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In the golden age of Hollywood filmmaking, RCA engineers developed what would become one of the most sophisticated ribbon microphones of its era: the KU-3A. This week, the Vintage King Tech Shop welcomed a pristine pair of these legendary ribbon mics for restoration.
"Both of these mics still have Wally Heider Studios stickers on them, and although we can't say what sessions they may have been used on, it is an iconic studio with a pedigree," said Tech Manager Cedric Yee. "These don't come through our shop very often, so it's very cool to see something like this that is from an iconic studio."
Developed in the late 1940s, the RCA KU-3A was designed specifically for film sound stages. At the time, motion picture studios needed a microphone that could capture dialogue with clarity while rejecting the mechanical noise of cameras and lights on set. RCA’s answer was the KU-3A.
Unlike RCA’s earlier bi-directional ribbon microphones, such as the 44 and 77, the KU-3A employed a cardioid polar pattern. This was achieved through a groundbreaking acoustic labyrinth built into the body of the microphone, which delayed sound waves entering from the rear before they reached the ribbon element. By introducing this delay, the KU-3A was able to combine the smooth, natural sound of a ribbon microphone with the focused directionality needed for film production.
Tailor-made for daily use on a sound stage, the KU-3A was built like a tank. The body was shock-mounted to reduce handling noise, the transformer was shielded against interference, and the capsule was built to withstand changes in temperature and humidity.
It was finished in a distinctive two-tone umber gray paint scheme, with a white reference stripe on the rear grille to guide boom operators in keeping the “dead” side of the microphone pointed away from the action.
Beyond its role in cinema, the KU-3A quickly found favor in music recording. Engineers discovered that its rich low end, smooth midrange, and silky top end sounded great on vocals, horns, and guitar. Its versatility helped establish it as one of RCA’s most revered ribbon microphones, bridging the worlds of film and studio recording.
The RCA KU-3A remains a highly sought-after piece of audio history. Hand-built in limited numbers, it represents both the technical ingenuity of its time and the enduring appeal of ribbon microphones. More than 70 years after its introduction, the KU-3A continues to be celebrated for its unmistakable sonic character.
This particular pair came into the shop with a few minor issues, and required re-ribboning and rebuilding the shock mounts, according to Cedric.
"The shock mounts are original, but we had to hunt down properly sized rubber to rebuild the elastic banding. The tan leather covers are for the head grill and I have been told are original to the microphone. The original connectors are still there to connect to the microphone output, but it has been terminated with newer cabling and XLRs."
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