Phil Allison wrote:
"Matti Adolfsen"

To get the best possible noise figure from a mic preamp, you must match
the amp input impedance to the microphone.


** Absolutely FALSE !!!

The best signal to noise ratio is obtained when the mic impedance is MUCH
less that the load impedance. This allows the mic to generate the greatest
signal voltage at the input while noise is a function of actual mic
impedance in the audio band.

Check your source information, there might be errors in your statement.
I'm talking about the thermal noise created in the resistive part of
the dynamic microphone. This is the metohd used by most european
broadcast companies to evaluate mic preamp performance. Best noise
figure can be obtained if the power matching is correct: source and load
impedances must be matched: Maximum power transfer from microphone to
preamp, and reduction of the noise source impedance to 50% of the
original. This theory is used in RF design too.

Btw, the best noise figure I've measured so far was from a TELEFUNKEN
V-76 tube amplifier. 0.5dB this preamp has high turns ratio input
transformer (about 1:50) feeding a pentode amplifier.

This is 600 ohms in most cases.

** Crapology.

Not so nice terminology, but this time correct information. Correct
impedance can be found from manufacturers websites and international
standards. I checked my sources:
- MIC input source impedance from 0 to 200 ohms (N10, page 6)
- MIC input source impedance for noise measrurements: 200 ohms (N10,
page 14)
- Nominal impedance of NEUMANN U-87: 200 ohms.
- Minimum load impedance for U-87 microphone: 1 k ohms (www.neumann.com).

Sources:
- Elements of infrared technology, page 236
- N10 Nordic technical recommendation 3rd edition, oct 1983

Further reading:
- Mathematical noise modeling and analysis of some popular preamplifier
circuit topologies. T.F. Darling, J.AES Vol 35, 1987
- Noise of sources, John Maxwell, National Semiconductor 1977

Matti