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Professional-grade audio amplifiers are designed for use in PA (public address) systems. Their job is simple: Take the audio signal from a mixing board and magnify it. Then they send that amplified signal to a speaker or a set of speakers.


PA amplifiers can withstand harsh operating conditions and a certain degree of unintended abuse. Most models have robust protection circuits, trouble-sensing signal limiters, and other safeguards. When pushed too hard, they protect themselves, save your speakers, and prevent audible distortion.

The continuous power rating is typically half of the program power rating. Think of it as the minimum amount of power a speaker needs in real-world situations. You want an amp that provides at least that much power. But you can safely use an amp with up to twice as much power.

For example, for a speaker with a continuous power rating of 200 watts, you want an amplifier that delivers between 200 and 400 watts RMS. The closer you get to the higher number, the better the speaker will sound.

The amount of power an amplifier generates depends on the impedance (or resistance) of the speakers it's driving. It'll put out different amounts of power to different impedance loads. So you might see an amp that puts out 1,000 watts at 8 ohms and 1,500 watts at 4 ohms.

Finally, most pro amplifiers do not have 12-volt triggers, or input-sensing auto turn-on circuits. This means they won't power up when you turn on your receiver or preamp/processor. You either leave your pro amps on all the time or manually turn them all on each time you want to watch a movie.

Typically, the front of the amplifier will have a gain/level knob and signal level indicator for each channel. Colored lights indicate when the amp is clipping. Proper "gain staging" starts with the settings on your mixer. To learn more about the process of setting levels at each link in your audio chain, read this article.

A high-pass filter (also known as a low-cut filter) lets you reduce the output of a speaker below a certain frequency. It lets the amp focus its power on the higher frequencies and not waste energy on the power-hungry bass frequencies.

Limiters are protection circuits that can help keep your amplifier from clipping and to prevent distortion in the sound. They help prevent distortion caused by an overdriven signal, a dropped microphone, or a short in an input jack.

In an actual listening room, you'd find that some speakers play louder than others when fed the same amount of power. In equal power mode, you'll hear these differences in loudness as they naturally occur between speakers.

A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound source or recorded sound or music. PA systems are used in any public venue that requires that an announcer, performer, etc. be sufficiently audible at a distance or over a large area. Typical applications include sports stadiums, public transportation vehicles and facilities, and live or recorded music venues and events. A PA system may include multiple microphones or other sound sources, a mixing console to combine and modify multiple sources, and multiple amplifiers and loudspeakers for louder volume or wider distribution.

The term sound reinforcement system generally means a PA system used specifically for live music or other performances.[1] In Britain any PA system is sometimes colloquially referred to as a Tannoy, after the company of that name, now owned by TC Electronic Group, which supplied many of the PA systems used previously in Britain.[2]

In 1910, the Automatic Electric Company of Chicago, Illinois, already a major supplier of automatic telephone switchboards, announced it had developed a loudspeaker, which it marketed under the name of the Automatic Enunciator. Company president Joseph Harris foresaw multiple potential uses, and the original publicity stressed the value of the invention as a hotel public address system, allowing people in all public rooms to hear announcements.[3] In June 1910, an initial "semi-public" demonstration was given to newspaper reporters at the Automatic Electric Company building, where a speaker's voice was transmitted to loudspeakers placed in a dozen locations "all over the building".[4]

A short time later, the Automatic Enunciator Company formed in Chicago order to market the new device, and a series of promotional installations followed.[5] In August 1912 a large outdoor installation was made at a water carnival held in Chicago by the Associated Yacht and Power Boat Clubs of America. Seventy-two loudspeakers were strung in pairs at forty-foot (12 meter) intervals along the docks, spanning a total of one-half mile (800 meters) of grandstands. The system was used to announce race reports and descriptions, carry a series of speeches about "The Chicago Plan", and provide music between races.[6]

Peter Jensen and Edwin Pridham of Magnavox began experimenting with sound reproduction in the 1910s. Working from a laboratory in Napa, California, they filed the first patent for a moving coil loudspeaker in 1911.[9] Four years later, in 1915, they built a dynamic loudspeaker with a 1-inch (2.5 cm) voice coil, a 3-inch (7.6 cm) corrugated diaphragm and a horn measuring 34 inches (86 cm) with a 22-inch (56 cm) aperture. The electromagnet created a flux field of approximately 11,000 Gauss.[9]

The first outside broadcast was made one week later, again supervised by Jensen and Pridham.[1][11] On December 30, when Governor of California Hiram Johnson was too ill to give a speech in person, loudspeakers were installed at the Civic Auditorium in San Francisco, connected to Johnson's house some miles away by cable and a microphone, from where he delivered his speech.[9] Jensen oversaw the governor using the microphone while Pridham operated the loudspeaker.

