Power Amplifiers

The audio power amplifier is a component whose function is - as its name implies - to increase the power of an audio signal, so that the signal can drive one or more loudspeaker. In sound systems, the power amplifier is always the final active component in the signal chain, located just before the loudspeakers.
In small portable sound systems, the power amplifier may be build into the mixpower, as a convenience.
Full comprehension of the function and application of power amplifier requires an understanding of electrical power and its relationship to voltage, to resistance or impedance, and to current. The details of these relationships, stated by Ohm's Law and associated equations, are beyond the province of this book. But some basic understanding may be imparted by way of a classic analogy:
Thing of water being forced through a pipe. The greater the pressure being applied, the more water will move through the pipe. The smaller the diameter of the pipe, the less water will flow through it. In drawing a comparison with electricity, the water represents electrons (negatively-charged atomic particles), which constitute the electricity itself. To flow rate of the water represent the electrical current (abbreviated I), measured in amperes (amps for short). The pressure represent the voltage (sometimes called electromotive force, abbreviated E), measured in volts. The resistance to flow of the pipe, which decreases as its diameter increases, represents the resistance (R), measured in ohms (symbolized by Ω); the greater the resistance (i.e., the smaller the pipe diameter), the less current (water) will flow.
Consider the DC electrical circuit of this figure:

This is a direct-current (DC) circuit, which means that the current flows in one direction only. A DC voltage E from sourceS (a battery) is applied across a resistance R. This resistance is termed the load. The current flow I is represented by an arrow in the diagram. Notice that this is indeed a circuit. That is, the current flows   "around" from the point of highest voltage (or potential)-the negative (-) terminal of the battery - to the point of the lowest potential - the positive (+) terminal. Should the circuit be broken, the current would cease to flow.
In speaking of alternating current (AC), such as an audio signal, impedance is substituted for simple resistance. Impedance, also measured in ohms, is sometimesabbreviated Z, or Z, rather than R. The chief distintion between resistance and impedance is that the impedance of a loadd (such as a loudspeaker) is different frequencies. Nevertheless, loudspeakers are clasified by their nominal impedance - a single figure, which is used. 
Consider the AC electrical circuit of this figure:

This figure represent a power amplifier (the AC souce) driving a loudspeaker (the load - abbreviated RL on some amplifier spec sheets).

Electrical power (abbreviated P or Po), measured in watts (W), is the energy used to do the work when an electrical current is made to flow through a load resistance or impedance. Power is proportionate to the square of the voltage or current, which accounts for the different increases in dB of a signal when the power is doubled, as compared to when the voltage is doubled.

With this rudimentary background, the specifications of power amplifiers can be discussed meaningfully.