For audio interfaces with Mic preamp, low level / high gain noise is a key performance parameter.

I therefore always measure EIN when reviewing an interface.

But what is EIN, and how do we measure it ?

I thought it was worth adding a proposed methodology that everybody could use.

Of course, this is subject to discussion and improvement.

Your Mic preamp gives some noise at the ouput, for a given gain.

Let's imagine for a minute a perfect preamp, adding no noise at all by itself, and giving the exact same gain.

The Equivalent Input Noise is the noise voltage that would give the same level of noise as your real mic preamp

if it was provided by a generator at the input of this ideal - noise free - preamplifier and amplified by the same amount of gain.

Why is this useful to measure a mic preamp noise ?

If you measure SNR on a mic preamp, the value will greatly vary with the preamp gain.

Example: At 60dB gain, your SNR will be 20dB lower than at 40dB, if the preamp self noise is the same.

Just because you'll amplify the noise more.

Therefore, to link SNR and actual preamp noise is not easy.

In real life, in practice, we'd like to know what a given source SPL, at a given distance, with a given mic,

will give us in terms of noise at the ouput of the preamp.

We want to compare this noise for different amplifiers.

And we'd also like to compare the noise for different gains.

That's what the EIN gives us.

First read the interface manual or specs

Let's take a RME UCX II as an example: RME says max input level is 18dBu at 0dB gain for Mic inputs.

Almost always, EIN is maximum at max gain.

So what's max gain ?

For the same RME UCX II, it's 75dB, as we may read above.

Then measure

Have gain set to max

Use REW or any FFT software, set RTA to 20Hz-20kHz BW, "rectangle" window, 32 averages, and measure Noise (in dBFS).

Notes:

[EIN (dBu) ] =

OK, now what if you want to get more accurate value ?

That will become a bit more complicated.

EIN is usually measured with an input load connected that ressembles a microphone.

Of course, we won't use a microphone, that would pickup ambiant noise.

We'll use a resistor instead.

Because each resistor generates its own thermal noise - a function of the resistance and of the temperature - we have to standardize the resistor value.

Usually, a 150 ohm resistor is used.

Also, the noise level is a function of the bandwidth you measure it for.

Usually, we'd consider 20Hz-20kHz

Some vendor consider 10Hz-30kHz or other BW.

IF the noise profile is flat and measurement is un-weighted, it's easy to estimate the BW impact.

If weighting is applied, it's a bit more tricky.

Finally, the temperature of the resistor should remain around a standardized value.

Usually 20°C.

This part is a bit tricky, but we'll see later how we can lower the impact of T° fluctations.

For the curious who don't already know, here is the formula for the Thermal noise for a pure resistance.

with, in our case,

This step is critical, since this will be your only reference to a voltage in the measurement.

If you want to get accurate measurement, you should not rely on manufacturer's specs in terms of max input level or exact gain.

You'd want to measure the actual max input level for the gain you're considering

In other words, you want to accurately measure [Max input level (dBu)] - [Gain (dB)] in above equation.

The good news is that you need to measure only one value.

The bad news is that this value is quite low.

In my RME UCX II case, at 75dB gain, the max input value is around 1mV.

And you want, say, 0.05dB accuracy. That's an error less than 0.6%.

Not easy if you don't own a proper, accurate, calibrated True RMS microvoltmeter.

Good ones are quite expensive.

So I don't.

Here is how I proceed, with a normal True RMS calibrated Multimeter.

This is based on the fact modern interfaces are quite linear vs level.

(You know, this linearity measurement Amir includes in each DAC review)

But we can't really rely on a DAC's level for accurate voltage measurements, since the actual voltage it will deliver will depend on its output impedance and the input impedance of the mic preamp we'll connect it to.

So I rather use a line-level interface input instead.

That will most likely be much more accurate than your standard DMM.

So we'll use it as a millivoltmeter, to measure the level with enough accuracy.

But we'll need to calibrate it with the DMM first.

