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Vintage Microphones, Part 2: Neumann M49, M50, U67 and U87
May 18, 2004 12:00 PM, Stephen Paul
Note: This article is a segment of a multi-part series detailing the history and technology behind some of the industry's most beloved and treasured vintage microphones. The autho—the late Stephen Paul—founded Stephen Paul Audio (www.spaudio.com), a firm that continues to offer specialized services in the restoration and hot-rodding of classic microphones. One of Paul's most enduring contributions to modern microphone technology was his research and application of ultrathin diaphragms (sometimes less than a single micron thick), which spawned a renaissance in the development of improved microphones from companies worldwide.
First published in 1989, this article series was one of Mix's most popular back-issue requests, and 15 years later, we are pleased to bring this work to a new generation of microphone fans.
Also, as a bonus, brochures of all the microphones in this article and other classic mics (in .pdf format) are available on this Website.—George Petersen
In Part 1, we started our series with an in-depth look at the famous Neumann U47, one of the most celebrated of the tube condenser microphones. We also covered a lot of ground on the operation of pressure gradient mics in general, and if you missed that issue…too bad. But serially folks, I am, from this point on, going to assume that you have at least a rudimentary understanding of the principles of physics involved in the construction of pressure and pressure gradient microphones. We also discussed the construction of the single backplate condenser capsule as made by Neumann since the late ‘30s. In this installment we will also be calling upon that knowledge, so sharpen your pencils—there will be a quiz after the show.
The stars of the show this time around will be the M49 (with a brief word about the M50), the U67, and its contemporary sister, still in production after 20 years, the U87.
They say old mics never die, they just get more expensive, but at least there are pockets of resistance to their demise here and there. Like in every major studio in the world, and a lot of minor ones, too! It is a fascinating fact that as microphones get more and more advanced in their physical construction, the old ones shine more and more in their very imperfection. We mentioned last months that all mics are echo chambers to some degree, and there are good ones and there are bad ones. Well, this fact bears repeating, that’s why I just did.
Keep it in mind as we explore the offerings from Neumann that kept the recording world simmering through the ’50s and ’60s. In our final debacle next month, we will look at the heavy-hitters from AKG that set the tracks of Ampex 300 Series machines on fire during that same era.
The M49 debuted in 1949 and was an instant success in German broadcasting. The microphone was designed on contract for the IRT, the governmental regulating arm of the German broadcast system. The salient feature of the M49 was that it was the first condenser microphone capable of remote pattern control, achieved through electronic means. By varying the rear-diaphragm voltage against the backplate reference, a full range of patterns, from omni to cardioid to figure-8, was accomplished. A Dr. Grosskopf held the patent on this method of pattern selection, and Neumann was contracted to build the resulting design.
The 49 had several other interesting features, including one of the first attempts to limit the effect of grille resonance on the response. Because the grille is slanted and presents a continuously varying profile to the capsule, there are fewer standing waves generated. It is also an interesting microphone because it was one of the first to incorporate a special ultra-miniature triode tube specifically designed for microphones. This was the MSC2, which was later refined and further miniaturized before seeing mass production as the AC701. It was a cute li’l bugger! It was designed to have a long life, low-current consumption, high-input impedance, and low microphonic and capacitance characteristics. In fact, this became the only tube permitted in a microphone that was going to be used on the German airwaves. It became understandably popular in Europe.
CORVETTES AND CADILLACS
The M49 used the same capsule as our old friend the 47, and yet, because of the completely different housing and the triode electronics, the sound of the mics bears few similarities. Both of them are superb vocal mics, though neither was designed for the purpose of close-miking. However, those crazy Americans, with their Corvettes and their Cadillacs, their loud ways and their rude rock ’n’ roll, discovered the magic of blasting a Neumann at close range, and thus began a love affair with proximity effect and large-diaphragm capsules that has endured to this day. One could even say that a love affair implies romance, and that level of what it is about these magic microphones that makes us music and engineering types so crazy. We recognize that sound. And we love it. All the more 'cause those days are gone. But the mics aren’t. Thank God.
