Sound Devices makes high-end audio mixers and recorders for television and film production at its factory in Reedsburg, Wisconsin. The company said because its tech center receives so many questions about, it created a white paper to address the issue.

Before discussing how to measure noise, it is important to identify the primary source of noise. All electronic components generate noise. Resistors, a key component in preamplifier circuitry, have several noise-generating mechanisms. Johnson noise (or thermal noise) is the primary component of a resistor’s noise.

Noise is primarily affected by the value of the resistor and by temperature. Higher temperatures and higher resistance result in higher noise. If a resistor could be cooled to absolute zero (-273.15 degrees F) it would be noiseless. The bottom line is all resistors make noise. They are a fundamental component in all electronics like microphone preamplifiers and the higher the resistance, the higher the noise.

The input impedance (resistance) of a typical microphone preamplifier is between 1k ohms and 10k ohms. This input resistance is intentionally much higher than the source impedance of a microphone (50-200 ohms) so that the preamplifier won’t load down the tiny signal from the microphone. A typical microphone with a source impedance of 150 ohms generates 0.22 two of noise (or stated another way, -133 dBV).

So why is turning up the preamplifier's gain control and listening to its noise misleading? First, an unterminated input (with no microphone or load connected) is a high impedance (resistance). And second, this is a common mistake when trying to determine the noise of a microphone preamplifier.

The problem here is that with no connection to the input, you are listening to the noise of the input impedance alone. The noise level is going to be much higher, which would never be the case with a microphone attached. The instant a microphone is plugged in, this noise drops significantly. This is also a good reason to turn down, or mute, unused inputs.

Different microphone preamplifiers (and subsequent gain stages like the fader, headphone amp, etc.) have different amounts of available gain. This sounds obvious, but it needs to be stated, “if a product being measured has more maximum gain (a good thing!) than another product, it will sound noisier when both are at maximum.”

Measuring noise

A valid noise measurement between several microphone preamplifiers requires a true “apples-to-apples” measurement. The steps below are a quick way to accurately compare two preamplifiers' noise performance.

First, calibrate the gain between units under test using a tone generator (or other source with a known output level) plugged into a mic input. Adjust the gain for the same level out of the headphone jack or line output. Doing this makes sure you have the same amount of total gain through both preamps.

Then, plug in a 150 ohm resistor across pin-2 and pin-3 of the XLR connector. This emulates a microphone, but without the acoustical output of a microphone. Alternatively, a dynamic microphone with an on/off switch will accomplish the same. Now listen to the difference between the units.

Many manufacturers specify preamplifiers using EIN (Equivalent Input Noise). This specification is helpful to evaluate different mic preamps. EIN is simply the noise at the output, less (minus) the gain of the preamp.

EIN is useful as it removes gain from the equation and makes apples-to-apples comparisons easier. EIN is expressed in dBV or dBu. The lower the number, the better the EIN. This number is properly measured using 150 ohms as an input terminator. The very best EIN that can be achieved is -133 dBV, because this is the resulting noise purely from a 150 ohm resistor.

We can measure noise well beyond human audibility, sometimes up to 1 MHz. Because noise is broadband and doesn’t just magically go to zero above 20 kHz, a meter with higher bandwidth will show higher amounts of noise. The important thing to keep in mind is that different pieces of test gear have different bandwidths. Always try to compare device performance using appropriate test gear.