What does a power amp do?
Audio in a system is conveyed as signal voltages. The levels of those voltages vary depending on their source.
For example, when sound waves hit the diaphragm in a microphone, the mic produces a varying voltage that corresponds to the air pressure variations in the sound wave. A mic usually produces a pretty small voltage—several millivolts, on average. An MP3 player or portable CD player generally will produce a signal averaging about a fraction of a volt.
A typical signal from a mixer will average around a volt, with peaks hitting perhaps 5 to 10 volts. This is commonly called a line-level signal. Usually, when analog audio signals are transported about in large sound systems, recording and broadcast studios, etc., they are at line level.
The power amp is part of the final stage in the audio signal stream. It boosts the line-level audio signal voltage into a higher signal voltage that can drive a loudspeaker. Even though it’s called a “power” amplifier, it is essentially a voltage amp—a multiplier of the input signal voltage. The amount of multiplication is called gain, and it can be described as a numeric multiplier (for example, 20×, in which the output voltage is 20 times greater than the input signal) or logarithmically (for example, 26 dB, which is equivalent to a voltage gain of 20×).
The output signal produced by a power amp will drive the loudspeaker to move in and out accordingly. That’s because the voltage causes current to flow in the loudspeaker’s voice coil, producing a magnetic field that interacts with the permanent magnet. The force of this interaction accelerates the cone inward or outward, which accelerates the air molecules in front of the cone. When the cone moves forward, the air molecules get pushed together and produce a higher air pressure. Conversely, when the cone moves backwards, the air molecules get spread apart, producing a lower air pressure. These cycles of alternating higher and lower pressure ripple out into the surrounding air as sound waves.