I set the mic input I want to measure to minimum gain (Phantom OFF)

I use one of my interface's balanced outputs. (Could be any decent DAC, actually)

With REW, I send a 1kHz sine wave through it.

I plug this output in parallel to one of the interface line inputs (that I'll use as a millivolmeter) and to my mic input, using an XLR Y cable.

At that stage, the mic input is still at minimum gain.

I set my output level just below the lowest range of my True RMS Digital Multimeter, because that's where the DMM is the most accurate.

In my case, that's 500mV, so I send, say, 499mV rms to my interface and measure the exact value in dBFS on my interface line input.

I note this value and the DMM Vrms value.

Then I lower the output level to around 10 dB below the expected max level of the mic input at max gain (see the quick method to estimate it).

In our example, I target a level of around -67dBu, or 0.3mV.

I then maximize the gain of the mic input and look at the level (in dBFS, from 20Hz to 20kHz) on both the Mic input and the line input.

(If your mic input has, say, more than 0.1% THD at that level, lower the level a few dBs.)

Given those measurements

[Calibration DMM level (V rms)]

[Calibration Line input level (dBFS)]

[Measure Line input Level (dBFS)]

[Measure Mic level (dBFS)]

My max input level is then

[Max Mic input level (dBu)] = 20 * LOG10([Calibration DMM level (V rms)]) - 10 *LOG10 (0.001*600)

- [Calibration Line input level (dBFS)]

+ [Measure Line input level (dBFS)]

- [Measure Mic level (dBFS)]

This should give you an accurate value.

This gives you [Noise (dBFS)]

You may then compute a more accurate EIN

[EIN (dBu)] = [Max Mic input level (dBu)] + [Noise (dBFS)]

Well, OK, but this values fluctuates with time.

Why ?

The temperature of the resistor.

The resistor's noise is far from negligible.

If the interface heats up, the resistor temperatur will also increase.

For a good preamp, it may actualy be higher than the preamp's noise itself.

But I can't measure it. So what ?

If we measure noise with the shorted plug, our "resistor"'s thermal noise is going to become very small.

Negligible if compared to the preamp's own noise.

With above's method, we'd get

[EIN Short (dBu)] = [Max Mic input level (dBu)] + [Noise Short (dBFS)]

then we may deduct the theoretical thermal nois eof a 150 ohm resistor to get a normalized EIN at 150 ohm 20°C, without impact of the temperature.

The thermal noise of the resistor is -130.92 dBu, or 220nV rms

[EIN (dBu)] = 10 * LOG10( 10^([EIN Short (dBu)]/10) + 10^(-130.92/10))

By the way, the preamp noise in V rms is then

[EIN Short (V rms)] = 10^(([EIN C/C (dBu)]+10*LOG10(0.6))/20)

(Of course, if you do step c., you don't need step b.)

But you're cheating ? A bit, sure.

But now we have

It's much more reproducible.

And still it matches exactly the value we would get if we were able to measure EIN directly while keeping the temperature of the resistor at exactly 20°C

and with a resistor value of exactly 150 Ohm

And if you want to compare to an EIN with another resistor value (like 200 ohm), you may as well compute it.

Here is a practical example, with the UCX II

Note that, here-above, I also measured EIN 150 Ohm directly (plot not shown).

I then recorded the ambiant temperature during the measurement and the actual resistor value.

After computing a compensation factor from those values, to normalize to standard 150 ohm / 20°C conditions, corrected measurement is almost identical to the value deducted from the Short measurement, at -127 dBu vs -126.70 dBu.

RME UCX II's internal noise at Max gain is 282 nanoV rms.

As a check, I also measured the Short with Virtins Multi Instrument 3.9.11.1

If I summarize the required steps:

You may want to repeat from 2. for a few other gains.

I like to measure for gains where the max input level is 10mV, then 100mV, for comparison between interfaces.

(EIN get worse with lowering gain)

If you measure more gains, you may get a comparison like this

(In the plot below, reference (0) gain is the gain that gives a maximum input level of 100mV for full scale)

I therefore always measure EIN when reviewing an interface.

