Connecting and reducing a multi turn wire wound resistor until no noise difference can be heard or measured between it and short circuit.
Attention must be paid to rf energy as it is too easy to listen to the radio!
The smaller this resistance the more sensitive or low noise the preamplifier is as it can read (distinguish between 0Ω) an extremely small thermal input voltage.
When short circuiting the input (from ground to coupling capacitor connected to grid) of a Pleiades V4 electron tube amplifier operating at 3.6V there is almost complete silence.
Notes:
Input transformers improve the noise performance as they convert, (transform) impedance.
When short circuiting the primary one is left with the DC resistance of the secondary.
This should be very low if a Nanocrystalinne Magnetec Nanoperm core is used as it permits constructing a given inductance with very few turns.
If further DC resistance reduction is needed it may be done with silver wire windings.
By making Pleiades transformers ie compensating for flat responce from vocal chords to listener's brain lower input inductance is needed (in order to remove increased perceived bass) so even less turns are needed.
When the best possible front end can be made it is then up to the microphone quality to dictate the signal to noise ratio according to its output at a given output impedance.
Then how close the source of sound is to the microphone.
Then how low noise is the ambience...
A very good front end amplifier should distinguish between short circuit at the input and a very low wire wound resistor connected to the input.
Small resistors emit less thermal or Johnson noise at a given absolute temperature.
Is this called equivalent input resistance?
How low noise would a Pleiades V series electron tube front end amplifier be in space operating with a small 3V solar cell and the cathode underheated to minimize all other noise effects including secondary emission?
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