Measuring Amplifier Power

Power ratings of amplifiers and their measurement methods

The output power of an amplifier is most of the time the only criterion to evaluate its performance and quality. For the sake of a quick comparison of different models and manufacturers we tend to forget other important factors like its ability to control EMF (Electro-motoric Force). Unfortunately this comparison at a glance is really misleading as most of the data presented have little or no value at all.

Bittner Audio always provided only one type of output power data: continuous output from 20 Hz to 20 kHz. This sine wave is not representing a real-world music signal, but it makes an excellent statement about the durability of the power supply and the efficiency of the amplifier.

Such a test easily reveals whether a transformer is too weak or that the power supply is converting too much energy into heat. If this is the case, the mains will be quickly overloaded or the amplifier gets so hot that it reaches its thermal capacity.

Continuous power output has been sufficient for many years to determine whether an amplifier is adequately powered for a given application. In fact, other testing procedures have been considered unprofessional and insufficient.

However, new developments require new methods of testing. High power amplifiers with an output of more than 1000 watts per channel are a commodity nowadays. Measuring an amplifier of for example 4000 watts will lead to serious problems if the traditional procedures are applied: driven with a sine wave into its nominal load (i.e. 4 ohms) it will draw more than 6000 watts or 25 A from the supplies. Most mains-fuses will blow during this test and it is most likely illegal to replace the fuse with the next higher value…

How can amplifiers of this magnitude be operated from regular mains at all? The answer is rather simple: even highly compressed music material like Rave/Dance or alarm signals are not comparable to a full power continuous output.

Therefore, most manufacturers decided to publish their data according to the following “real-world” testing procedure: the maximum output power of an amplifier will be measured with a 1 kHz sine wave with a 33.3 ms on-time followed by 66.6 ms off-time. This test is closer to music program and will indicate a higher power, however, there will be no excessive draw on the mains, and fuses will stay intact. As a continuous power output 1/8 or 1/3 load is chosen.

Although this is closer to reality it also delivers values that cannot be verified applying the traditional testing procedures: a 2000 watts per channel amplifier tested with a burst signal delivers a continuous power output of approximately 650 watts (1/3 load) or 250 watts (1/8 load).

Unfortunately, it is still not possible to compare the products of different manufacturers. Neither are similar methods applied, as it is not really clear what a burst signal is made of, nor are the conditions known under which the results have been achieved. Therefore, if continuous output power data are available, they are still a good means to evaluate performance and durability. Please keep in mind that professional power amplifiers should be tools that have been designed for untroubled continuous operation to justify the investment.

Bittner Audio will therefore provide all data in the accustomed fashion i.e. continuous output power to demonstrate the capacity and stability of the products. In addition, the output power will also be indicated using the described testing method of a 33.3/66.6 ms burst signal to allow an easier comparison of different models and brands.

However, care should be taken while dimensioning fuses and or uninterrupted power supplies (UPS). In this case we recommend applying the power consumption of 1/8 or in the worst case 1/3 load. This also holds for determining the cooling capacity of air conditioned technical rooms.

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    • Joachim Schwarz
      September 2007