Valve
Microphones - VM-951,
Design and Construction |
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The
idea of producing my own "High Quality" valve microphone, always
appealed to me. Having built various valve microphones, for use in my own
studio, I felt it was now time to design a model suitable for the
"Professional" user.
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At
this time, 1995, there was very little in the way of "New" valve
microphone production taking place. The only "High Quality"
valve microphones available, were the old AKG's, Neumann's etc. It was not
until some years later, that we saw the resurgence in valve microphone
production, that has now (2005) escalated out-of-control.
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It
would all start, with the construction and testing of the various
prototype microphones. Carrying out measurements on a microphone, might
well give you all the technical data you require, however ..... you
still need to know what it sounds like. After all, that is the reason for
using it for !
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Fortunately,
being involved in the local music scene, I was able to "Test"
the various prototype microphones at various practice sessions. The music
would be in the form of "Orchestras", through "Brass and
Big Bands", to "Rock Groups".
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Once
I had overcome the various problems, that were encounter along the way, I
was ready to design the "Production" version. A process that
would take very much longer, then I had originally anticipated.
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I
was lucky enough to have the use of a friends workshop. This was fully
equipped with "Industrial" lathes, together with a milling
machine. Machining the various parts, required for the case and internal
mountings etc, would take many hours. Having to "Correct" any
problems along the way. However, the final was design eventually completed
and production could commence.
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Externally,
the prototype VM-951 looked very similar to the AKG C12. Indeed, I used a
modified C12 type "In-line Connector", for the microphone
base/connector. This enabled the microphone to be used with a standard AKG
C12 "Stand mount" type of connector. The microphone was quite
long, allowing room for plenty of various component changes that were to
take place during the testing stage.
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On
the prototype, brass tubing was used for the housing. This proved to be
very "Workable" and acted as a very efficient electrical screen
for the microphone electronics.
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Therefore,
the same brass tubing is used in the production version of the VM-951.
This time, with a "British Racing Green" powder paint finish.
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External
view of the
"Prototype" VM-951 |
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As
this was only a "Prototype", then the quality/principle of
construction was not of great importance. Therefore it was decided to have
two 6BA threaded brass rods, running parallel the length of the
microphone. Being fixed in the connector assembly at the base and the
capsule mounting lugs at the top. Small tag boards could then be used, to accommodate
the majority of the components.
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The
layout, through need rather then anything else, following that of
microphones such as the AKG C12 and Schoeps CMV51. Using such a layout,
enables the valve connections, to be located as close as is practically
possible, to the capsule wiring. Thus keeping inter-connection capacitance
down to a minimum, and maintaining an excellent transient response..
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Inside
the "Final" version of the "Prototype" VM-951 |
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The
"Original" output matching transformer, used during testing, was
of a BBC design. This proved to be a very good match for the chosen
valve. The core, of this transformer, had quite a large "Cross
sectional area",. Something not often found in microphone designs,
other then the likes of the Neumann U47, M49 and M50. Not forgetting, the
early versions of AKG C12.
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This
large "Cross sectional area", enabled the design to have an
excellent overload performance at LF. Therefore, the prototype VM-951, was
able to capture the "Weight/Force" of the sound being recorded,
with great ease.
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The
"Output Matching" transformer used in production version of the
VM-951, is of the same design as that used in the prototype, be it of a
slightly different appearance..
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Out-put
matching transformer, in the
"Prototype" VM-951 |
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Having
finally arrived with a circuit design that "Match's" the
microphone design requirements, it was time to "Re-design" the
microphone, suitable for mass production. Aspects of the microphone
assembly, would also require some consideration.
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It
was decided to use a PCB, for component mounting, rather then the larger
and bulky tag strips as used in the prototype. This could be easily
"Pre-populated", ready for fitting into the microphone. The
design of the PCB has changed over the years, to enable a "Common"
PCB to be used in other microphone design's.
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The
valve used in the VM-951, is mounted on a valve base, which in turn, is
then wired to the PCB. In future microphones, a wire ended valve could
easily be soldered directly to the PCB.
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PCB used in the VM-951 |
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For
reasons already mentioned, it is wise to keep the
"Interconnection" between the capsule and the valve grid, as
short as practically possible.
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The
valve used in the VM-951, is of a "Plug-in" type, and the
associated valve socket, is mounted directly beneath the capsule, on the
capsule mount. Therefore meeting the requirements above.
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The
microphone design, requires the capsule mount to "Anchor" the
top/upper end of the PCB. Forming part of the main internal structure of
the microphone.
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The
"Prototype" version was engineered from aluminum. This being easily
machined and modified, until correct.
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Capsule
Mount, "Prototype"
and "Production" version's |
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The
lower end of the PCB being "Anchored" to the transformer mounting frame,
which in turn, is mounted to the base/connector assembly. The transformer,
as mentioned above, is a snug fit into the ABS transformer frame.
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A
modern 6-pin "Cannon" connector is fitted into the base of the
VM-951. The base/connector of the microphone being machined from solid aluminum.
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The
microphone "Housing Tube", being a slide fit over the entire
microphone. Being secured in place, by three Grub Screws. Much like
the principle that is used on the AKG C12 and many other microphones.
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Microphone
assembly, lower section |
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My "Sincere
Thanks" go to the following companies, for their time and attention
to detail, during the manufacture of the respective components.
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Norwich
Plastics (Norfolk, Gt. Britain) - ABS Moldings |
One
Way Circuits (Norfolk, Gt. Britain) - Printed
Circuit Board |
Sowter
Transformers (Suffolk, Gt. Britain) - Matching
Transformer |
Spray
UK 2000 (Leicestershire, Gt. Britain) - Powder
Paint Finish |
Yellow
Metal Racing (Norfolk, Gt. Britain) - Base
Assembly |
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