8TELEFUNKEN Elektroakustik 9Diamond Series
The 251 and C12 both feature the CK12, but the capsules are polarized
very dierently in the two models. In the C12, the front membrane is always
grounded, and the back membrane varies from 0 to 120 volts, whereas in the
251, both the front and back membranes vary between -60 and +60 volts.
Though the overall range is identical, the dierence in approach does account
for a discrepancy in how the capsule transfers sound energy into an electrical
signal. The polarizing network itself was also moved inside the microphone
in the 251, with the switch now found on the front of the microphone. It is
speculated that this was done to imitate the look of the U47. The number of
polar patterns was also reduced to three: Cardioid, Omnidirectional and Figure
8. There are also obvious dierences in the mechanical assembly, body tube
diameter and headgrille size and construction between the two microphones.
The sonic dierences between the 251 and the C12 can largely be attributed to
the headgrille design, change in polarizing network, and change in tube bias,
since they share the same capsule, tube, and output transformer.
It is generally estimated that approximately 2500 C12’s were made during
AKG’s 10-year production run from 1953 until 1963. In 1962, AKG released the
C12A, which also featured a CK12 capsule but with a dierent tube, circuit, and
transformer. The later version of the famed CK12 capsule is also found in early
versions of the C 414 which was initially released in 1971. The modern “VR”
version was released in 1994 as a replica of the original C12, but aside from
the 6072A tube, it has little in common with the capsule, sound, or circuit of the
original C12. A stereo version, the C24, was introduced in 1959 and stayed in
production years after the C12 was discontinued.
The TELEFUNKEN Elektroakustik replica C12 features a historically accurate
CK12 capsule, made by hand in South Windsor, CT, a 6072A vacuum tube in
self bias conguration, and a T14/1 output transformer made by the original
European manufacturer, Haufe. The polar pattern box has been combined
with the power supply into one unit. This does not aect the sound in any way,
but greatly improves the functionality by limiting the number of interconnects,
cabling, and equipment needed for operation. Mechanically, most parts found
in the replica are compatible with vintage units though t cannot be guaranteed
due to subtle changes in the original design. While the original units feature a
plated body with an engraved AKG logo, the Telefunken replica features a gray
powder coated body and classic Telefunken Diamond badge.
C12 History
The C12 was introduced by AKG, or Akustische und Kino-Gerate (Acoustic and
Film Equipment), in 1953. It was designed by AKG engineer Konrad Wolf to
compete with the already-popular U47. While both microphones feature a large
diaphragm capsule, vacuum tube, and output transformer, little else is similar.
Unlike the U47, the C12 features nine remotely switchable polar patterns from
an external switching box connected to the microphone’s power supply. Initially
intended for capturing classical performances and broadcast applications, it
quickly found its way into the recording eld. AKG’s edge terminated CK12
capsule produced a more open and detailed sound compared to the U47’s mid-
forward and present center-terminated M7 capsule.
The CK12 capsule is probably the most famous microphone capsule ever
produced and was a major breakthrough in design at the time. Though many
changes were made during the original production years, all true CK12’s feature
dual-backplates with edge-terminated diaphragms. Those changes include
diaphragm materials and weights, as well as changes to the size of the chamber
between the backplates. There were also changes to the output transformer
used over the production run. Early versions have the V2148 transformer, while
the later have the more famous Haufe T14/1 transformer, which in itself varied
over the years. There have also been some minor cosmetic and mechanical
changes to the microphone, including the shape and height of the headgrille,
and variations in hardware, cable assembly, and power supply design.
When Telefunken stopped distributing the U47 in 1958, AKG was contracted to
develop a new agship condenser microphone. AKG’s design was the ELA M
251, essentially a modication of the C12. The C12, basically unchanged, was
also re-branded as the Siemens SM203 and SM 204. Though the 251 is based
on the C12, there were many modications made to the original design that
account for the functional and sonic dierences between the two models. A
major change found in the 251 is the self-biasing of the vacuum tube, compared
to the xed bias conguration found in the C12. With self bias there is no
calibration necessary when changing tubes and it is generally thought of as
sounding more open and natural. It is a very common modication to convert
AKG C12’s to self-bias. The TELEFUNKEN Elektroakustik C12 is self biased.
Another dierence is that the tube socket is inverted in the 251, making the high
impedance line from capsule to tube grid signicantly shorter.
