Curio's
- AKG's
"Brass"CK12 capsule
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A
quick look at it's history
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The
"Brass" CK12, must surely rank as one of the most famous
capsules ever designed.
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First
used in the legendary AKG C12 valve microphone,
and then in the C12a/b valve (Nuvistor) microphones that followed.
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Not
forgetting it's deployment in the the excellent Telefunken
(Elam) M250 and M251 series of valve microphones.
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Of
course, the "Brass" CK12 capsule was also found in AKG's
"Large Capsule" FET microphones i.e.
C412, C414 and early examples of the C414EB and
C414E1.
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The
"Brass" CK12 capsule, was finally
replaced by the original "Teflon"
version of the CK12. Being very much cheaper to
produce, but never sounding anything like the
original "Brass" CK12 that it replaced.
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AKG went on
to design and manufacture another capsule, in an
attempt to "Emulate" the qualities of
the original "Brass" CK12 capsule. This
capsule being used in the C414TLII and the
C12VR microphones.
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The
original "Brass" CK12 capsule
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What
makes the "Brass" CK12 so special ?
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Those
"Magical" qualities of the
"Brass" CK12 capsule
..... maybe it's
just down to the excellent design and manufacture
of this "Precision" component. The
"Brass" CK12
capsule is probably the most complex of any
microphone capsule ever manufactured.
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If we
take a closer look at the design and assembly of
the "Brass" CK12, then maybe we can
begin to appreciate
why it sounds so good !
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Sectional
views of "Brass" CK12 capsule
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Note
- For reasons
of clarity, fixing screws, together with their
locating holes, are not shown.
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Let
us start by looking at "The Parts", that
are utilized in the capsule assembly.
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Diaphragm
Mounting Ring
and Diaphragm
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Diaphragm
to Backplate, Insulated Spacing Ring
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Insulated,
Backplate, Mounting Ring
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Primary
Backplate
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Secondary Backplate
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Note
- The Secondary Backplate is engineered
differently, on the front and rear capsule
sections, see later
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When the
above parts are fitted together, we have one half
of the capsule assembly. Two assemblies, front and
rear, are required to form the complete capsule.
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Front
half of capsule
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Backplate
to Backplate, Insulated Spacing Rings
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Rear
half of capsule
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Finally,
the front and rear halves of the capsule are
assembled, together with the associate spacing
rings, thus forming an "Critically
Tuned" air-tight unit.
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The finished
"Brass"
CK12 capsule assembly
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Let us
now dismantle the "Brass" CK12 Capsule, and see what we find !
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Initially
we have the complete capsule
assembly, removed from the associated microphone
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Once the
capsule supporting bracket and wiring connector
tags have all been removed, we are left with just
the capsule. Firstly we need to loosen the 3
screws that hold the two halves of the capsule
assembly together. These are located on one face of the
capsule only. The two halves of the capsule are now
held together by a Locktite type
of adhesive, used during manufacture.
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Next, the 24
diaphragm retaining screws, 12 on each face of the
capsule, need to be very carefully loosened,
ready to be removed.
The diaphragm mounting ring, together with the
diaphragm, can now be removed. Along with the very
thin insulated "Spacing Rings", that are
used to determine the exact distance between rear
of the "Diaphragm" and
"Backplate".
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These
"Spacing Rings" effecting the
sensitivity of the capsule e.g. thinner the ring,
the greater the sensitivity. The rings also having
an influence on the frequency response and polar
pattern of the final assembled capsule.
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The "Backplate
Assembly", with the two halves of the
capsule still intact, can now be separated for
closer inspection.. Not forgetting to remove
the very thin insulated "Spacing Rings",
used to keep the two halves of the capsule assembly,
the correct distance apart. These spacing rings
are different from those used for
determining the gap between the diaphragm to the
backplate.
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These
"Spacing Rings" effecting the frequency response
and polar pattern of the assembled capsule.
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The
backplate, have a very fine pitch screw
thread around
the circumference , and is
screwed into the Perspex mounting
ring. Again,
this assembly is held firmly in place with a Locktie type of
adhesive. At this stage of manufacture, the
working face of the backplate assembly is now
ready for final machining. There is, and has to be, a "Seamless"
edge to the front face of the mounting ring and
working surface of the
backplate. The engineering tolerance's at
this stage, are very tight indeed.
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To gain
electrical access to the "Brass" backplate, a long
fixing screw
goes through the Perspex insulating
ring and makes
contact with the threaded hole, just
visible,
within the thread around the backplate. In the case of the
C12 microphone, and other AKG models that use the
"Brass"
CK12 capsule, a coupling
link is then used to electrically
connect the two backplates together. In the case of
the Telefunken 250/251 series, then the backplates
of the "Brass" CK12 capsule, are not electrically
connected together.
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If we take a
closer look at the front and rear halves of the
capsule, we can see that the holes in the rear of
the backplates, are machined
differently. This is all down to the design of
the "Brass" CK12, and the required placement of the holes
to obtain the desired frequency response and polar
pattern of the capsule assembly.
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Indeed, the
design of the each backplate is so intricate, that
they are machined and assembled from two separate
parts (I don't know of any other backplate
assembly that is this complex). The holes in the front/working
face, of the backplate, is
the same for both halves of the capsule. However,
the holes in the rear of the back plates are
different, as mentioned in the last paragraph. The
two parts of each backplate being screwed together (or
dismantled) again via a very fine pitch screw
thread on the circumference and inside edge of the
backplate parts. The two components, that make up
each backplate, are very fine examples of "Precision"
engineering, and one of the reasons why it would
be so expensive to re-create the "Brass" CK12 capsule
today. Indeed the engineering requirements of any
item that goes into making a capsule, require very
high standards of precision and design detail. The
"Brass" CK12 is a fine
example of such craftsmanship.
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Note
- Whilst dismantling the "Brass" CK12 capsule, for
photographing, I made a note of the number of
"Parts" used to make the capsule.
There being no less than 55 items.
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