The invention of the vacuum tube more than 100 years ago made many things possible –radios, televisions, even computers. And, of course, audio power amplifiers to drive the speakers in them… and also professional audio systems. Since then, after the transistor started to replace the vacuum tube in the 1970s, power amplifiers have grown from a few watts for driving small speakers up to several kilowatts driving high-powered line arrays.

Since the 1970s, the market for sound reinforcement systems has matured, offering thousands of different loudspeaker cabinets and separately-sold power amplifiers to match any application. But this has introduced a challenge: what power amplifier to select for what speaker?

As with all component selection issues, the trick is to work backwards from the end of the design process. In the case of a sound reinforcement system, this is the targeted desired Sound Pressure Level (SPL) at the listener’s position. If there’s only one listener then it’s relatively easy to come up with a solution, but in public address (PA) systems - which are designed to amplify sound for a group of listeners (an audience) - there are as many listener positions as there are listeners, so multiple loudspeakers may be required (but that is another topic).

Let’s assume that we have only one full-range loudspeaker, already selected for whatever reason, and that the goal is to generate a target SPL at a single listening position. For high quality systems, the peak (maximum) SPL should be used to ensure that the speaker and power amplifier deliver sound without distortion, even at the highest level. The selection of the power amplifier is then easy, just follow the three steps below:

Step 1: Calculate back from target peak SPL to speaker peak SPL at one metre.

By dividing the target peak SPL at the listening position with the logarithmic distance between the listener and speaker, we can calculate the peak SPL needed at one metre from the loudspeaker.

Step 2: Calculate back from speaker SPL at one metre to power.

Because all loudspeaker specifications include a ‘sensitivity’ parameter (which predicts what SPL at one metre from the loudspeaker is generated with one watt of power), we can easily calculate what peak power we need from the amplifier to drive the loudspeaker to its peak SPL.

At this point it’s advisable to double-check if the loudspeaker can handle that power. If not, you have to select another, more powerful, loudspeaker.

Step 3: Select the power amplifier to support the peak power.

Read the peak (maximum) power rating of the power amplifier for the impedance of the selected speaker - most commonly specified for 4ohm and 8ohm.

At this point it’s advisable to double-check if the power amplifier can also support the continuous power needed in the system.

Of course this is not the whole story by far, but it works as a quick rule of thumb when choosing power amplifiers for high quality sound reinforcement systems, to always produce the highest possible power output / quality at peak level.

Besides these three steps, we also need to check terminal levels, noise levels, the crest factor of the loudspeaker, amplifier, music genre and many more things.