Determining The Amount Of Amplifier Power For A Variety Of Systems

Many loudspeakers are also rated in continuous or average power handling.

If the loudspeaker’s continuous power handling is 500 watts, 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 percent).

If the amp’s continuous power is higher than 125 percent, you risk damaging the loudspeaker with too much power in the event of feedback. If the amp’s continuous power is lower than 80 percent, 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-watt continuous power handling. The amplifier’s continuous average power should be twice that, or 400 watts (within 80 to 125 percent, 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 percent).

For example, if a loudspeaker is rated at 400 watts program, the amp should provide 400 watts continuous (x 80 to 125 percent).

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 percent).

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 watts continuous average power. You might run it at 50 watts on the average so that occasional peaks can reach 800 watts without clipping. In other words, you allow 12 dB for signal peaks (refer to Figure 2).

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.