It is important that the efficiency of audio amplifiers is measured correctly. Good test signals and adequate measurement procedures are crucial to make fair comparisons between amplifiers and reliably predict the dissipation in practical situations. This is also a vital condition for judging the usefulness of new amplifier topologies.
In literature, the efficiency of amplifiers is usually measured with sinusoidal signals. For amplifiers based on a class D topology, this gives approximately the same results as for audio signals, as long as one bears in mind that the average output power of an audio amplifier while playing normal audio signals is much lower than its maximum sine output power. Some high efficiency audio amplifiers, however, need specific audio characteristics to obtain a high efficiency. Well known topologies in this field are the class G and class H principles. The amplifiers which use knowledge about either the amplitude or the frequency distribution of average audio signals, measurements with sinusoids can give pessimistic results.
The best signal would be a real audio signal, but this has several disadvantages. The question is which audio signal should be taken. Speech? Music? What kind of music? This is not standardised. Furthermore, at least several seconds of audio are necessary to get a good impression, which is not very practical for simulations. Also, a music signal does not give stable readings on meters. In practice, more creative ways were found. Either the efficiency was measured indirectly by measuring heat sink temperatures, or an ad hoc measure is defined. Another possibility is to use the IEC-268 "simulated programme material". The spectral distributions of programme material were measured, and the latter also investigated whether the IEC test signal is useful for evaluating the power rating of loudspeakers. There is, however, no standard test signal intended for measuring or predicting amplifier efficiency. However, we can try to find such a signal. For that reason, please refer to the articles Characteristics of Audio Signals and The IEC-268 Test Signal.
Completeness
Despite the good characteristics of the IEC signal, one can still wonder if these characteristics are complete, do they fully determine amplifier dissipation? To answer this, three amplifiers were built: a standard class AB amplifier, a class H amplifier (an amplifier that lifts the power supply during signal peaks by means of an electrolytic capacitor), and a class D + AB amplifier (an amplifier that has a class AB and a class D amplifier in parallel). The class AB amplifier is only sensitive to the amplitude distribution of its output signal, the frequency is not important. The class H amplifier dissipation is "to some extent" frequency dependent, because charging and discharging the capacitor is not lossless, so the total dissipation of this amplifier depends on both the volume and the frequency of the output signal. Finally, the class D + AB amplifier is also sensitive to both characteristics. The class AB part in this amplifier has to support the output current for high d(Iout)/dt, starting at 1 kHz full scale signal swing. All amplifiers have a maximum output power of 30 W, and have identical heat sinks.
Input to the amplifiers are both the IEC signal and a music fragment that is selected because it has almost identical characteristics (a fragment of "Me and Bobby McGee" by Janis Joplin). Measured are the heat sink temperatures as a function of time of all amplifiers. The results are depicted in Picture 1. The average output power was 2 W, at which the amplifiers were clipping a negligible part of the time. The difference in dissipation between the two signals is insignificant. When the average output power is increased to 10 W, the music and the test signal are clipping a considerable part of the time. Even then, there is hardly any difference between the two, as is shown in Picture 2. The differences that do occur can be explained by measurement inaccuracies or slight differences between the amplitude distributions.
Picture 1: Heat sink temperatures for three amplifier classes and two signals at an average output power of 2 W (no clipping)
Picture 2: Heat sink temperatures for three amplifier classes and two signals at an average output power of 10 W (heavy clipping)
With these results it seems that the amplitude and frequency characteristics fully determine amplifier dissipation, also under clipping conditions. Thus we can trust that the dissipation of audio fragments with the same characteristics as the IEC signal will also cause the same dissipation.
Accuracy
Although the IEC characteristics are a good average, individual fragments can have characteristics that are quite different. The question arises if these fragments produce amplifier dissipations that are also significantly different. To answer this question, it is necessary to measure the dissipation of the three amplifier classes for all audio fragments. Direct measurement of amplifier efficiency for audio signals, however, is difficult. One possibility is measuring the heat sink temperature, as was done in the previous section. This requires a constant ambient temperature and is very time consuming. Another (complicated) possibility is sampling the output voltage and the supply current, and calculate the dissipation. To circumvent these drawbacks, behavioural models of the amplifiers are used, and the dissipation is simulated with C programs, evaluating the dissipated energy per audio sample. With the proper models, it is easy to calculate the dissipations for the various audio fragments. The models were developed with the IEC signal measurement results as reference. To demonstrate the validity for real audio signals, Picture 3 shows the simulated dissipation for both the IEC signal and the fragment of Janis Joplin. The dissipations are practically the same, as they should be. Furthermore, the ratios between amplifier dissipations at 2 W and 10 W deviate less than 15 % from the ratios of the extrapolated increase in heat sink temperatures of Picture 1 and Picture 2.
Picture 3: Simulated dissipation of three amplifier classes (for the IEC test signal and a fragment of Janis Joplin)
After all audio fragments were scaled to equal power, the dissipation they caused was calculated for all amplifier classes. Picture 4 shows the results as a histogram. It has a logarithmic x-axis. The distance between the left border and the right border of each bar is a factor 1.05. The height of the bar indicates how many audio fragments cause a dissipation in that range. The vertical lines indicate the dissipation for the IEC signal. It appears that all fragments have dissipations within +/- 20 % of the dissipation predicted by the IEC test signal. One fragment stands out because it causes a high dissipation in both the class D + AB and the class H amplifier. The large high frequency contents decreases the efficiency of the two amplifiers. Although this is an exceptional case, it is important to realise that the good predictive qualities of the IEC signal might not be valid for an amplifier which is more sensitive to the frequency contents of its input signal. In general, however, the IEC signal is representative for a wide range of audio signals.
Picture 4: Histogram of the simulated dissipation of all audio fragments in 3 amplifier classes (Vertical lines indicate the dissipation for the IEC signal)
Conclusions
For the tested types of high-efficiency amplifiers: a class AB, a class H, and a class D + AB amplifier. The power, the amplitude distribution and the frequency distribution of the output signal fully determine the amplifier’s dissipation. The Peak-to-Average ratio of the signal is not very significant.
The dissipation for a variety of real-life audio signals of constant volume deviates only 20 % from the dissipation caused by the IEC 268 test signal at the same output power. Therefore, this signal is very suitable for measuring audio amplifier efficiency. This must be verified for new amplifiers types, that may be more sensitive to amplitude or frequency distribution deviations.
Two alternative test signals are proposed. For simulation and test purposes, a simple test signal can be used for amplifiers with near frequency independent dissipation (A Simple Periodic Test Signal). When the frequency contents is also important, an IEC look-alike test signal can be used which has the same characteristics as the IEC signal (An IEC Variant), but is easier to generate in simulation and hardware.
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