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WPI Technical Theatre Handbook: Power Amplifiers
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Power Amplifiers

The outputs of a sound board are typically line level signals which can not directly drive speakers. Thus, a power amplifier must be used between the outputs of a mixer and the speakers. Power amplifiers come in a variety of sizes, each with different capabilities and features.

Power amplifiers are functionally simple units. All amplifiers have a set of line-level inputs and a set of speaker-level outputs. Most have a power switch and volume controls as well. The outputs of a sound board are typically connected to the inputs on the amplifier, and speakers are connected to the speaker outputs. The job of the amplifier is to take the small line-level signal and boost it to the levels required to drive speakers. One characteristic of better amplifiers is that they more faithfully reproduce the small signal at the speaker outputs. Inexpensive amplifiers tend to color the sound by inaccurately reproducing the line-level signal at the speaker outputs.

One important rating for amplifiers is the amount of power they can supply, specified in watts. Given sufficiently capable speakers, the more power an amplifier can supply the louder the overall sound system can be before it begins to distort6.2. System distortion is not desirable as it sounds bad and can damage speakers. It is important to note that some low-end professional equipment, and quite a bit of consumer gear is improperly rated. Knowing that the general public tends to buy amplifiers based solely on the number of watts they can produce, some companies go out of their way to artificially bloat these figures. Caveat emptor.

The power rating for an amplifier is usually specified in terms of driving a certain load. The load (in most cases a speaker) has a characteristic impedance. Most speakers have an impedance of four or eight ohms, and not by accident, most amplifiers specify their ratings for four or eight ohm loads. It is important to pay attention to loading because it's possible to blow up an amplifier if it is not rated to handle the load being driven.

next up previous contents index
Next: Speakers Up: Output Previous: Output   Contents   Index
Steve Richardson 2000-07-06

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