But what is EIN, and how do we measure it ?

I thought it was worth adding a proposed methodology that everybody could use.

Of course, this is subject to discussion and improvement.

**Equivalent Input Noise**Your Mic preamp gives some noise at the ouput, for a given gain.

Let's imagine for a minute a perfect preamp, adding no noise at all by itself, and giving the exact same gain.

The Equivalent Input Noise is the noise voltage that would give the same level of noise as your real mic preamp

if it was provided by a generator at the input of this ideal - noise free - preamplifier and amplified by the same amount of gain.

Why is this useful to measure a mic preamp noise ?

If you measure SNR on a mic preamp, the value will greatly vary with the preamp gain.

Example: At 60dB gain, your SNR will be 20dB lower than at 40dB, if the preamp self noise is the same.

Just because you'll amplify the noise more.

Therefore, to link SNR and actual preamp noise is not easy.

In real life, in practice, we'd like to know what a given source SPL, at a given distance, with a given mic,

will give us in terms of noise at the ouput of the preamp.

We want to compare this noise for different amplifiers.

And we'd also like to compare the noise for different gains.

That's what the EIN gives us.

**How is EIN measured ?***To measure EIN, you'll need a test "load"*

So you'll first need toSo you'll first need to

*Prepare test loads**Take 2 XLR-3 male plugs.**Solder a 150 ohm metal film resistor between pin 2 and 3 in one plug and mark it "150"**Solder a copper conductor between pin 2 and 3 in the other plug and mark it "Short"*

*You'll need a software to measure noise too.*

In this example, I'll use REW. But any good FFT measurement software should work.In this example, I'll use REW. But any good FFT measurement software should work.

**1. The quick, not accurate, simplified, method**First read the interface manual or specs

**a. What's the maximum input level of the interface without pad or trim, and for what gain ?**Let's take a RME UCX II as an example: RME says max input level is 18dBu at 0dB gain for Mic inputs.

**b. What's max gain ?**Almost always, EIN is maximum at max gain.

So what's max gain ?

For the same RME UCX II, it's 75dB, as we may read above.

**Make sure Phantom is OFF for the mic input you want to measure**Then measure

**c. Insert the "150" XLR plug in the Mic input**

Have gain set to max

Use REW or any FFT software, set RTA to 20Hz-20kHz BW, "rectangle" window, 32 averages, and measure Noise (in dBFS).

Notes:

- There is no signal, so SNR won't give you anything. Don't even look at the Noise level value. Look at the total dBFS rms value only.

So for REW, use the top-right value. - It's been suggested to use a 0.5m mic cable to connect the plug to the Mic input, to avoid impact if interface's own temperature fluctuation.

This, indeed, provides more stable results

**d. Compute EIN**[EIN (dBu) ] =

[Max input level (dBu)] - ( [Measured Gain (dB)] - [Gain for Max Level (dB)] ) + [Measured Noise Level (dBFS)] = 18 - (75 - 0) + -70 dBu = -127 dBu |

**Simple, isn't it ?**

NotesNotes

- Here, I measured un-weighted.

If you measure your Noise with A-Weighting, your EIN will be A-weigthed.

In the example above, REW gives you both: Noise level is -72 dBFS (A), so [EIN (A) (dBu)] = -129 dBu (A)

(which, by the way, is 1 dB better than the Specs)

- This is an approximation, so don't get excited about the figures after the dot.

OK, now what if you want to get more accurate value ?

That will become a bit more complicated.

**2. The accurate method***For this, you'll also need a (ideally calibrated) True RMS Digital Multi Meter (DMM)***Precautions**EIN is usually measured with an input load connected that ressembles a microphone.

Of course, we won't use a microphone, that would pickup ambiant noise.

We'll use a resistor instead.

Because each resistor generates its own thermal noise - a function of the resistance and of the temperature - we have to standardize the resistor value.

Usually, a 150 ohm resistor is used.

