Home audio power amplifiers come in a wide variety of configurations and circuits, and can have a significant overall impact on the sound of your stereo. While many consumers think watts are an important measurement, watts are misleading, because it is simply a function of voltage multiplied by current. While two amplifiers may both measure 100 watts, for instance, one with a better power supply will be able to supply high current and high voltage and avoid clipping, and thereby sound much better.
Tubes
Tubed power amplifiers, which dominated the home audio industry up till the 1960s, came back into vogue in the 1990s. Many audiophiles feel that tubes, which may have higher distortion measurements than solid state amplifiers, produce an even order distortion which doesn't sound as bad to the human ear as solid state type distortion. Tubes will need to be replaced now and then, and that can be expensive.
Solid state
Solid state amplifiers, which use transistors, became popular in the late '60s and early '70s, in part because tube circuits were very unreliable. Solid state amps run cooler, and tend to have better control over bass frequencies than tube amps, which are more noted for a magical sounding mid-range. Solid state circuits are also less efficient than tube circuits.
Circuit types
Amplifiers are available in a variety of circuit types, including Class A, Class B, Class AB, and Class D. Tube amps can also employ single-ended triode designs, ultra-linear designs, and more. A Class A circuit is less efficient, but produces very little distortion. A Class A amp reproduces the entire input wave at the output end. A Class B amp is more efficient because it idles at zero current, whereas Class A conducts the same amount of current all the time. Class AB amps split the difference, helping to eliminate crossover distortion; in a Class AB amp, one circuit or tube is driven until the plate cuts it off, when it switches to the other. Class AB designs are the most popular for high-end stereo power amplifiers. A Class D amplifier is different, in that the output stage switches between "on" and "off" states, so the current is either running or not. The amplifier can be controlled by analog or digital circuits; because the output stages are sometimes "off," smaller heatsinks can be used, and the amplifiers themselves are smaller.
With tube amplifiers, single-ended triode amps use a single triode to produce an output, and operate in Class A. They typically have a lower power output, but many audiophiles praise their midrange clarity. An ultralinear design uses beam tetrodes or pentodes, and has taps on the output transformers. The taps connect to the screens of the tubes, driving the tubes with part of the output signal and lowering the overall distortion significantly. Ultralinear designs are used in tube amps with higher power outputs.
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