Thermal noise & preamp EIN

Johnson noise of a resistor in µV and dBu, and how close a preamp's EIN is to the physical limit.

Resistance (source impedance)

Bandwidth

Temperature

°C

Noise voltage (RMS): 0.222254µV
Noise level: -130.845dBu
Noise level: -133.063dBV
Noise density: 1.57157nV/√Hz

Judge a microphone preamp's EIN

Preamp EIN from datasheet

dBu

Usually measured with a 150 Ω source, 20 Hz – 20 kHz.

Source resistance used for the spec

Bandwidth of the spec

Noise figure (how far above the theoretical minimum): 2.84452dB
Theoretical minimum (resistor noise alone): -130.845dBu

Every resistor produces Johnson–Nyquist noise. A 150 Ω microphone over 20 kHz cannot be quieter than about −131 dBu, so preamps quoting EIN around −128 to −130 dBu are within 1–3 dB of perfect.

How it works

Any resistance generates noise that grows with temperature, resistance and bandwidth. A microphone's own impedance therefore sets a floor that no preamp can beat.

Formulas

V_n = √(4 k T R B)
noise figure = EIN − V_n(150 Ω, 20 kHz)

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