Replacement
Microphone Body - RMB-2021, Design
and Construction |
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Requirements |
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The RMB-2021 is to be a valve based
replacement for the "Calrec" CB20 and CB21 microphone body
(electronics). Thus giving existing owners of Calrec 2000 and 2100 series
microphones, the
option, of acquiring the warmth and smoothness that is available from a valve
design.
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The RMB-2021 would be required to
work with existing "Calrec"
2000 and 2100 type capsules, as used with the CB 20 and CB21 bodies, together
with other capsules in the
"Calrec" range that utilized the same fitting i.e. CC700 and
CC750.
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For cosmetic reasons, and
convenience of use, the RMB-2021 should be of similar dimensions to that of the original
"Calrec" CB20 and CB21 bodies.
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A suitable power supply unit for the
RMB-2021 would also be required.
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Original
"Calrec" CB21 body, fitted with a CC03 capsule
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"Cathode
Follower" Output |
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It
was decided from the very conception of the design, that the valve would
be operated as a "Cathode Follower", with the associated output
matching transformer having a somewhat lower turns ratio then that found
in "Anode Load" designs.
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An
excellent
example of a "Cathode Follower" design, can be found in the
wonderful little AKG C60. Here, the
electronics are designed around the famous Telefunken AC701
valve. The output matching transformer being mounted in the power supply
case.
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Another
"Good" small microphone, based around a "Cathode Follower",
is the NWDR designed Hiller
M59. The valve being a MSC2,
forerunner of the AC701.
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The
RMB-2021, will have the associated output matching transformer, fitted inside
the "Housing Tube". Well
away from any stray magnetic fields (Hum) that could be emanating from the
associated mains transformer.
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"Cathode
Follower" designed AKG C60's
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Using
a Nuvistor in a "Cathode
Follower" design ? |
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Another
example of a Cathode Follower design being the AKG C12A and C12B. Both
based around the 7586
"Nuvistor, which is great for many applications.
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"Sonically"
the 7586
qualities are somewhat "Dry and Grainy", almost
"Clinical". Sounding somewhat like a "Semiconductor".
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Other
microphones that use the 7586 being the AKG C28c, Fi-cord 1200, Hammond
M100, Neumann U64, to name but a few. Some
Neumann U47 / U48 microphones use a "Nuvistor", type 13CW4. Again, the 13CW4 is not favored by all, for the same
reasons as the 7586.
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"Nuvistors"
are small and convenient devices, but for
various reasons, some of which have already been mentioned, a Nuvistor will not be used
in the RMB-2021.
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"Cathode
Follower" designed AKG C12B
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Fitting it all in
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It is
intended, that the RMB-2021 is to offer that "Warm and Plumy" sound, liked by so many musicians and
engineers. So, a valve of suitable sonic quality and physical size i.e. ML-1955
is to be used.
As is that found in the RV-14m,
a direct replacement for the Telefunken VF14.
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As
stated earlier, for cosmetic reasons and convenience of use, I decided to
keep the physical size of the RMB-2021, very similar to that of the
original "Calrec" CB20 / CB21. However, by the time the
"Base bush" was fitted to the "Housing tube", there
was very little space left inside to fit the components. Everything having
to be squeezed neatly into an area just 95 mm long x 21 mm in diameter
..... a tight fit !
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There
were many prototype electronic assemblies built, in an attempt to find the
desired qualities.. These were very "Basic" in structure, with
the majority of the components being soldered directly to the valve holder
and other parts left "Floating" around inside the housing tube !.
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Note
- The capsule "Pick-up" contact (Brass pin) is very close to
valve "Grid" Input, to help reduce "Signal Loss", due
to stray capacitance, and "Improve Transit Response".
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Part of the
original "Prototype" electronic assemble
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Typical 1950's / 1960's
Construction technique |
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After
many circuit design / component changes, together with hours of rigorous
electronic and listing tests, the "Final Design" for the
RMB-2021 is completed. This included selecting the "Ideal"
working voltages for the HT and LT supplies, together with that of finding
the "Appropriate" polarizing voltage, for the associated
capsules that are to be used with the RMB-2021.
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A
PCB would not be used, one reason being the lack of space, therefore the majority of
the components being fitted directly to the valve holder. Much like the
technique used during initial testing. This type of construction was used
on many early "Commercial" designs from the 50's - 60's e.g.
Altec 150, Neumann U47,
Sony C37. Indeed, for "Sonic" reasons, many engineers and
musicians, prefer this type of construction.
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Electronics of the
final "Test" version
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Bodywork |
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The
"Housing tube" for the RMB-2021, is the same as that used for the
original Calrec CB20 and CB21 microphones. However, the original "Base
bush" (socket / connector carrier) as used for the Calrec CB20 and CB21, could not be utilized, at it offers insufficient space inside the
new RMB-2021.
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The
"Base bush" for the RMB-2021, is the same as that used on the Calrec
CM15 (Klark
Teknik AT1) "Test and Calibration" microphone. Due to the
physical position of the connector insert, this part offers considerably
more space within the RMB-2021 "Housing tube".
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Fortunately,
through my association with Calrec microphones, I have copies of the "Engineering
Drawings" used for the Calrec CB20 and CB21 "Housing tube" together with that
for the CM15 "Base
bush". Thus enabling the parts needed for the RMB-2021, to be
machined as required.
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After
spending many hours of "Drawing"
on the PC,
a suitable colour for the replacement body was eventually found. At last,
the end was in sight, "Powder
Coating" the housing assembly could finally commence !
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Machined
"Brass" RMB-2021 Socket / Connector carrier
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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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Grace
Manufacturing (Leicestershire, Gt. Britain) - Powder
Coating (Microphone) |
One
Way Circuits (Norfolk, Gt. Britain) - Printed
Circuit Board (PSU) |
Sowter
Transformers (Suffolk, Gt. Britain) - Matching
Transformer (Microphone) |
Yellow
Metal Racing (Norfolk, Gt. Britain) - Machining
(Microphone Housing Tube Assembly) |
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