Vintage Microphones, Part 3: AKG C-12, C-24 and Telefunken 250/251

Note: This article is the final segment of a multi-part series detailing the history and technology behind some of the industry's most beloved and treasured 5/18/2004 8:00 AM Eastern

Note: This article is the final segment of a multi-part series detailing the history and technology behind some of the industry's most beloved and treasured vintage microphones. The author—the late Stephen Paul—founded Stephen Paul Audio (, a firm that continues to offer specialized services in the restoration and hot-rodding of classic microphones. One of Mr. Paul's most enduring contributions to modern microphone technology was his research and application of ultra-thin 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

Well, gang, this is it—the last in this three-part series of scientific fact and unbridled, unmitigated opinion. Of course, though opinion enters into it, it is an opinion garnered over the years through my own personal interaction with all of you crazed users out there, and, thus, it is really a synthesis of all your opinions.

Now that we’ve got that disclaiming cop-out out of the way, let’s take a break from our friends at Neumann and go across the border to Austria, the land that brought us Mozart and Freud. Obviously, these guys know how to party.

AKG is the focus of our attention here, in case you haven’t guessed. AKG is fascinating because the company was ultimately responsible for introducing one of the most important innovations in capsule design, yet, this is not well-known at all. AKG was noted throughout the ’60s and ’70s for the brightness of its microphones and its distinctly different color from those of Neumann. Let’s have a look at some of these brilliant performers and try to discover some of the reasons they sound the way they do. (As Richard Heyser once said, “It pains me to give away in tem minutes what it took me ten years of work to discover,” but what the hell, eh?)

The first of the AKG large-diaphragm, tube-equipped condenser microphones was the C-2, introduced in 1952. As with the earliest M49 mics, it was equipped with the Hiller MSC2 tube, the forerunner of the AC701. A year later, in 1953) the glorious year in which the Corvette was introduced), AKG designed the C-12, identical to the C-2 in every way except for its 6072 industrial triode. This is an internationally available miniature twin triode tube and was still used much later in AKG's The Tube microphone. However, when we restore or modify C-12s or 251s at Stephen Paul Audio, we equip the microphones with a 5751, a tube we have found to have preferable sonic characteristics—it is extremely quiet and consistent.

To understand the innovative AKG sound, we have to investigate the heart of the mic—the capsule. On page 94 we see a photograph of the CK-12 capsule in its original incarnation. The two separated halves are shown next to each other. The unseen inner half has the round pattern of holes at a different location from the center, this ensuring a minimum offset of 1.25mm between the inner sets of holes with the backplates are assembled. As in the 67 backplate discussed last month, this offset has a number of functions, one of which is to set up the delay of a wave traveling through the capsule. This delay must be tuned precisely to the same value as the diffractive delay around the capsule. In other words, when sound from the back of the microphone travels around the capsule and pressurizes the front diaphragm, the wave going through the capsule system must arrive at the back of the front membrane at precisely the same moment, in the correct phase relationship, in order to cancel sound coming from the rear. A small gap between the backplates, in conjunction with the offset of the holes, sets up this delay.

AKG was the first manufacturer to build a dual backplate design. If you recall from the 67 article, we discussed how the two electrodes also act as a lowpass filter and transform the microphone into an omni receiver above a certain frequency. This is so high frequencies will be picked up better when off-axis to the mic. The other advantage of the dual backplate design was that it enabled the polar pattern of the mic to be adjusted without changing its on-axis sensitivity. This backplate scheme was the invention of Mr. Kalusche and Dr. Spandock, two engineers from Siemens and Halske. It was patented in 1951, and the following year AKG modified the design and used it in the C-2. AKG’s version of the device was patented in 1954. This became the basis for all subsequent CK-12 capsules until about 1981.

If we really want to unravel one of the greatest secrets of the CK-12 capsule, we will have to travel down into the darkness of its secret chambers. Yes! Here, deep within the bowels of the electrode itself, we will find walls teeming with the hidden mysteries of Acoustical Design. These perforated sheets of brass, inscrutably performing their miracle of transformation, compress and reverberate the acoustic waves traveling through them in enigmatic and ineffable ways. And, thus, we perceive the magical sound of the C-12 and 251.

