People should perhaps be aware that the U 67 and the various Neumann stereo mikes based on its same K 67 capsule sound different from one another. It would be good to know which approach is actually planned for this stereo mike, since it can't be both at the same time.
The U 67 is a mellow, smooth-sounding tube mike but its capsule has an inherently rising high-frequency response on axis. The U 67 circuitry contains a very fancy feedback system that does a number of things, such as providing a low-cut filter before the signal even reaches the grid of the tube. It also rolls off the top-end response of the capsule a total of 7 dB (!) by 16 kHz.
German broadcasting at the time the U 67 was introduced (1960) was still a government monopoly, and all the German stations received their technical direction from a central organization which was the forerunner of today's IRT. Since the early 1950s, German broadcasters had settled on the Telefunken AC 701k tube as standard equipment in condenser microphones, so there was unhappiness about the fact that the U 67 used a different tube that needed special power supplies. Many broadcast studios had N 52 power supplies for AC 701-based microphones built right into their mixing desks.
About a year later, Neumann agreed to produce a broadcast version of the U 67 called the M 269, which not only used the AC 701k but, like the M 49 before it, also offered remote pattern control. (It defaulted to cardioid when used with an ordinary power supply, but if used with the special power supplies built for the M 49, it could be remotely controlled.) And since the broadcast users expected to use the M 269 as a "main microphone" for recording entire broadcasts with only one or two mikes--i.e. not as a spot microphone or a vocal soloist's microphone, the way it's mainly used in studios today--they were content with an EQ curve that rolled off the response a little less on both ends. (Bandwidth control was again a standard feature of broadcast mixing consoles, so it didn't need to be in the microphone so much.) Thus the M 269 not only has different features and specifications from the U 67, but also a slightly different sound quality.
Then Neumann superseded their SM 2 and SM 23 stereo microphones with a model based on two K 67 capsules and two AC 701 tubes. Since this was intended for more distant use--overall recording rather than spot miking--and since space was at a premium in the combined amplifier, Neumann simplified the feedback arrangement and removed the circuit branch that rolled off the high frequencies in the microphone. As a result, the SM 69 and its solid-state successors (the SM 69fet and USM 69), have a rising high-frequency character; they're the opposite end of the spectrum from the U 67 as far as this series is concerned. The spectrum goes like this: U 67 (= 7 dB rolloff by 16 kHz) <-> M 269 <-> U 77 <-> U 87 and U 87 A (= 4 dB rolloff) <-> the three stereo mikes (= no high-frequency rolloff).
Thus my point: A stereo microphone can't faithfully follow the U 67 sound if it sounds like the stereo mikes that are based on the same capsule. I for one would prefer the U 67 sound--the SM 69fet and USM 69 definitely need some high-frequency restraint at times, and the whole notion of directional microphones needing a tipped-up high end to compensate for recording distance doesn't make much sense to me anyway.
--best regards
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