RCA 77-A


Type Ribbon
Polar pattern Variable pattern
Diaphragm Small
First decade of production 1930s



This is the RCA Type 77-A, forerunner of the 77-DX. Grand-daddy of the ribbon microphone, the 77-A is among the rarest of the RCA ribbon mics. Designed by Dr. Harry F. Olson, RCA’s lifelong resident audio genius during the late 20s and early 30s, the 77-A set the performance benchmarks for all RCA ribbons to follow for the next four decades. It is rumored that proto­types actually existed in 1929 and 1930; however, the 77-A wasn’t announced until 1932. It featured two vertical in-line ribbons and an acoustic labyrinth inside the case, which enabled it to be uni-directional. The 77-A is a huge microphone resembling a cannon shell with a large perforated windscreen on the top portion. It is gimballed at its center of gravity in a U-shaped fork.

Design, Specifications and Technical Data

Output Impedance: 30,150, and 250 ohms

Load Impedance: Open circuit

Effective Output Level at 1,000 Hertz (all output connections):

  • Bi-Directional (B) –54 dbm
  • Uni-Directional (U) –57 dbm
  • Non-Directional (N) –60 dbm
  • L-1, L-2, L-3 between –54 and –57 dbm

Output Level for Speech at Two Feet (all output connections):

  • Bi-Directional (B) –61 vu
  • Uni-Directional (U) –64 vu
  • Non-Directional (N) –67 vu
  • Hum Pick-up Level: –125 dbm

Dimensions and Weight:

  • Length: 11½ inches
  • Width: 3¾ inches
  • Depth: 2½ inches
  • Weight: 4½ pounds
  • Mounting: ½-inch pipe thread

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