The two main schools of thought surrounding capsule systems have been called aperiodic and resonator-equipped. These are their incomprehensible names, passed down from High Priest to High Priest. Well, actually, it’s almost that esoteric. I mean, just finding out about all of this stuff took some serious Inspector Clouseau tactics. Let’s hope that they don’t come after mo for spilling the beans. Quick, before they realize it! Here’s the story.

As we hack our way through the jungles of capsule design, keep in mind that there are many ways to skin an explorer. Without going over the high side, let’s quickly review a couple of salient points about acoustical systems. When I first put on my Indiana Jones hat and sallied forth into the forest of design, I was amazed to discover that the languages of these vastly different cultures, Electronics and Acoustics, have a common ground called dynamical analogies. Very simply put, if we stick a hole in something, its behavior in a sound field can be modeled by comparing it to various electrical elements. For example, a hole drilled through a piece of brass can be modeled as a resistance and capacitance (acoustical, in this case.). Depending on the size of the tube formed by the hole’s passage through the material, there’s also a bit of inductance as well. (Again, this is acoustical inductance, of course.) Every hole, slot and chamber found in microphone design has its electrical equivalent circuit. If your can figure out what analogies apply to a given physical setup, you can model the capsule’s behavior and get some idea of what it will do in reality.

In the case of the CK-12, there is a set of chambers behind the perforate surface that lies beneath the diaphragm. This is why in the photograph you can see that the pattern of holes on the outer surface is different from the round pattern found on the inner surface. In between these two walls is a chamber. This chamber has a resonance that falls within the audio range, thus creating a resonator-equipped design. It is interesting that Neumann built only aperiodic designs in its large-capsule format. Neumann did, however, make use of the resonator design in its smaller mics, notably the nickel and aluminum diaphragm types. Aperiodic designs feature elements (holes, slots, tubes, etc.) that are dimensioned in such a way that their individual resonances lie above the highest operational frequency of the capsule (also known as the boundary frequency.)

In the original AKG design, these chambers were very small, and their resonance was fairly high. It almost qualified as an aperiodic design. A production change expanded the volume of this chamber, and this brought its resonance squarely into the high-frequency range. The fascinating thing is that the change was only made to increase the overall sensitivity of the capsule and not to adjust its frequency response. The additional rise in top end was considered a negligible side effect in those days. The engineers wished to have a higher-sensitivity mic because FM broadcasting began, and with the improved specification for signal-to-noise, the mics were required to be quieter. To lessen the damping forces applied to the diaphragm, they increased the air gap behind it and enlarged the volume of the chambers. This bought them roughly a 3dB increase in sensitivity. In other words, the output of the mic for a given sound pressure rose by this amount.

Isn’t this wonderful! For it’s the sparkle and the edge that attract today’s users to these products, and these characteristics were acquired quite incidentally. One of the other main differences displayed by the AKG mics that set them apart from the Neumann stuff is that Neumann put a screw in the center of their large-diaphragm capsules, and the membrane vibrates in the form of a ring. The free air resonance of the diaphragm is also set quite a bit lower that AKG’s, all of these considerations create a vastly different sound character.

AKG used a fairly tightly stretched diaphragm that was largely friction-controlled.; Neumann tunes its diaphragm quite a bit lower and uses a combination of friction and mass control.

AKG has put its diaphragms on a diet several times over the years. The first generation of the CK-12 was made of 10-micron Stryroflex plastic. The next generation was 9-micron mylar, and in the late ’60s the design changes to 6-micron mylar. This was the last version available from the factory. We, of course, make this capsule available with films down to either 3- or 1.5-microns thick. As AKG found, when you lower the mass of the membrane, things really start to improve up top. Naturally, we aren’t satisfied until we take things right to the edge, and then the CK-12 is equipped with these ultra-thin films, the transient response improves dramatically. Our modification also has a tendency to lower the “Q” at resonance, and this has the subjective effect of smoothing out the high-frequency range quite a bit. The thicker film has a tendency to be peaky in this area, and is a little edgier.

