RCA BK-5A Cardioid Ribbon Microphone #4287 (Vintage)

The RCA BK-5A is a classic ribbon microphone. Great on horns, guitars, vocals. Rich lows, silky smooth mids and highs. The dual layer filter protects the ribbon from high SPLs, so record bass cabs and drums without fear! A must have for every mic locker - from RCA, the inventors of the ribbon mic.

History

A ribbon mic works by suspending a thin piece of corrugated aluminum, (the 'ribbon'), between two poles of a magnet. The compression and rarefaction of air vibrates this ribbon in the magnetic field, which creates voltage changes that are run through an output transformer to the external amplifier of your choice. Like a dynamic mic, a ribbon requires no tubes, batteries or power supply of any kind. In the case of the RCA BK-5A, the chassis is built to absorb mechanical shocks, the output transformer is shielded against magnetic fields, and the capsule is resistant to changes in humidity and temperature.

From the original brochure:

"The RCA Type BK-5A Uniaxial Microphone is a dependable, high- quality ribbon instrument possessing an improved unidirectional characteristic, and is designed for broadcast use in AM, FM, and TV stations. The microphone has a frequency response that is essentially uniform from 50 to 15,000 cycles. Its excellent response and frequency range, combined with its unexcelled cardioidal directional characteristic, makes it ideal for reproducing both speech and music.

The microphone has been engineered with the television studio in mind. Since maximum sensitivity lies on the major mechanical axis, it is a one-axis, or uniaxial type micro­phone. This directional characteristic simplifies micro­phone and camera placement problems. Incorporated in the unit is a blast filter which reduces damage to the micro­phone from gun blasts and other violent noises. In addition, the small size, light weight, unobtrusive yet attractive gray finish and appearance render it especially suitable for television, but it is also admirably suited to general broadcasting and high-fidelity sound systems.

The Type BK-5A Microphone is a unidirectional micro­phone in which the moving element is a thin corrugated metallic ribbon clamped under light tension to cause it to vibrate at its own resonant frequency. The ribbon is placed between the pole pieces of a magnetic circuit. One side of the ribbon is open to the atmosphere and the other opens on an acoustical labyrinth which has phase-shift openings giving the instrument its improved unidirectional characteristics. The labyrinth of the micro­phone houses an impedance matching transformer and switch for selecting response characteristics for voice or music.

A unique feature of the BK-5A is a blast filter consisting of two separate cloth layers supported by perforated metal screens. The filters reduce damage to the micro­phone from gun blasts and other loud noises encountered in broadcast programming. In addition, the transformer is exceptionally well shielded against stray magnetic fields and can perform satisfactorily in high hum fields. As further protection for the sensitive vibrating ribbon, a wind screen is available for use with the instrument. Its use is recommended if the instrument is to be used outdoors."