How about using a source of a very small signal voltage with very low output impedance?
One could use a signal generator, eg a Wein bridge oscillator or even the output of a CD player, in series with a resistor and then a 1 ohm resistor shunt.
This potential divider will attenuate the signal to extreme low levels, attenuating the noise as well.
The minute signal of 1Ω output impedance (virtually no thermal noise from 1Ω) can feed the input of the microphone preamplifier. The best preamplifier would be the one that can discriminate the minute signal from its self generated noise.
Other additions to part II.
The input transformer steps down the equivalent input noise resistance (of an active device) at its primary which becomes the input of the preamplifier.
A 200 ohm microphone with 14dB equivalent acoustic noise input behaves as a higher than 200ohm resistor in terms of noise when the mic is at 0dB acoustic input?
Would an amplifier of 200ohm equivalent input impedance be fine in this case?
Additional references:
Electrometer Tubes for the Measurement of Small Currents - John A. Victoreen - Proceedings of the I.R.E.
Introduction to Electronice for students of biology, chemistry and medicine - Suprynowicz - p.158 - Addison Wesley
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