Recording - October 2012 - 30

BY SCOTT DORSEY

AEA KU4 Unidirectional Ribbon Microphone
A rare vintage mic makes a stunning entrance into the 21st Century
Most of the time, reviewing replica and reissue products is easy. You get the “Pultec-like” equalizer, you put it on the bench next to a Pultec, and compare the two. With microphones it can get a bit more difficult, since sometimes microphones change with age (and sometimes people like the changes with age and sometimes they don’t), but you still have a reference point... The problem with doing that in this review is that with the KU4, Wes Dooley of Audio Engineering Associates (AEA) has adapted the design from the RCA KU3A microphone, and hardly anyone has ever seen an actual KU3A. Whereas the 77 and 44 microphones could be found in every radio station and studio in the country, the KU3A was a small production product of which RCA Photophone made two runs of under 300 units each for the film industry. Researching I remember hearing people in the 1980s talking about the “MI-10001” microphone for scoring stage use, as if it was some rare legendary Hollywood thing. That was the catalogue number for the KU3A, I now learn, “MI” meaning Manufacturing Inventory. What I do know about the KU3A is that it was originally intended for dialogue recording on film sets. It is considered one of AEA’s Big Ribbon family, but its ribbon is 1.25" long vs. the 2.35" ribbon on the figure-8 AEA mics like the 44, 84, 88 and 92. Rather than being suspended in free air, it has a labyrinth behind the ribbon and some venting in order to give the microphone a very tight cardioid pattern. Because these mics were made in very small quantities they didn’t get the kind of careful cost-engineering that mass production products did, and some parameters on them seem to have been tweaked by hand. Now, when you make a replica of a vintage product, a lot of what goes into the reissue is knowing what needs to be precisely machined and what does not. Machine a part to too tight a tolerance and you increase the cost unncessarily. Machine it with tolerances that are too loose, and you degrade performance. The problem is that I suspect the original engineers of the KU3A didn’t put much effort into this; when you’re making a small handful of products one at a time by hand, you don’t have a chance to find out where the tolerances can be relaxed vs. where they need to be tightened. Folks who have used the KU3A confirm that many of them have “file-until-it fits” hand work in places, and a modern reissue cannot do this. The RCA 77 was an adjustable pattern microphone. It was a figure-8 ribbon but with an acoustical delay line (which RCA called the labyrinth) connected to the rear, and a plate that allowed the rear vents to be open or closed to change the mic pattern. Looking at the KU4, to some extent it looks to me like someone took a 77 design and enlarged and re-engineered the labyrinth to reduce internal reflections at the expense of having the pattern fixed. Then they replaced the grille with one that has fewer internal reflections. It is a large and imposing microphone and much heavier than the 77DX, and upon listening to the mic, I find it also has more clarity and detail than a 77DX... which is not something I had expected. Peeking First of all, this microphone, although it’s a part-for-part replica, is not identical to the original RCA. AEA uses Phillips-head screws, for instance, which were still patented (and therefore avoided by RCA) at the time the originals were made, and they are locked into place with modern RTV rather than the glyptol that RCA would originally have used. Some of the metal finishes and the electroplating job on the yoke
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RECORDING October 2012

Recording - October 2012

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