Across the operating life of an amplifier the average power output typically remains 20 to 50dB below full scale even with material that has been compressed and limited to maximise apparent loudness. In linear power terms this means output power is 100 to 100,000 times less than the peak output. So, the efficiency of the amplifier in this region, if we take the circuit represented in Figure 1 as an example, is typically much lower than 50%.
Part of the problem with conventional amplifier topologies is that they are designed for signals rarely (if ever) encountered in real-world audio: pure sine waves. Audio, whether music, speech or sound effect, exhibits very different characteristics to a single, fixed-frequency sine wave. Strong transient peaks and complex waveforms are features of most audio programme material.
Music typically has a crest factor or content peak-to-average power ratio (CPAPR) between 10 and 20dB scale - 10 and 100 in linear power terms. The CPAPR for audio from TV or movies is even more extreme, typically exceeding 20dB. The table below highlights several examples.
Figure 2: Table of peak to average power ratios for six classical and pop music tracks.
However, an audio amplifier has to cope with a significantly greater dynamic range than just the CPAPR, with the user volume control adding another significant element of power variation. Very few consumer amplifiers are driven for long periods at maximum volume, so we must also consider the gain peak-to-average power ratio (GPAPR). We define this as the ratio between the volume gain when the amplifier is playing at its lifetime average power output, and the volume gain corresponding to the onset of clipping of the amplifier. GPAPR typically varies between about 10 and 30dB in consumer systems.
For example, lets examine an audio system delivering up to 10W peak with an efficiency of 90dB(SPL) at 1W and 1m. This will have an operating lifetime CPAPR in the region of 15dB. And, we can assume a lifetime sound pressure level of 73dB(SPL) at 1m, a level commonly used by consumer audio manufacturers in battery lifetime tests. At maximum – provided by driving the amplifier at its 10W peak – the effective output volume thanks to the CPAPR will be 100dB minus 15dB, providing 85dB average sound pressure level at 1m. As the target SPL is 73dB, the GPAPR will be 12db (85dB minus 73db).
The CPAPR and GPAPR can be brought together in an overall measure called the lifetime peak-to-average power ratio (LPAPR), calculated as the sum of the two dB ratings. As the typical CPAPR lies in the range 10dB to 20dB and GPAPR is normally within 10 to 30dB, the LPAPR will be between 20dB and 50dB. Maximising amplifier efficiency over this range – representing levels as much as 100,000 times lower than the peak output – is a major challenge. But it is one that can be solved.