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CEA 490-A Consumer Electronics Association Standard Test Methods of Measurements for Audio Amplifiers

Primary Rating & Disclosure For power amplifiers, integrated amplifiers, tuner/receivers (or such section), the primary ratings shall be: a) Power Output b) Frequency Response c) Input Impedance d) Total Harmonic Distortion (THD) The primary rating should be disclosed in the format indicated in the following examples: a) Monaural Single channel:120 watts RMS at 8 ohms, 1 kHz, and 1% THD b) Stereo Two channels: 100 watts RMS per channel at 8 ohms, 1 kHz, and 1% THD c) Multichannel i) Equal power for all channels Example: Five channels 125 watts RMS per channel at 8 ohms, 1 kHz, and 1% THD ii) Unequal power among all channels Example: Five channels 80 watts RMS per front chanel (L, C, R), 40 watts RMS per rear channel (L,R) - at 8 ohms, 1 kHz, and 1% THD In each instance, the following information should be included with the primary rating disclosure: at 8 ohms,1 kHz, and 1% THD.

What to Look For: The Good In the absence of ironclad laws governing how receiver, amplifier, and HTiB manufacturers define power output capabilities, there are a few telltale signs to look for that can help disclose what's going on with a specific product. One is a power rating for all-channels driven, a spec that lets you know how much power an amp can deliver with all five, six, or seven channels engaged, as opposed to a single channel. This spec, when available, doesn't exactly tell the whole story - in the real world, program material that engages multiple channels at full or near-full power simultaneously is rare. (Surround sound pipe-organ music, anyone?) But with the soundtracks of major movie-studio releases tending more toward the manic with each passing year, it's good to know that your system can handle a full multichannel assault. For that reason, it's also good if manufacturers provide single-channel, stereo, and all-channels-driven power ratings in their specs.

Another thing to look for when comparing amps or receivers is the load used for conducting power measurements. This number, which gets specified in ohms, is used to represent the speaker load presented to the amp's output circuit. For example, a typical power measurement might read "100 watts per channel at < 0.07% THD from 20Hz-20kHz into 8 ohms." What this spec basically tells you is that the receiver can deliver 100 watts of continuous power across the entire audio bandwidth with less than 0.07% total harmonic distortion (THD) into an 8-ohm resistance. (A maximum measured distortion level below 1% across the full frequency range is likely to be inaudible on normal program material, such as movie soundtracks or music.)

Although 8 ohms has long been considered the standard load for measuring amp power (most speakers are spec'd at 8 ohms), an actual speaker usually presents a fairly complex load to an amplifier, with its impedance varying widely over the frequency spectrum. That's why it's helpful to see both 4- and 8-ohm power specs - you'll have increased confidence that the receiver can be mated with a broad range of speakers without overheating and shutting down.

A more rarefied power rating you'll find on some high-end receivers and amps is dynamic headroom. In some ways, this number is more significant than a "continuous" rating because it defines a product's ability to unleash additional power reserves in short bursts. Amp manufacturers issuing dynamic-headroom ratings - NAD, for example - state them in dB, expressing decibels of power available above and beyond the amp or receiver's "continuous power" specification for transient bursts (20 milliseconds). So if an amp's dynamic headroom is rated as 3 dB, that means it can deliver twice its rated power (doubling power delivers just a 3 dB loudness increase) for quick transients in music.