RCA Type 77-DX microphone
The RCA Type 77-DX microphone is a poly-directional ribbon microphone, or pressure-gradient microphone, introduced by the RCA Corporation in 1954. Both it and its predecessor, the Type 77-D, are listed in RCA's 1955 catalog, and the 77-DX appears as late as 1967. Its design has inspired a stereotypical microphone icon. Throughout the 1950s and 1960s, phonograph record album cover photos show artists singing into the side of a 77-DX.
The directionality of the 77-DX is variable. A rotating backshutter on the acoustic labyrinth, controlled by a screwdriver-operated slot at the rear of the microphone, allows the user to vary the microphone's pattern from omnidirectional in the fully closed position, to cardioid, to figure-8 (bidirectional) in the fully open position. A 3-position switch at the bottom end of the microphone allows the user to control the desired amount of low-frequency rolloff .
Response and output characteristics
At 1 kHz, the 77DX has an output of -50 to -56dbm, depending on the pattern selection. The output impedance is user-selectable; factory preset at 250 ohms, and changeable to 30 or 150 ohms. The microphone has a fairly flat response. In the cardioid (unidirectional) pattern, it is level from approximately 150 Hz to 2 kHz, with a slight rise peaking at just under 5 kHz, then dropping approximately 3dB/octave to 20 kHz.
The 77-DX has been used on countless vocal recordings by Bing Crosby, Kate Smith, Frank Sinatra, Elvis Presley, Johnny Cash and many others. It is still sought after today for use in recording brass instruments.
The foil ribbon element in a 77-DX will respond to gravitational force as the microphone is positioned or tilted. Audio engineers experienced with the 77-DX typically suspend, hang or position the microphone at various angles to achieve a variety of desired frequency response characteristics. For example, with the microphone suspended, a slight tilt on the horizontal axis will cause the ribbon to 'sag' slightly, resulting in a noticeable boost in the lower-midrange frequencies.
The ribbon is positioned inside the housing in such a way that fricatives may cause popping sounds, due to the explosion of air directly into the ribbon. A common solution to this problem has been to attach a pencil vertically across the front of the microphone. The pencil deflects a vocalist's breath as it reaches the microphone, and prevents it from hitting the ribbon directly.
- S.O. Coutant, Microphones, retrieved 2011-01-15