The chronology of the more celebrated AKG mics might be of some interest here. So let’s look at the years in which our favorites debuted.

As mentioned earlier, the C-12 was introduced in 1953. It had a 10-micron capsule and a fairly flat response. The earliest versions were equipped with the original CK-12, which has the smaller chambers and therefore less sensitivity and less high-frequency rise. These first editions were also equipped with a large non-humbucking transformer that had a number of impedance options built in. The early power supplies looked rather like large toaster, and the separate pattern control box was somewhat reminiscent of a Lionel train power transformer throttle. Ah, the good stuff, eh? The power connector also looked as though it was a refuge from a theater lighting setup, and the entire thing weighed about 25 pounds. A beige crinkle-finish paint completed the beautiful, solid look of these earliest systems.

Somewhere around 1959, Telefunken commissioned AKG to build a microphone that would be larger in diameter than the C-12 and would have the pattern control on the microphone itself rather than attached to the power supply. It has been rumored that this request was a result of Neumann having decided to distribute its own product directly. In the past, Telefunken had distributed Neumann’s microphones worldwide, and this is why some U47s say Telefunken on them. When Telefunken was left without a professional condenser microphone in its product line, the company contracted AKG to build the 251. Telefunken wanted it to be somewhat like the 47, so AKG came up with the 250. The 250 and the 251 are essentially the same mic, the 251 having the addition of a figure-8 pattern.

The 251 also pioneered a pattern-switching scheme that Neumann would later use in the U87, that of insulating the two backplate halves from each other and switching them in various combinations to achieve three major patterns.

There was a 251E in addition to the 251. The “E” stood for export, and this mic was delivered with the 6072 installed. The non-E version has the ubiquitous AC701 in it and could be used on the Gernal airwaves. The tube in the 251 was inverted so that the pins were closer to the capsule. Unlike the C-12, which had a fairly long wire run to the tube, the 251 was set up to minimize the length of the leads to the capsule, thereby reducing the stray capacitance to the input stage and ensuring maximum output from the transducer.

In 1964 the C-12a appeared, and this was a radical redesign of the entire system. For one thing, the case was tiny compared to the C-12 or the 251, and the mic employed a 7586 nuvistor tube in a cathode follower configuration. The C-12a looked essentially like the 412 and the 414, with its angled grille and hand-sized body. The cathode follower circuit ensured very low distortion, but had the disadvantage of extremely low gain by condenser mic standards. It also had a relatively dense nylon mesh in the grille, which lent the top end a pastel character.

The C-12a was the last of the AKG vintage tube mics to be manufactured, and in 1971 the 412 was introduced, the first FET large-capsule mic from AKG. This was followed by the 414 and its subsequent additions, which have continued to this day. The construction of the CK-12 has been radically updated, and today the sound bears only a slight resemblance to the heyday for the hotsy-totsy, top-end screamers that made the tube systems so unique.

AKG has certainly “fixed” the problem of the top-end rise, but many of us feel that those problems are the essence of enjoyment for many artists. Se we work hard to keep that old sound alive for the many who feel that there is no substitute for the kick-ass top end of these large-diaphragm, resonator-equipped beauties. The concession that we make to modernity is purely in the ultra-thin film of our version, which brings mics into the ’90s with killer rise-time specs.

After all, we keep talking about this thin film we use… Sorry, couldn’t resist. Actually, a little discussion of the relative merits and the subjective effect of these different models might be somewhat informative and throw some light one the more obscure aspects of these wonderful tools.

The C-12 is revered for its top end, strangely enough. It’s funny—I’ve got a drawer full of the older type of small-chambered flat capsules, and I can’t give the damn things away! I’ve tried. I took one out of George Massenburg’s C-12, and just for a joke, when George had me mod the mic with the 1-micron film, I stuck in one of the big chamber capsules with a 6dB rise at 12 kHz. I figured he’d hate it and ask me to make the mic more like it had been.

