Microphone RCA 4A, also called the Microphone RCA 4AA, Microphone RCA 4AP, Box camera mic, and Microphone RCA 4A-1 is an early RCA Corporation condenser microphone used in the Golden Age of Radio. The box-style RCA 4A microphone was introduced in 1928. The RCA 4A had a built-in RCA AA4088 three vacuum tube audio amplifier. The AA4088 audio amplifier used three RCA 864 triode tubes, also labeled the UX-864. The "A" was for "Announce". The 4AA microphone came with a desk stand for use by radio announcers. The RCA 4AP came with a floor-mounted mic stand. Broadcast radio stations widely used the RCA 4A in the 1930s. The RCA 4A has a 7-wire cable that connects to the 19-inch rackmount vacuum-tube power supply and the microphone output electrical connector. The microphone has a capacitor condenser unit, RCA model UZ-4083. The output is from a balanced audio transformer. The microphone tube power supply, RCA Type II-A, feeds 180 volts, 90 volts, and 6 volts for the tube filament heaters. A WADX Microphone Type RCA 4AP was used in the closing scenes of the 1932 movie The Big Broadcast, in which Bing Crosby and Stuart Erwin sing the song Please, a 1932 song by Bing Crosby, at a radio station.
