Power Amplifier Specifications
Now that we understand signal levels, let’s see how they relate to power amp specifications.

Continuous Average Power
The power specification of most amplifiers is continuous average power. That’s the maximum power in watts that the amplifier can produce at rated distortion, at a certain load impedance, when playing a continuous sine wave. That sine wave signal is either a 1 kHz tone or is a range of tones from 20 Hz to 20 kHz.

For example, one amplifier puts out 1,800 W per channel into 4 ohms in stereo mode at 0.1% THD at 1 kHz. Another amp produces 8,000 W into 8 ohms in bridge-mono mode at 0.35% THD from 20 Hz to 20 kHz.

Many loudspeakers are also rated in continuous or average power handling. If the loudspeaker’s continuous power handling is 500 W, that means it can handle 500 watts of pink noise continuously without mechanical or thermal damage.

To generate that power-handling spec, the loudspeaker is usually fed “AES pink noise”, which is pink noise that is clipped so that its crest factor is 6 dB. Normally pink noise has a crest factor of 12 dB. The AES loudspeaker test signal IEC 268-5 specifies band-limited pink noise with a crest factor of 6 dB.

I recommend that the amplifier’s continuous power be twice the loudspeaker’s continuous power handling (within 80% to 125%). If the amp’s continuous power is higher than 125%, you risk damaging the loudspeaker with too much power in the event of feedback. If the amp’s continuous power is lower than 80%, you will probably clip the signal in an attempt to get enough volume out of the loudspeaker. Clipping a signal produces strong high frequencies which can burn out tweeters.

For example, suppose a loudspeaker has 200 W continuous power handling. The amplifier’s continuous average power should be twice that, or 400 watts (within 80% to 125%, or 320 watts to 500 watts).

Program Power
Some loudspeakers are rated in program power (music power), which is usually twice the continuous average power. A suitable amplifier should have a continuous average power that matches the loudspeaker’s program power (within 80% to 125%). For example, if a loudspeaker is rated at 400 W program, the amp should provide 400 W continuous (x 80% to 125%).

Peak Power
This is the power, in watts, that an amplifier can produce during short peaks or transients. Typically, peak power is 1 to 3 dB higher than the continuous power. Peak power depends on the amplifier’s power reserves (energy storage). If the amplifier’s power supply has a bank of large filter capacitors, they can store energy that can be released during short peaks.

Many loudspeakers are rated in the peak power they can handle. Ideally, the power amplifier’s peak power should not exceed the loudspeaker’s peak power rating. If the power amp does not have a peak power spec, just make sure that the amp’s continuous power is twice that of the loudspeaker (within 80% to 125%).

Creating Headroom For Signal Peaks
In practice, you don’t run the amp at full continuous power because that does not allow extra power for signal peaks, which might be 6 to 24 dB above the average level. Instead, the amp’s average power is typically 6 to 24 dB below clipping to allow for undistorted signal peaks.

For example, you might turn the amp down so it puts out 1/16th of its rated continuous power on the average. Occasional peaks will make the amp produce its rated continuous power during those peaks.
Suppose an amp is rated at 800 W continuous average power. You might run it at 50W on the average so that occasional peaks can reach 800W without clipping.

In other words, you allow 12 dB for signal peaks (Figure 2, bottom). Actually, peaks can be a bit higher than that because an amplifier’s peak power is typically 1 to 3 dB higher than its continuous power.
In this case, crest factor (dB) = 10 * log (800/50) = 12 dB.

The power specs below are continuous average power:

• An 800 W amp run at 400 W allows for 3 dB peaks above the average level. In this case, the only undistorted signal you could play would be a sine wave or square wave.

• An 800 W amp run at 200 W allows for 6 dB peaks above the average level. This is adequate for highly compressed rock music or a rock broadcast.

• An 800 W amp run at 100 W allows for 9 dB peaks above the average level.

• An 800 W amp run at 50 W allows for 12 dB peaks above the average level. This is adequate for speech.

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