Well, it’s a good thing that I don’t speculate in the stock market! George discovered what so many others already had. The damn thing was fun! It looked like those backplates might not be coming back after all. I decided to go by the Complex and see what was up. I figured I’d better be sneaky about it, so I pretended I was just there to visit Linda Rondstadt, who was recording the Canciones album with Shawn Murphy. Linda was using an early 1-micron 67 (the first-ever commissioned, in fact), and I happily sat and listened to her incredible vocal work. I figured George, who was in the next room recording Toto, would eventually swoop in and hopefully tell me how awful the extra top-end on the mic was.

Well, sure enough, here comes out boy, and he be wagglin’ his finger at me. The conversation went something like this:

“So, George, how’s the C-12?” (Gulp!)

“Stephen, that thing is the best mic I ever heard in my life. I don’t need any EQ on it. I just plug it in, go right to tape, and it sounds unbelievable. I don’t know what you did, but whatever it is, you did it right. The mic is perfect!”

Normally, such high praise from someone like George would’ve warmed the cockles of my heart. In this case, my stomach sank. What was I going to say, “ Sorry, George, those aren’t your backplates”? Yes, and another hard lesson learned in Hollywood!

Anyway, the point is, the C-12 is especially revered for its sparkle. The resonator system may not be as accurate technically (it has more overshoot and ringing than aperiodic design), but it sure does produce the results. Use the C-12 on those vocals where you really want to capture the “air” and the silvery edge of a singer’s voice. They are also wonderful overhead mics, as well as being quite killer on guitars, both electric and acoustic. Jackson Browne used a pair of 1-micron C-12s on acoustic piano on the World in Motion album with beautiful results.

The 251 had the same capsule with different circuitry and a very different grille and body. The grille has an additional layer of mesh internally, which tends to smooth out the high frequencies and adds a peak at 5 kHz. It also tends to make the resonance at 12 kHz a bit narrower. The 251 also was designed to have less bottom end, and rolls off starting at about 100 Hz or so, and this gives the mic a good characteristic for female vocals. Use the 251 when the C-12 sounds a little too strident but you still want that AKG sound. It’s also quieter than the C-12 and can be used in situations where this is more critical.

The C-12a, while not an enormous success as a vocal mic, has been used when a soft, silky quality is desired, and has certainly been used as a wonderful string microphone.

Most of us are familiar with the 414s built before the capsule change, and many, many great vocals have been recorded with these. The only thing to really watch out for on these mics is that with solid-state electronics, the edginess of the top end can be a bit much on certain singers. A windscreen can help create a compromise that will tune in those difficult artists.

So there we have it, ladies and gentlemen. The great vocal mics exposed! It should certainly be said here that many mics have been used on vocals that were not the focus of these articles, and this is not to say that they are not valid. Whatever works is valid, and we all know that.

Rather, I have tried to give some insight into the workings and history of the true classics in this field, and, of course, this precludes listing every single mic that has ever been used on a vocal! Many companies have built some wonderful mics that were not covered in these chapters—Sony, B&K, Sanken, Milab, etc. But if we really look at the vast majority of professional vocal recordings, the truth is that the microphones we have talked about have had the most profound influence on the history of the art. It is thus fitting that we should turn our attention to these in particular.

The fact remains, however, that we should never be afraid to try anything, because, as we can see from out experience with specifications and the real world, the real world always wins. This is perhaps the great lesion to be learned form all of this.

Whether you’re a tube freak or just after that perfect sound, hopefully, in these pages you have discovered some of the secrets that you’ve always wondered about. Put them to good use, keep recording and chasing that rainbow of perfection, and keep those classics alive!

(Special thanks to Stephen Peus at Neumann, Norbert Sobol of AKG and Jerry Graham and Juergen Wahl for their help.)

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