RCA Uniaxial BK-5

In our previous article on the Electro-Voice dynamic cardioid microphones we briefly mentioned, at the top, the RCA Uniaxial mic, the BK-5. For this article I want to go into some of the details of this very interesting microphone.


The prototypes of this mic were developed in the early 1950s by RCA and, as I recall, were used on the speaker’s platform of the national political conventions in 1952.. The idea was to use the same generating principle as was used in the model 77 series of unidirectional mics…but in a more modern looking and somewhat smaller package designed for TV. The result was called the model BK-5A “Uniaxial” microphone which was followed by the BK-5B…but both were almost identical in appearance.

Unlike it’s older brother, the Model 77DX, the BK-5 had only one pickup pattern…unidirectional. However, the BK-5 did not replace the 77DX and they were both offered for sale up until the end of the RCA Broadcast division around 1980.

By making the BK-5 with just the one pattern the RCA engineers were able to optimize the frequency response for that single pattern. As good as it was, the model 77 wound up being a compromise in order to offer the multiple patterns in one microphone. The BK-5 had a much smoother and extended high frequency response than the model 77 in the unidirectional pattern setting.

RCA said that the BK-5 was especially engineered with the TV studio in mind. Since the maximum sensitivity lies on the major mechanical axis, it is a one axis, or uniaxial type microphone. This directional characteristic simplified microphone and camera placement problems when used on the overhead mics booms in TV. Incorporated into the unit was a blast filter which effectively reduces damage from gun blasts and other violent noises.

The BK-5 had a short fork-type swivel that attached to the identical rubber cushion mounting that was used on the 77DX which utilized the standard RCA half-inch pipe thread to the stand. There was also a special TV boom mounting and a large ball-type windscreen option. At the bottom end was the same type three position voice-music equalization switch as used on the 77DX.

RCA claimed that the BK-5 was admirably suited to general broadcasting and high-fidelity sound systems. This mic saw a lot of use in radio control rooms as an excellent announce mic. In fact, as I recall, both KHJ in LA and KFRC in San Francisco used this mic in their on-air studios during their “Boss radio” heydays.

Dr. Coutant tests the BK5-A Ribbon Microphone:

The BK-5 was a ribbon mic and utilized the same basic acoustic principle as in the model 77 series of velocity unidirectional microphones. Further detailed information on this mic is available on the Stan  Coutant  microphone website .


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