Selecting The Right Preamp For Your Microphone

This means the noise is given in units of dB SPL.

A noise spec might read 14 dB SPL equivalent, A-weighted, or shortened to just 14 dB-A (bad terminology, but common).

This is interpreted to mean that the inherent noise floor is equivalent to a sound source with a sound pressure level of 14 dB.

Problems arise trying to compare the mic’s noise rating of 14 dB SPL with a preamp’s equivalent input noise (EIN) rating of, say, -128 dBu.

Talk about apples and oranges!

Luckily (again) tables come to the rescue. Table 3 provides an easy look-up conversion between a microphone’s output noise, expressed in equivalent dB-SPL, and its sensitivity rating, in mV/Pa, into output noise expressed in dBu, A-weighted.

Using Table 3, a direct noise comparison between any microphone and any preamp is possible.

The example shown by the blue column and row is for a microphone with a noise floor of 14 dB-SPL and a sensitivity rating of 20 mV/Pa, which translates into an output noise of -112 dBu, A-weighted.

Table 3. Output Noise for Condenser Mics (dBu)

Similar to Table 1, you can use Table 3 to map out a preamp’s A-weighted noise to show the combinations that add insignificant noise.

If you use a -10 dB difference figure as a guide, then the preamp’s noise amounts to less than 0.4 dB increase. The red-shaded triangle area in Table 3 shows an example of this.

The areas not shaded represent all possible combinations of microphone sensitivity and noise specifications that can be used with Rane’s MS 1S Mic Stage, for instance, without adding significant noise.