The following year, Jensen and Pridham applied for a patent for what they called their "Sound Magnifying Phonograph". Over the next two years they developed their first valve amplifier. In 1919 this was standardized as a 3-stage 25 watt amplifier.[9]

This system was used by former US president William Howard Taft at a speech in Grant Park, Chicago, and first used by a current president when Woodrow Wilson addressed 50,000 people in San Diego, California.[11][12] Wilson's speech was part of his nationwide tour to promote the establishment of the League of Nations.[13] It was held on September 9, 1919 at City Stadium. As with the San Francisco installation, Jensen supervised the microphone and Pridham the loudspeakers. Wilson spoke into two large horns mounted on his platform, which channelled his voice into the microphone.[13] Similar systems were used in the following years by Warren G. Harding and Franklin D. Roosevelt.[9]

By the early 1920s, Marconi had established a department dedicated to public address and began producing loudspeakers and amplifiers to match a growing demand.[9] In 1925, George V used such a system at the British Empire Exhibition, addressing 90,000 via six long-range loudspeakers.[9] This public use of loudspeakers brought attention to the possibilities of such technology. The 1925 Royal Air Force Pageant at Hendon Aerodrome used a Marconi system to allow the announcer to address the crowds, as well as amplify the band.[9] In 1929, the Schneider Trophy race at Calshot Spit used a public address system that had 200 horns, weighing a total of 20 tons.[9]

Engineers invented the first loud, powerful amplifier and speaker systems for public address systems and movie theaters. These large PA systems and movie theatre sound systems were very large and very expensive, and so they could not be used by most touring musicians. After 1927, smaller, portable AC mains-powered PA systems that could be plugged into a regular wall socket "quickly became popular with musicians"; indeed, "... Leon McAuliffe (with Bob Wills) still used a carbon mic and a portable PA as late as 1935." During the late 1920s to mid-1930s, small portable PA systems and guitar combo amplifiers were fairly similar. These early amps had a "single volume control and one or two input jacks, field coil speakers" and thin wooden cabinets; remarkably, these early amps did not have tone controls or even an on-off switch.[14] Portable PA systems you could plug into wall sockets appeared in the early 1930s when the introduction of electrolytic capacitors and rectifier tubes enabled economical built-in power supplies that could plug into wall outlets. Previously, amplifiers required heavy multiple battery packs.

In the 1960s, an electric-amplified version of the megaphone, which used a loudspeaker, amplifier and a folded horn, largely replaced the basic cone-style megaphone. Small handheld, battery-powered electric megaphones are used by fire and rescue personnel, police, protesters, and people addressing outdoor audiences. With many small handheld models, the microphone is mounted at the back end of the device, and the user holds the megaphone in front of her/his mouth to use it, and presses a trigger to turn on the amplifier and loudspeaker. Larger electric megaphones may have a microphone attached by a cable, which enables a person to speak without having their face obscured by the flared horn.

The simplest, smallest PA systems consist of a microphone, an amplifier, and one or more loudspeakers. PA systems of this type, often providing 50 to 200 watts of power, are often used in small venues such as school auditoriums, churches, and coffeehouse stages. Small PA systems may extend to an entire building, such as a restaurant, store, elementary school or office building. A sound source such as a compact disc player or radio may be connected to a PA system so that music can be played through the system. Smaller, battery-powered 12 volt systems may be installed in vehicles such as tour buses or school buses, so that the tour guide and/or driver can speak to all the passengers. Portable systems may be battery powered and/or powered by plugging the system into an electric wall socket. These may also be used for by people addressing smaller groups such as information sessions or team meetings. Battery-powered systems can be used by guides who are speaking to clients on walking tours. 041b061a72

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