The Ratings Game: Getting A Handle On Amplifier Power Ratings

Current is cumbersome to measure, but the power equation lets us state the amplifier’s output power using more easily measured quantities, like voltage and impedance. The impedance variable gets too complicated with real-world loudspeakers, so amplifiers are tested into resistive loads, which can be rated with one number. Eight ohms is eight ohms and unlike a loudspeaker’s impedance it is independent of frequency.

So, the amplifier is connected to a dummy resistor, fed a test signal, and the RMS voltage across the load is measured. Square the voltage and divide it by the resistance of the load and you have a power rating. While that yields a “one number” rating for the amplifier, what is really needed to understand what an amplifier can do are the voltage and resistance values that produced the power rating. Only then can you see how the amplifier really behaves under different loading conditions, which can be completely masked by a wattage-only rating.

The Common Amplifier Format
Meaningful amplifier selection (and comparisons) requires a complete picture of an amplifier’s performance. The burst test is a piece of that pie, but there are at least two others.

The Common Amplifier Format (CAF) was created to address this issue. Rather than just giving a “one number” watts rating, it provides all of the input/output information needed to deploy a power amplifier. This includes the voltage and power performance for burst signals, continuous sine waves, and noise, all in a concise matrix.

Figure 4 shows a section of the Voltage, Power I/O Matrix. The details about the CAF are available from www.cafgroup.org. It is not a standard. It is a suggested, defensible way to specify the output capability of an audio power amplifier based on what sound system designers need to know. It is free for any manufacturer to use.

Figure 4

In addition to the traditional one-number watts rating, the CAF gives the output voltage of the amplifier for various signal types across various load resistances, allowing you to see how the wattage rating was determined. This can be an eye-opener.

Three Signal Types
The CAF includes the CEA2006 burst test for 1 kHz and 50 Hz, for the resistances that the amplifier is rated to drive. This is typically 8, 4, and sometimes 2 ohms. No, with the CAF you can’t invent your own burst signal.

The continuous sine wave rating is still an important metric, if for no other reason than the fact that the amplifier may have to produce a full-scale sine wave for more than a few seconds to reproduce some types of music, signal tones, or tsunami warnings. If two competing amplifiers have similar burst ratings, then the continuous rating may differentiate between them.

The CAF includes a 15-second continuous sine wave test into the same resistances as the burst test. This is where many “2 ohm” ratings bite the dust. An amplifier that can sustain a sine wave into its minimum rated impedance does so because it was designed for it. That costs the manufacturer money, and it adds value to the product. Burst ratings alone do not give credit where it is due.

Noise ratings reveal the amplifier’s performance for music-like and speech-like program. The CAF includes noise test results for 1/4-power and 1/8-power (ref. burst power). The waveform is BS EN 50332-1 noise, which is spectrally-shaped pink noise that is soft-clipped to a 6 dB crest factor. This is necessary to prevent the triggering of the protection or limiting circuits of the amplifier during the test.

One-fourth power simulates how an amplifier performs for heavily compressed program, such as in a touring rig. One-eighth power simulates slightly compressed typical program, and is the most appropriate stimulus for the majority of applications.

It should be noted that the 1/8-power noise test is not a measure of the amplifier’s capability. It is not particularly stressful to the amplifier. This test reveals the current draw of the amplifier (for sizing circuit breakers) and the BTU output of the amplifier (for designing cooling systems), both of which are included in the complete CAF data matrix. Sizing either of these using burst or continuous sine wave ratings would result in over-designed auxiliary systems and wasted money.

The personality of an amplifier changes completely depending on the output configuration (e.g. mono, stereo, bridged, parallel). Change that and it’s a different amplifier. The CAF requires a separate data matrix for each output configuration, so a manufacturer can’t just publish the highest watts rating from all supported configurations.

The output of an amplifier also depends on how many channels are used, since in some designs they all share a common power supply. The CAF requires a separate data matrix for one channel driven and all channels driven.

Conclusion
As an industry, we have become sloppy in our technical practices. We have learned nothing from the ‘70s, other than that a different path was needed to exaggerate amplifier power ratings. “Peak power” was out, but now it is coming back in, and along with “burst power” is being used to sway consumers. In a world where more is perceived to be better, it is hard to resist the temptation to provide it, not by product design, but by playing the ratings game.