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Power amplifiers can most readily be recognized by their faceplates. They usually look very bare. In fact, some only have a power switch. Although users can listen to music using just a power amplifier, a single program source (such as a cd player), and speakers, power amplifiers of all types are intended to be used with preamplifiers, which provide users the option of listening to more than just a single program source.
For the past decade power amplifiers were one of the least purchased components among audio equipment. Recently, however, there has been a resurgence in popularity. This is especially apparent in the low budget high performance market and in the "home theater" market (more on this under Surround Sound Power Amplifiers).
Popular features include speaker switches, power level meters, provisions for bridging power, output level controls, and switches for power protection. A few in the market include d-a converters.
Speaker switches are provided on power amplifiers that can drive two or more pairs of speakers. The purpose of the speaker switches is to allow the listener to select any pair or combination of pairs of speakers for listening.
Power level meters are on relatively few power amplifiers. Basically, they display the sound level of the listening source in terms of wattage (a unit of power) or decibels (a unit of sound level) or both wattage and decibels. The meters can come in the form of a pair of swinging needles, which usually show the average sound level, or illuminated peak level meters, which are a pair of segmented lights showing the highest level of sound at any point in time. There is nothing to do on the listener's part but observe.
Many of today's amplifiers have a switch on the back panel that allows users to bridge power. For example, if a stereo power amplifier with a power rating of 100 watts per channel had a switch on the rear panel for bridging power, flipping the switch would combine the power of the two stereo channels into one more powerful channel producing maybe about 240 watts per channel. The rationale behind this feature is to allow the user the ability to deliver louder and cleaner volume, which is useful for inefficient speakers requiring a lot of power or very large rooms. Since both channels are combined into one for increased power, however, another amplifier would need to be used to reproduce the other stero channel.
Output level controls are useful when speakers are either unusually efficient or inefficient. Somewhat like volume controls, they allow you to turn down the sound level of the power amplifier to compensate for speakers that are unusually loud or turn up the sound level for unusually soft sounding speakers. Since the listener routinely uses the volume control on the preamplifier (see preamplfiers) to adjust the volume, the output level controls on the power amplifier can enable the listener avoid hearing wide volume changes with just a small turn of the preamplfier's volume control. Conversely, with inefficient speakers turning up the power amplifier's output level controls can enable the listener to avoid the necessity of routinely turning the preamplifier's volume control up high.
Any power amplifier can damage itself or the speakers if it is driven beyond its intended power capability. To reduce the risk of damage some amplifiers have special "anti-clipping" circuitry. When activated, this circuitry enables a power amplifier to reserve unused power or automatically lower power in times when it's about to be driven beyond its capability. A switch often labeled "clipping" or "soft clip" activates this feature.
Critical listeners may want built-in d-a converters. This feature can often improve the sound of digital components such as compact disc players, DA T tape decks, DCC tape decks, and Minidisc recorders (see d-a converters, coaxial and optical under Compact Disc Player Features in Chapter 7).
While surround sound receivers may have obvious advantages over power amplifiers designed for surround sound applications, such as integration and generally lower costs, they often do not deliver the performance that power amplifiers can.
