Amplifiers come in different classes and different "sizes," or output capabilities. The class refers to the internal workings of the amp and isn't related to output. The power rating is measured in watts. This indicates the maximum output that is possible while still maintaining high quality. Load impedance is measured in ohms, with standards at eight, four and two ohms. Usually, the lower the ohms, the higher the wattage.
Signals enter a power amplifier from a source such as an electrical instrument, a microphone, a preamplifier, a signal processor, a CD player or a tuner. The power amplifier expands the sound and puts out a stronger signal that is carried to the speakers.
When choosing a power amp, it is necessary to understand the power level of the input device and the capabilities of the speakers. A solid state amplifier produces the least amount of distortion in the sound as it is amplified, and is generally preferred over tube amplifiers. Some distortion occurs with any amplifier and any speaker, but tube amplifiers tend to have more "clipping," or distortion, as they reach the upper end of their power capability.