Also, the noise level is a function of the bandwidth you measure it for.

Usually, we'd consider 20Hz-20kHz

Some vendor consider 10Hz-30kHz or other BW.

IF the noise profile is flat and measurement is un-weighted, it's easy to estimate the BW impact.

If weighting is applied, it's a bit more tricky.

Finally, the temperature of the resistor should remain around a standardized value.

Usually 20°C.

This part is a bit tricky, but we'll see later how we can lower the impact of T° fluctations.

For the curious who don't already know, here is the formula for the Thermal noise for a pure resistance.

with, in our case,

**a. Measure Max input level at max gain**This step is critical, since this will be your only reference to a voltage in the measurement.

If you want to get accurate measurement, you should not rely on manufacturer's specs in terms of max input level or exact gain.

You'd want to measure the actual max input level for the gain you're considering

In other words, you want to accurately measure [Max input level (dBu)] - [Gain (dB)] in above equation.

The good news is that you need to measure only one value.

The bad news is that this value is quite low.

In my RME UCX II case, at 75dB gain, the max input value is around 1mV.

And you want, say, 0.05dB accuracy. That's an error less than 0.6%.

Not easy if you don't own a proper, accurate, calibrated True RMS microvoltmeter.

Good ones are quite expensive.

So I don't.

Here is how I proceed, with a normal True RMS calibrated Multimeter.

This is based on the fact modern interfaces are quite linear vs level.

(You know, this linearity measurement Amir includes in each DAC review)

But we can't really rely on a DAC's level for accurate voltage measurements, since the actual voltage it will deliver will depend on its output impedance and the input impedance of the mic preamp we'll connect it to.

So I rather use a line-level interface input instead.

That will most likely be much more accurate than your standard DMM.

So we'll use it as a millivoltmeter, to measure the level with enough accuracy.

But we'll need to calibrate it with the DMM first.

I set the mic input I want to measure to minimum gain (Phantom OFF)

I use one of my interface's balanced outputs. (Could be any decent DAC, actually)

With REW, I send a 1kHz sine wave through it.

I plug this output in parallel to one of the interface line inputs (that I'll use as a millivolmeter) and to my mic input, using an XLR Y cable.

At that stage, the mic input is still at minimum gain.

I set my output level just below the lowest range of my True RMS Digital Multimeter, because that's where the DMM is the most accurate.

In my case, that's 500mV, so I send, say, 499mV rms to my interface and measure the exact value in dBFS on my interface line input.

I note this value and the DMM Vrms value.

Then I lower the output level to around 10 dB below the expected max level of the mic input at max gain (see the quick method to estimate it).

In our example, I target a level of around -67dBu, or 0.3mV.

*Note*

I often use Shure A15AS for that. SNR is then increased by several dBs.

Look at the linearity for the loopback for a 1kHz signal over a full 20kHz Bandwidth below.

The 20kHz BW increases significantly the impact of noise on the result.

(Here, I intentionally zoomed the Y scale to 0.05 / -0.05dB to really assess the accuracy we will get. Amir's plot is ususally +/-5dB)

We'd like less than +/- 0.01dB linearity error over 80dB for our measurement.

Without the passive pad, we are borderline.

By adding the passive resistor, we'll basically shift the X axis 0dBFS by 25 dB to the right, and only use down to -55dBFS.

Linearity is then well beyond 0.01dB.**:**To improve this low level signal's SNR, you may want to reduce the level by using a passive balanced pad after the interface output.I often use Shure A15AS for that. SNR is then increased by several dBs.

Look at the linearity for the loopback for a 1kHz signal over a full 20kHz Bandwidth below.

The 20kHz BW increases significantly the impact of noise on the result.

(Here, I intentionally zoomed the Y scale to 0.05 / -0.05dB to really assess the accuracy we will get. Amir's plot is ususally +/-5dB)

We'd like less than +/- 0.01dB linearity error over 80dB for our measurement.

Without the passive pad, we are borderline.

