what means exactly EIN and why is this criteria so important ?
One of the most audible aspect of amplifying small signals - like the output signal from a low sensitivity microphone when the source is far or not very loud - is noise.
In usual HiFi DACs and ADCs, we measure noise at a fixed "gain": That's what the Dynamic range value gives you.
But with a microphone preamp, this gain is not fixed.
The very goal of a microphone preamp is to amplify the signal to bring it above the ADC's noise level by a margin.
EIN (Equivalent Input Noise) helps us to understand how big this margin can be, comparing directly the noise as a voltage (usually expressed in dBu), so independently from the gain factor.
The noise is coming from 2 sources:
The source's noise : In the EIN, usually. a resistor of 150 Ohm is used as a "source".
This value is similar to a dynamic microphone impedance.
Of course, we don't want a microphone signal to mess the measurement, so we use a fix resistor instead.
The resistor itself is subject to Thermal noise (see
here), so it contributes to the EIN noise.
At 20°C, a 150 Ohm resistor will have a thermal noise for a 20hz-20kHz bandwidth of 220 nVrms, or -130,92 dBu unweighted.
Then the preamplifier adds its own noise, which is what we want to evaluate.
As you may see in my measurements, the EIN at maximum gain under the same circumstances is at -128.2dBu.
If we deduct the resistor thermal noise, that's approx. 206 nVrms of noise added by the preamp itself (I compute this value and show it in the bottom right on each EIN measurement).
So less than the resistor's own noise.
As the gain decreases, this value will increase, since you'll be more and more limited by the ADC's noise.
In an ideal world, an ADC with an infinitely low noise level would not need a preamp.
Above a certain gain, the benefit of increasing the gain becomes too small, since we reach the preamp electronic's limits.