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One tube microphone from Berlin
and one from Barstow:
By David Royer |
1 2 3
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Dave and the finished mic.
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The Neumann U-47 microphone
is a much sought-after and expensive tube microphone, which does its job very
well. However, the prospect of spending thousands of dollars for a microphone
is out of the question for many people. When the Marshall
MXL 2001 condenser microphone appeared on the market, I bought one with the
idea of reworking the internal electronics to a circuit similar to that in the
U-47, to see how well the microphone could be made to work. The Marshall/MXL mic
with redesigned electronics works very nicely and more than justifies the effort
and expense. Tube microphones obey the same laws of physics whether they are made
in Berlin or in Barstow – so let’s look at a Neumann U-47 and then at
my modification to the Marshall MXL-2001. The U-47, like any condenser microphone,
can be divided into three parts:
1. the condenser microphone capsule
2. the head amplifier
3. the power supply | The
Capsule The “capsule” of a condenser microphone consists of
a metal disc called a “backplate” and, spaced approximately 40 microns
in front of it, a metal or metal plated (usually gold sputtered) plastic diaphragm
- which is typically about 6 microns thick. This diaphragm is clamped around its
edge like a drumhead, so that the central part is free to vibrate when acted on
by sound waves. When the diaphragm is “driven” by a sound wave, it vibrates
and the space between it and the backplate changes as the diaphragm is moved toward
and away from the backplate under the influence of the sound wave.
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Figures 1a and 1b are photos of a Neumann M7 capsule
and the capsule in the Marshall MXL 2001 respectively. | |
The diaphragm and the backplate are electrically insulated from each other
and they form the two plates of a capacitor (condenser). The capacitance of this
capacitor is directly proportional to the size and spacing of the diaphragm and
backplate. Since the spacing is a variable quantity, which corresponds to the
sound waves actuating the diaphragm, an audio signal can be picked off of the
capsule if a suitable electrical circuit is included to translate the capacitance
change to an AC voltage change. The original condenser microphone designs
had one diaphragm and a solid backplate - such a microphone will be omni-directional.
On the other hand, if the backplate is drilled so that there are holes that go
all the way through it, things can be arranged so that a pre-determined amount
of sound can “leak” to the back of the microphone’s diaphragm,
just enough so that the pickup is “nulled” from the rear. To this day,
several small capsule condenser microphones are available in alternative omni
and cardioid versions, which differ only in this crucial detail. |
Finally, if two diaphragms are used, one on each side of a common backplate,
the result will be two capsules, each with a cardioid pattern. By electrically
combining the outputs of the two “halves” of the capsule in pre-selected
phase and signal level combinations, it is possible to get a selection of several
patterns. The Neumann M-7 condenser microphone capsule (used in pre-1958
U-47s and M-49s) was a fine microphone capsule, which, when paired with the Neumann
electronics, worked very well indeed. The Marshall MXL 2001 uses a capsule that
is copied from the Neumann KK67/87 capsule, which was used in the Neumann U-67,
U-87, SM-69 and M-269. The copied capsule works darn well in its own right, if
it is mated with well-designed electronics. |
From
Tape Op: No. 25 • Sept/Oct 2001