By adding the passive resistor, we'll basically shift the X axis 0dBFS by 25 dB to the right, and only use down to -55dBFS.

Linearity is then well beyond 0.01dB.

I then maximize the gain of the mic input and look at the level (in dBFS, from 20Hz to 20kHz) on both the Mic input and the line input.

(If your mic input has, say, more than 0.1% THD at that level, lower the level a few dBs.)

Given those measurements

[Calibration DMM level (V rms)]

[Calibration Line input level (dBFS)]

[Measure Line input Level (dBFS)]

[Measure Mic level (dBFS)]

My max input level is then

[Max Mic input level (dBu)] = 20 * LOG10([Calibration DMM level (V rms)]) - 10 *LOG10 (0.001*600)

- [Calibration Line input level (dBFS)]

+ [Measure Line input level (dBFS)]

- [Measure Mic level (dBFS)]

This should give you an accurate value.

**b. Measure the noise with the "150" plug in the mic input**This gives you [Noise (dBFS)]

You may then compute a more accurate EIN

[EIN (dBu)] = [Max Mic input level (dBu)] + [Noise (dBFS)]

Well, OK, but this values fluctuates with time.

Why ?

The temperature of the resistor.

The resistor's noise is far from negligible.

If the interface heats up, the resistor temperatur will also increase.

For a good preamp, it may actualy be higher than the preamp's noise itself.

But I can't measure it. So what ?

**c. Measure the noise with the "Short" plug in the mic input**

If we measure noise with the shorted plug, our "resistor"'s thermal noise is going to become very small.

Negligible if compared to the preamp's own noise.

With above's method, we'd get

[EIN Short (dBu)] = [Max Mic input level (dBu)] + [Noise Short (dBFS)]

then we may deduct the theoretical thermal nois eof a 150 ohm resistor to get a normalized EIN at 150 ohm 20°C, without impact of the temperature.

The thermal noise of the resistor is -130.92 dBu, or 220nV rms

[EIN (dBu)] = 10 * LOG10( 10^([EIN Short (dBu)]/10) + 10^(-130.92/10))

By the way, the preamp noise in V rms is then

[EIN Short (V rms)] = 10^(([EIN C/C (dBu)]+10*LOG10(0.6))/20)

(Of course, if you do step c., you don't need step b.)

But you're cheating ? A bit, sure.

But now we have

**a stable an accurate measurement**that may be compared to any EIN 150 value.It's much more reproducible.

And still it matches exactly the value we would get if we were able to measure EIN directly while keeping the temperature of the resistor at exactly 20°C

and with a resistor value of exactly 150 Ohm

And if you want to compare to an EIN with another resistor value (like 200 ohm), you may as well compute it.

Here is a practical example, with the UCX II

Note that, here-above, I also measured EIN 150 Ohm directly (plot not shown).

I then recorded the ambiant temperature during the measurement and the actual resistor value.

After computing a compensation factor from those values, to normalize to standard 150 ohm / 20°C conditions, corrected measurement is almost identical to the value deducted from the Short measurement, at -127 dBu vs -126.70 dBu.

RME UCX II's internal noise at Max gain is 282 nanoV rms.

As a check, I also measured the Short with Virtins Multi Instrument 3.9.11.1

If I summarize the required steps:

- Calibrate the line input you'll use to measure the low level signal using your multimeter and a signal close to its lower range upper limit
- Set the gain on the Mic input for the gain you want to measure (usually, we start with Max gain)
- With the line input plugged in parallel with the Mic input you want to measure, calibrate the Mic input's maximum level, using a signal 6-12dB below the expected max input level
- Plug the "Short" XLR plug and measure noise
- Compute EIN for various resistor values

You may want to repeat from 2. for a few other gains.

I like to measure for gains where the max input level is 10mV, then 100mV, for comparison between interfaces.

(EIN get worse with lowering gain)

If you measure more gains, you may get a comparison like this

(In the plot below, reference (0) gain is the gain that gives a maximum input level of 100mV for full scale)

**Comments welcome...**
Last edited: