The Complete Handbook of Power Amplifiers

Chapter 1: Understanding Power Amplifiers

A power amplifier is the final active stage in the audio chain. Its job is simple to state: it takes a low-power audio signal and increases its voltage and current capability so that it can physically move a loudspeaker driver. Everything you do before that—the microphone, the mixer, the effects processor, the crossover—is creating a voltage representation of an acoustic event. That voltage is too weak to move a coil of wire inside a magnetic gap with any force. Here is a concrete example: the line-level output of a mixing console might be +4 dBu, which is about 1.23 volts. A typical 8-ohm loudspeaker needs about 30 volts to produce a moderate 112 dB SPL from a single meter away. The amplifier is the device that takes that 1.23 volts and provides the current to make the 30 volts happen across that loudspeaker terminals.

It is tempting to think of the amplifier as a "bigger" version of the signal. That is not accurate. A good amplifier does not color the sound; it accurately reproduces the shape of the input signal but with a much larger amplitude. It is a voltage source that, under normal operating conditions, outputs a voltage that is the input voltage multiplied by a fixed gain factor. The mystery is not in the gain; gain is just a resistor ratio or a digital multiplication factor. The difficulty, and where engineering and design matter, is maintaining that accurate gain across the wide range