Power amplifiers
Power amplifiers boost a
low-voltage level signal and provide electrical power to drive a
loudspeaker. All speakers require power amplification of the low-level
signal by an amplifier, including headphones. Most professional audio
amplifiers also provide protection from overdriven signals, short circuits
across the output, and excess temperature. A limiter is often used to
protect loudspeakers and amplifiers from overload.
Like most sound reinforcement equipment products, professional amplifiers
are designed to be mounted within standard 19-inch
racks. Many power amplifiers feature internal fans to draw air across
their heat sinks. Since they can generate a significant amount of heat,
thermal dissipation is an important factor for operators to consider when
mounting amplifiers into equipment racks. Active
loudspeakers feature internally mounted amplifiers that have been selected
by the manufacturer to be the best amplifier for use with the given
loudspeaker.
In the 1970s and 1980s, most PA amplifiers were heavy Class
AB amplifiers. In the late 1990s power amplifiers in PA applications have
became lighter, smaller, more powerful and more efficient due to
increasing use of switching power supplies and Class
D amplifiers, which offer significant weight and space savings as well as
increased efficiency. Installations in railroad stations, stadia and
airports, their high efficiency allow them to run with minimal additional
cooling and with higher rack densities compared to older amplifiers.
Digital loudspeaker management systems (DLMS) that combine digital
crossover functions, compression, limiting, and other features in a single
unit have become popular since their introduction. They are used to
process the mix from the mixing console and route it to the various
amplifiers in use. Systems may include several loudspeakers, each with its
own output optimized for a specific range of frequencies (i.e. bass,
midrange and treble). Bi, tri, or quad amplifying a sound reinforcement
system with the aide of a DLMS results in a more efficient use of
amplifier power by sending each amplifier only the frequencies appropriate
for its respective loudspeaker. Most DLMS units that are designed for use
by non-professionals have calibration and testing functions such as a pink
noise generator coupled with a real-time
analyzer to allow automated
room equalization.
The amount of amplifier power used in a performance setting depends on a
number of factors, such as the desired Sound Pressure Level of the
performers, whether the venue is indoors or outdoors, and the presence of
competing background noise. The following list gives a rough "rule of
thumb" for the amount of amplifier power used in different settings:
- "Small Vocal" system - About 500
watts
- "Large Vocal" system - About 1,000
watts
- "Small Club" system - About 9,000
watts
- "Large Club" system - About 18,000
watts
- "Small Stadium" system - About
28,000 watts
