mobanmarketAt the end of 2020, the Federal Trade Commission proposed eliminating what had come to be known as the “Amplifier Rule,” which had been in effect since 1974. Then-FTC commissioner Christine S. Wilson wrote, “Freeing businesses from unnecessarily prescriptive requirements benefits consumers.”
To me, that made no sense. Far from imposing “unnecessarily prescriptive requirements” on amplifier manufacturers, the Amplifier Rule had long forced manufacturers to clean up their acts.
As I wrote in an article published on the Stereophile website in 2021, 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 with little to do with reality (footnote 1). 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 original Amplifier Rule was revised in 2000, imposing the following conditions on manufacturers:
• Power output was to be expressed as “average continuous power in watts.”
• Load impedance should be 8 ohms.
• All associated channels must be driven while the measurement was made. (This requirement was later dropped for multi-channel home-theater amplifiers, where only the main L and R channels needed to be driven.)
• The power bandwidth or power frequency response must be cited.
• The maximum THD from 0.25W to the rated power must be cited.
• The amplifier must be preconditioned with a 1kHz sinewave at 1/8 rated power for 60 minutes before measuring. This was a revision from the original requirement, which mandated preconditioning at 1/3 power, a level that results in the maximum heat dissipation in a class-AB amplifier’s output devices, for 60 minutes. Concerned about the impact of the original rule on multichannel home-theater amplifiers, the Consumer Electronics Association (CEA) changed the preconditioning time from one hour to 30 minutes in its own standard (CEA-490-A). The FTC adopted the 30 minutes requirement in 2008.
The FTC invited public comments for those concerned about the effect of eliminating the Amplifier Rule. Some 550 people responded. One of those comments, submitted by Ralph Karsten, the founder of amplifier manufacturer Atma-Sphere and for many years the main man there, summed up my own feelings: “Back in 1974 when this rule was enacted it transformed the Wild West of amplifier sales to an even playing field. It makes no sense to stop now.” I made a similar point in a telephone conversation with the FTC’s Jock K. Chung in May 2021, in which I described amplifier power testing in detail.
After evaluating the comments received, in July 2022 the commission issued a notice of proposed rulemaking and sought additional comments about its proposals (footnote 2), which would standardize certain test conditions for measuring amplifier power output, and on the parameters of consumers’ normal use of multichannel home theater amplifiers. Then, in July 2023, the agency issued a supplemental notice of proposed rulemaking (SNPRM) for comment. Finally, in July 2024, the FTC published what it called the “Final Amendments to Amplifier Rule to Make Testing Methods More Useful to Consumers.”
The new rule does not require amplifier manufacturers to disclose maximum power output—but those that do must follow the new standards. The revisions are listed in a pdf entitled “Part 432—Power Output Claims for Amplifiers Utilized in Home Entertainment Products.” The preconditioning period is specified: 1/8 power with an 8 ohm load for one hour. In a stereo amplifier, both channels must be driven. Most significantly, after the input signal has been continuously applied at full rated power for at least five minutes, the amplifier needs to be able to operate “at any power level from 250mW to the rated power at all frequencies within the rated power band of 20Hz to 20kHz without exceeding 1.0% of total harmonic distortion plus noise (THD+N) at an impedance of 8 ohms.” In simplest terms, the maximum rated power is that at which the amplifier’s output does not exceed 1% at any audioband frequency.
As a practical matter, for most amplifiers, maximum output power will be determined by a 20kHz power measurement instead of a 1kHz measurement.
In a typical amplifier circuit, the open-loop bandwidth—the bandwidth before negative feedback is applied—can be as low as a few hundred Hz (footnote 3). The amount of corrective feedback available gets smaller as the frequency rises, so at 1% THD+N an amplifier is likely to deliver a lower maximum power—often much lower—at 20kHz than it is capable of delivering at low and middle frequencies.
Manufacturers that continue to report maximum power output must comply with this new rule, and the new specification must be presented prominently. The manufacturer may, in addition, present power measurements that do not comply with the Amplifier Rule, but such specifications must be accompanied with, “This rating does not meet the FTC standard.”
The new Amplifier Rule took effect in August 2024—about the time this issue was going to press (though it may take a while for manufacturers to adjust their specifications). Traditionally, Stereophile has measured maximum output power at 1% THD+N at 1kHz. Going forward, to accord with the new FTC standard, we will add measurements at 20Hz and 20kHz into 8 ohms.
Props to the FTC for not only keeping the Amplifier Rule but even making it tougher.
Footnote 1: I gave an example of this exaggeration in an article I wrote in 1989 in which I both discussed the FTC’s Amplifier Rule and announced that Stereophile was to include measurements in its reviews. “Running from, say, a 14V car supply, a typical single-ended amplifier can swing around 12V peak–peak into a 4-ohm loudspeaker. This voltage is equivalent to an RMS voltage of 12/(2×root 2) = 4.24V RMS. This is the DC voltage equivalent to that peak-peak AC voltage when considered as a source for resistive heating. By Ohm’s Law, into a 4-ohm load, that 4.24 RMS voltage will raise 4.24×4.24/4 W, ie, just 4.5W! I know, I know, you’re not going to sell many 4.5W amplifiers come crank-it-up season, so let’s use the power that would be generated if that peak-peak voltage were, instead, an RMS figure . . . That gives, let’s think up a fancy label, a “Maximum Peak Burst Power” rating, for example—no, replace the word “Burst,” it implies that the amp can’t keep it up—a “Maximum Peak Music Power” of 36W!”
Footnote 2: One of those comments included several references to Stereophile‘s amplifier measurements.
Footnote 3: See fig.3 here.