As we have pointed out before, today’s offerings from the Mighty Manufacturers Over There are light years ahead of these crusty old dinosaurs in terms of specmanship and thinkology; but the fact is, for some reason these new starships just can’t seem to shake the oldies. It’s kind of like having the Red baron on your tail. Naturally, new classics like the TLM170 and the Sanken CU-44x are bound to prove themselves over time, and in some arenas they will doubtless prove superior not only in specifications, but in performance under certain conditions. However, the pure thrill of belting a ballad into a 47 tube will never die. The 49 is especially good on female vocals. Not having quite the “nasality” that the 47 is known for, it has a smoother high-frequency range, and the size of the grille forms a larger-volume enclosure giving the capsule more breathing room and contributing to its “openness.” Many female singers, including Barbra Streisand, use this microphone. It has a way of rendering the female vocal with an immediacy and presence not found in any other design. No, this is not a Neumann brochure. The damn thing works!
GREAT BALLS OF FIRE
Because there was also a demand for a microphone that would compensate for the loss in high-frequency response incurred by long distances, the M50 was born. High frequencies roll off as the square of the reciprocal of the distance from the sound source due to frictional effects. This means that the viscosity of the air is sufficient to dissipate sound energy in the form of heat, and as there is less power in the treble range, it suffers the most from this effect. Therefore, when a mic is hung high above an orchestra in the auditorium, a boost in the response above 5 kHz is desirable. Even today, these microphones still make some of the most wonderful orchestral recordings in the world. We have successfully modified this model with a one-micron diaphragm and they sound quite wonderful.
We will touch on them briefly, but as they were omnidirectional only and not often used on vocals, they are beyond the scope of this diatribe. Just for the archives, we have included a photograph of one without its grille, so the vastly different construction (and the meaning of this subhead) from the 49 can be seen easily. It confuses a lot of people that these two mics are almost identical on the exterior. You can tell a 50, as it always has a white jewel or dot on the front, and the 49 has a red one.
So much for the 49. We now turn to a significantly difficult microphone to pin down, the famous (or infamous if you prefer) U67.
SO, YOU WANT A CLOSE-UP MIC?
Sometime in the mid-'60s, someone at Neumann had a great idea—“Hey, let’s build a close-up microphone! Yeah!” And so, into the pages of history was written the U67. Now this mic set the stage for Neumann’s entrance into the future. In fact, an original brochure for this mic proclaimed, “U67, Sound of Tomorrow!”
In a way, it did turn out to be somewhat prophetic. The U67 was a real turning point for Neumann and for recording in general. The 67 was a genuine attempt to create a tube mic that could withstand sufficient levels to really allow close-miking a voice or loud instrument without some of the problems that could occur.
First and foremost, the 67 was designed with a low-frequency roll-off that was always active, a sort of electronic rumble and pop filter. It also had a high-frequency roll-off built into the amplifier and a boost built into the capsule. This was a sort of acoustic pre-emphasis and electronic de-emphasis curve. The idea was that by using lots of feedback and feedforward, they could control the dynamic range and signal-to-noise of the mic, enabling it to handle high levels at close range. (Feedback means taking a portion of the output and mixing it back into the input out of phase, and feedforward means mixing it back to the input in phase.) This had the added benefit of making the mic quieter and less subject to sibilance. However, there are some who feel that this did not do a w hole lot to make the mic sound “open,” and the heinous crime of negative feedback is enough to make today’s audiophiles keel over their Conrad-Johnson tube amps. Oh well, since when have we pros cared a hoot about wheat the audiophiles think?
The fact is that there are some who feel that the mic is perfect for vocals where a smoother sound is desired. We do build a version of the 67 with a one-micron diaphragm and some of this feedforward-back adjusted for more top end, and it has been used on artists such as Linda Ronstadt with very satisfying results. The thinner diaphragm seems to smooth out the peak at 12 kHz sufficiently to allow us to use that silky energy range with no penalty in signal-to-noise. Be that as it may, either version of the mic is still widely accepted for its “warmth.” When we modify it to add clarity, we are careful to keep this defect intact.
THE LABYRINTH OF THE MINOTAUR
An interesting feature of the 67 is that it was the first large-diaphragm capsule built by Neumann that used the “labyrinth” acoustic lowpass filter dual backplate design. What a mouthful! The photo on page 119 shows the two parts of this backplate. The electrode is shown with one in a front orientation and the other showing what the inside facing surface looks like. This pattern of holes is offset when the capsule is assembled, and no one hole looks directly into any other hole. They are all spaced apart from one another about 1.5mm, and the two halves are assembled with a spacer that creates a slot between them.
What happens here—without getting too technical—is when a sound wave below about 4 kHz hits one side of the capsule, it goes through the hole, across the slow and out the other hole. This causes a time delay, which is adjusted to equal the delay of a wave that went around the capsule. If they both get there at the same time, they will cancel, and the mic is a cardioid.
Above 4 kHz, the offset holes act as a lowpass filter and roll off the sound pressure. The reason this is done is when the wave becomes smaller than the diameter of the capsule, the physical wall of the rings and backplates becomes an obstacle to the sound, and the mic automatically acquires a cardioid characteristic. However, since the ideal cardioid will pick up all audio frequencies equally up to the 90-degree boundary and then shut everything off as we walk behind the mic, we have to control the angle of pickup at high frequencies very carefully. If the mic is designed as a cardioid even at high frequencies where the wave is shadowed anyway, the mic will attain a very narrow pickup of high end, considerably less than 90 degrees. By shutting down the cancellation mechanism at the wavelength/diameter transition point, the mic is transformed into an omni and picks up high end quite a bit better off to the side, where it eventually disappears anyhow owing to the blocking wall of the capsule itself! Get it? Neither do I, but perhaps if we read it a few times it will come to us. It is to Neumann’s credit that the U87 is still manufactured, the solid-state sister to the 67. A modified 87 has the same advantages as the 67 and the added feature of reliability.
Newer mic designs attempt to address this off-axis problem, and though they have certainly succeeded, for some strange reason an awful lot of artists prefer the old chestnuts, which are loaded with flaws. I guess there’s just something about these poor, old-fashioned, outmoded designs that people just plain like better! (Well, on some vocals, anyway.)
Certainly, the cardioid/omni approach is a big, fat compromise at best, but on the viscerality index, big is beautiful. We like those 32mm diameter mics. The only way to correct the problem is to build a smaller capsule, and many have been so manufactured. But in the end, it’s these older, larger, less-perfect systems that have garnered the huge following of the vocalist crowd. Surely this cannot just be ignorance on the part of all these artists and producers.
In this series we have tried to get a rare glimpse of the secret insides of these sensitive devices and explore the subjective aspect of the almost fanatical devotion that many recordists have to these older mics. The shapes of the mics themselves have a lot to do with their responses, and the various types of amplifiers and tubes contribute to our impressions of them.
We can see that in some cases these devices may be looked at as “colors” that the engineer can use, as though having a magic paint box from which to select the perfect “tone” to capture a sound. When we do a modification to any of these classics, we are not trying to change the character the mic possesses, but simply trying to bring the system to the pinnacle of performance that is its particular shape and hardware underpinnings render it capable of.
In the end, the most successful of us have allowed our ears to be the judge, and when the artist says “Wow!” you know you got a good one.
So, until next month, when we delve into the mysteries and some of the history of the AKG series of classic vocal mics, may all your vocals be colored and crystalline; yes, and unashamedly so.
Stephen Paul founded Stephen Paul Audio, a Burbank, Calif.–based firm specializing in the repair, modification and refurbishing of contemporary and vintage microphones.
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