Stereophile‘s reviews with measurements in 1989 (footnote 1). For example, though there aren’t many true high-end class-AB amplifiers that fail this test, our publishing the temperature of the heatsinks and chassis does reveal amplifiers that should not be enclosed in a cabinet or stacked.
And in his recent book on amplifier design, Bob Cordell recommends that heatsinks should be designed so that their temperature shall not exceed 60°C when operated at 1/3 power into 8 ohms. “The FTC metric may seem conservative,” he writes on p.186, “but things get worse rapidly when the amplifier is called on to deliver its power into the many loudspeakers that exhibit much lower impedances than 8 ohms.”
I asked Cordell for his thoughts on the proposed ending of the FTC Amplifier Rule:
“My first reaction is that it is silly for them to be revisiting this much-needed rule. I think it is important to recognize that there are numerous facets to this rule, so repealing the whole rule because of one facet is not necessary. For example, the preconditioning rule is probably the one that bothers all the manufacturers the most, and yet has the least to do with sound quality, and has only modest relevance to quoting power honestly in average (‘RMS’ [footnote 2]) watts.
“That part of the rule could probably be loosened up without too much angst. But continuous average power over a short interval remains important. For example, there is little real-world reason to spec it in such a way that the heatsinks have that huge amount of time to reach nearly final temperature. I would be happy with 1/8 continuous average power over as little as 5 minutes.
“The gross abuses that took place before this rule (like ‘music power’) actually had little to do with heatsink capacity.”—Bob Cordell
Comments (footnote 3)? Has the Amplifier Rule outlived its relevance, particularly when so many amplifiers now use class-D output stages?
Footnote 1: We have recently been performing both the original preconditioning and the CEA’s 2008 revision. If the amplifier being tested passes the 30 minutes at 1/8 power into 8 ohms test without the heatsinks becoming very hot, I then operate it at 1/3 power into 8 ohms for 60 minutes. I monitor both heatsink temperature and the THD+noise percentage during this time to make sure that the amplifier is still operating correctly.—John Atkinson
Footnote 2: “RMS” is in quotes because there is no such thing as “RMS power.” This was the subject of discussion in the early 1970s—see, for example, “RMS Power: Fact or Fancy” by John Eargle and Bart Locanthi, J. Audio Eng. Soc., Vol.20, p.45 (Jan/Feb 1972). While the RMS (Root Mean Square) values of voltage and current waveforms can be calculated, when these are used to calculate power, the result is an average value.—John Atkinson
Footnote 3: The deadline for sending comments to the FTC on the proposed ending the Amplifier Rule is February 16, 2021. Guidelines for sending comments can be found here.—John Atkinson
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