In Pro Audio Review, October 2001 Ted Spencer wrote:

In the few brief decades of sound recording history, there are perhaps only two vintage pro audio products that have reached "holy grail" status, at least if the sky-high prices presently being paid for them are any indication. One is the Fairchild 670 limiter (currently $20,000 to $25,000) and the other is the Telefunken ELAM 251 microphone (currently $13,000 to $18,000).

You may ask, "What the heck is so special about this thing that it went from costing $650 in 1965 to $20,000 in 2001?" In the '50s, before the introduction of the ELAM 251, Nemann had been building U47s exclusively under subcontract with Telefunken, which was distributing them outside Germany under its own name. In 1963 Neumann decided to end its association with Telefunken and begun distributing U47s for the first time outside Germany under the Neumann name. This left Telefunken without a high-end condenser mic, so the company turned to AKG, who agreed to subcontract a new design.

The goal was to exceed the U47's sound quality, and arguably, AKG succeeded. (...) By the time AKG subcontract with Telefunken ended in 1969, only about 3,000 of the ELAMs had been sold worldwide. Thus we have the recipe for a classic collector's item: high intrinsic quality (at least sonically) and extreme rarity.

AKG C-12/ELAM 250/251, from Twelve Microphones That Changed History by Jim Webb, Mix Magazine:

AKG, which stands for Akustische und Kino-Gerate (Acoustic and Film Equipment) was formed in 1947 in Vienna, Austria. AKG developed the C-12 condenser microphone in 1953 based on a dual backplate/dual-membrane idea patented by Kalusche and Spardock in 1951. The modified version of this idea became the basis for the CK-12 capsule.

AKG was the first to manufacture a split electrode microphone. The original capsule membrane was 10-micron-thick PVC, which was later changed to 9-micron-thin Mylar. The amplifier design was based on the 6072 tube, and the C-24 stereo edition of the mic with two CK-12 capsules utilized this dual triode to its full extent.

The C-12, like the M49, had a remotely controlled pattern selection from omni to bi-directional via the selector switch located in a box between the microphone and the power supply. The C-12 remained in production until 1963. In 1964, the C-12A appeared with a 7586 Nuvistor tube amplifier and a physical shape foreshadowing the design of the 414 Series. In 1959, after the U47 had been withdrawn from Telefunken distribution, Telefunken commissioned AKG to develop a large-diaphragm condenser microphone. This became the ELAM 250.

This design incorporated the CK-12 capsule in a wider body with a thicker wire mesh grille. A two-pattern selector switch (cardioid to omnidirectional) was placed on the microphone. The ELAM 251 added a third bi-directional pattern to the switching arrangement. The "E" designation after the 251 indicates export, incorporating a 6072 tube amplifier. Non-E designations indicate the standard German AC701K tube amplifier.

The immense popularity of the C-12 and ELAM 250/251 microphones has made them the most expensive and highly desirable vintage tube mics on the market today.

After signing a contract with Telefunken, AKG seized the production of the C12. During the 10-year manufacturing period, only about 2,000 of the C12s had been made.

In 1994, AKG started production of the C12 VR (Vintage Revival) which was designed to be a copy of the original C12. Due to a different circuitry and a completely different manufacturing process of the capsule, the new VR is not even close to the performance of the original. The original CK12 capsule was hand assembled and tuned with a screw adjustable rim, contrasted to an automated assembly with an inferior glued-in diaphragm. The same is the case with the earlier attempt by AKG, known as The Tube.

EQ Magazine in the December 1994 issue describes the AKG C12 microphone from the collection of Allen Sides, Ocean Way Recording:

This particular mic was used by Lionel Ritchie on three of his albums. Mr. Sides has personally used the mic on Frank Sinatra, Ray Charles, James Ingram, Denise Williams and Johnny Mathis. It was also one of six or so of Ocean Way's C12 used on "We are the World".

The C12 is sort of the standard overhead drum mic of choice at Ocean Way, and it's fair to say that 80 percent of all recordings done at our studios use them. They are unparalleled for background vocals and make excellent lead vocal mics provided sibilance is not a problem, as they are very bright.

Conventional measurement techniques do not always tell us the realities of one mic sounding better than another. A new mic that shows lower distortion, lower noise, and grater SPL capacity may still sound considerably less impressive than its tube counterpart. The things that we as engineers and musicians look for - size, richness, and what we call a musical sound - are difficult to define in purely technical terms.