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

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,

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

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

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

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

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

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

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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