EIN. Equivalent Input Noise or Input Referred Noise
What is tested? Equivalent input noise, or input referred noise, is how noise is spec'd on mixing consoles, standalone mic preamps and other signal processing units with mic inputs. The problem in measuring mixing consoles (and all mic preamps) is knowing ahead of time how much gain is going to be used. The mic stage itself is the dominant noise generator; therefore, the output noise is almost totally determined by the amount of gain: turn the gain up, and the output noise goes up accordingly. Thus, the EIN is the amount of noise added to the input signal. Both are then amplified to obtain the final output signal.
For example, say your mixer has an EIN of -130 dBu. This means the noise is 130 dB below a reference point of 0.775 volts (0 dBu). If your microphone puts out, say, -50 dBu under normal conditions, then the S/N at the input to the mic preamp is 80 dB (i.e., the added noise is 80 dB below the input signal). This is uniquely determined by the magnitude of the input signal and the EIN. From here on out, turning up the gain increases both the signal and the noise by the same amount.
How is it measured? With the gain set for maximum and the input terminated with the expected source impedance, the output noise is measured with an rms voltmeter fitted with a bandwidth or weighting filter.
Required Conditions. This is a spec where test conditions are critical. It is very easy to deceive without them. Since high-gain mic stages greatly amplify source noise, the terminating input resistance must be stated. Two equally quiet inputs will measure vastly different if not using the identical input impedance. The standard source impedance is 150 ohms. As unintuitive as it may be, a plain resistor, hooked up to nothing, generates noise, and the larger the resistor value the greater the noise. It is called thermal noise or Johnson noise (after its discoverer J. B. Johnson, in 1928) and results from the motion of electron charge of the atoms making up the resistor. All that moving about is called thermal agitation (caused by heat -- the hotter the resistor, the noisier).
The input terminating resistor defines the lower limit of noise performance. In use, a mic stage cannot be quieter than the source. A trick which unscrupulous manufacturers may use is to spec their mic stage with the input shorted -- a big no-no, since it does not represent the real performance of the preamp.
The next biggie in spec'ing the EIN of mic stages is bandwidth. This same thermal noise limit of the input terminating resistance is a strong function of measurement bandwidth. For example, the noise voltage generated by the standard 150 ohm input resistor, measured over a bandwidth of 20 kHz (and room temperature) is -131 dBu, i.e., you cannot have an operating mic stage, with a 150 ohm source, quieter than -131 dBu. However, if you use only a 10 kHz bandwidth, then the noise drops to -134 dBu, a big 3 dB improvement. (For those paying close attention: it is not 6 dB like you might expect since the bandwidth is half. It is a square root function, so it is reduced by the square root of one-half, or 0.707, which is 3 dB less).
Since the measured output noise is such a strong function of bandwidth and gain, it is recommended to use no weighting filters. They only complicate comparison among manufacturers. Remember: if a manufacturer's reported EIN seems too good to be true, look for the details. They may not be lying, only using favorable conditions to deceive.
Correct: EIN = -130 dBu, 22 kHz BW, max gain, Rs = 150 ohms
Wrong: EIN = -130 dBu
