Rupert Neve Shelford 5052: Mic Pre / Inductor EQ - Manual (Page 7)

MIC PRE DESIGN NOTES FROM MR. RUPERT NEVE
In former years, before the introduction of solid state amplifiers, transformers
were necessary to step down from the very high input impedance of tubes, and to
provide a balanced input for the microphone line. An input impedance of 1,000
or 1,200 ohms became established for microphones having a source impedance of
150 or 200 ohms, with connection being made on a twisted twin screened cable
(This type of cable, while excellent for low impedance work, has high capacitance
between its conductors and between each conductor and screen. Resultant high
frequency losses are excessive with high impedance sources like piezo pickups
and may cause resonances with magnetic pickups.). Condenser microphones
worked off high voltage supplies (250V!) on the studio floor which polarized the
diaphragms and powered a built-in pre-amplifier.
More and more microphones were needed as “Pop” music gained ground and
this led to the popular and efficient method of 48-volt “Phantom” powering that
was built into the multi-channel recording console – in place of numerous bulky
supplies littering the studio, a miniature pre-amplifier now being fitted inside the
microphone casing.
The 48-volt supply was fed to the microphone through balancing resistors so it
was impossible for all of this voltage to actually reach the microphone, resulting in
low polarizing volts to the microphone capsule and virtual starvation of the little
pre-amp inside the microphone. Nevertheless amazingly good microphones were
designed and made, becoming the familiar product we use today.
If a low value resistive load is connected to the output of an amplifier, that amplifier
has to produce power in order to maintain a voltage across that load. Obviously
if we want more voltage (output from the microphone) we need to provide a
larger supply for the amplifier or design amplifiers with a higher input impedance,
and therefore a lighter load. A microphone is a voltage generator, not a power
amplifier. Most microphones give their most accurate performance when they are
not loaded by the input impedance of a traditional preamplifier. If the microphone
uses an electronic circuit (transformerless) output, a low value of load impedance
can possibly stress the little microphone pre-amplifier, causing decreased slew rate
and compression at high levels.
On the other hand, a high value of load impedance allows the microphone to
“breathe” and give of its best, this being particularly advantageous with very high
level percussive sounds. If the microphone has an inductive source (such as would
be the case if it has a transformer output) a low value of load impedance causes the
high frequencies to roll off due to leakage inductance in the transformer in addition
to the above amplifier distortion (This can be an advantage with some condenser
microphones!).
For this reason we have provided a high value of input impedance that will load
microphones to the smallest possible extent and makes the best possible use of that
limited “Phantom” 48-volt supply.