<P> By the early 1920s, Marconi had established a department dedicated to public address and began producing loudspeakers and amplifiers to match a growing demand . In 1925, George V used such a system at the British Empire Exhibition, addressing 90,000 via six long - range loudspeakers . 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 . In 1929, the Schneider Trophy race at Calshot Spit used a public address system that had 200 horns, weighing a total of 20 tons . </P> <P> 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 . 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 . </P> <P> 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 . </P> <P> 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 . </P>

Difference between analog and digital public address system