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AEA KU4 Unidirectional Ribbon Microphone
where the peaks and valleys in the top end are, I’m prepared to believe they’re in the same places as those of the original RCA design. The response is actually very smooth with direction, with a clean dropoff of level as you move to the side, then an abrupt polarity change at the null and then you get to the rear lobe. The nulls aren’t really very clean nulls and they are kind of rough, but they’re much closer to real nulls than you’ll get with a conventional cardioid microphone. Actual testing in the studio with a monitor wedge in the null bears that out; you can use these mics in this situation if you’re careful about levels and reflections from walls. Plenty of mics don’t work when you do this. A 1W walkie-talkie at 146 MHz does not result in appreciable interference. Listening On a first listening test the first thing that really jumped out at me was the microphone pattern. It’s a supercardioid, which is to say it’s a figure-8 whose rear lobe has been reduced in size but still exists. The front lobe is kind of wide, and there is a very abrupt polarity change as you move across the two nulls, which are 120 degrees offaxis. These nulls aren’t as deep as with a figure-8 mic, but they are deep enough that they can be used to reduce leakage from a monitor wedge, or from a nearby instrument in the studio, very effectively. It’s a very, very clean vocal sound, and my first thought about the mic was to use it for male vocals. This isn’t surprising

are different than what you’d find on 1940s productions. None of this is bad— in fact, from the standpoint of having a product that is reliable and maintainable it’s all very good. Inside, the motor assembly looks a little like a 77DX, but with a tube leading from a sealed chamber behind the ribbon down into a hidden box in the bottom half of the microphone. There’s also a lot of baffling in there which isn’t in the 77DX, presumably to improve directionality at high frequencies. Underneath that is the acoustic delay assembly, which is a huge aluminum piece that has been machined with multiple holes and grooves in order to form a long mostly-nonresonant tube. My guess is that the two bumps in the midrange frequency response have to do with the length of this tube and possibly with the way the line from behind the ribbon is routed. My guess is also that with modern manufacturing methods it’s probably done in a very accurate and repeatable way so that all the KU4 mics sound the same. This is something that was not really possible in the 1940s and again will result in a reliable and repeatable mic in a way the KU3A could not be. The transformer appears to be an RCAstyle one, and the wiring from the motor to the transformer is heavy and carefully

artifacts from the rear lobe become evident. Still, I can get a rough overall response at low frequencies and better narrowband response in the top two octaves. Frequency response is (as in all large ribbon mics) very irregular in the top octave but it changes less as the source moves than the 77 and 44 mics, probably due to the improved ribbon design. The bass end is a little light and there are

Underneath the KU4’s motor assembly is the acoustic delay assembly (shown here), a huge aluminum piece that has been machined with multiple holes and grooves to form a long mostly-nonresonant tube.
twisted. My demo mic had an odd open splice in the wire which might result some magnetic noise pickup in a poor environment, but I couldn’t hear any problem in any of my tests and I am assuming that this will not be the case in the production units. The wiring is not supported but it’s heavy-gauge enough that vibration of the wiring should not be a serious problem, and my guess is that adding additional supporting would be deviating too much from the original RCA design (as would using a modern transformer). Measuring Since this mic is a supercardioid, it has a small lobe in the rear which makes measurement in my small hemianechoic chamber a bit more difficult, since measurement
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RECORDING October 2012

a couple of midrange dips and a little presence boost. I found this kind of surprising, actually, as in the listening tests (see below) I didn’t notice any of the little midrange blips at all, and I thought the low end response sounded more rolled off than it measured. That change in low end between my perception and the curve may have something to do with the microphone pattern. Looking at the published curves for the KU3A on the coutant.org microphones website, the midrange dips are at just the same location, as is that little presence peak. The coutant.org curves are smoothed “marketing department” curves from RCA, though, and at high frequencies measurements are very touchy about direction, so although I see some differences in

given that it was intended for dialogue recording. So the first thing I did was to put it up with a bluegrass group that wanted to all play around a single microphone, and it worked very well. The low end was a little bit light and had to be EQed up a notch to balance the upright bass, but vocals on-axis were clean and the instruments off-axis were detailed and clear without being too bright. On horns it was lovely, if anything a little bit too bright until I pulled it back a few more feet than I would normally put a ribbon. The mic’s character and eventual positioning smoothed the horn sound out without losing any of the blattiness. The KU4 also worked very well on electric guitar. That’s not a huge surprise— I have never used any microphone that

http://www.coutant.org http://www.coutant.org

Recording - October 2012

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