How to measure EIN (Equivalent Input Noise) of an interface's Mic preamp

[Blumlein 88]

I think taking into account preamp input impedance is needed. I thought my readings were a bit on the high side. Not impossible, but a bit high. When I took into account the effect of loading by the input impedance they look more believable even though it was less than 1 db correction.

 

[AnalogSteph]

On the flipside this means that if you want to build a super-low-noise preamp that is pushing the very limits of physics, it better have input impedance about as high as phantom power will allow. That one obviously poses an upper limit at 13.6 kOhms. The common choice of low Z inputs has more to do with practicality, they don't want the input to go into all out mad snake mode when left open I guess.

 

[Blumlein 88]

On the flipside this means that if you want to build a super-low-noise preamp that is pushing the very limits of physics, it better have input impedance about as high as phantom power will allow. That one obviously poses an upper limit at 13.6 kOhms. The common choice of low Z inputs has more to do with practicality, they don't want the input to go into all out mad snake mode when left open I guess.

If you are using phantom power it is a waste of time anyway. It means you are using condensers with phantom power and condensers have self noise higher than the EIN noise by enough EIN has to be terrible for it to matter. There are a few expensive mic pres that have 10 k ohm inputs with phantom power on while using 47 k ohm to 100 k ohm with phantom power off.

I do know of one condenser which tackles the problem a bit differently. Marek Design makes a transformer-less tube condenser the RS-1 which puts out line level signals and has self noise of only 4 db (1.5 db A-wtd). You don't need a mic preamp.

Here is one preamp example with high input impedance. AEA TRP.

https://aearibbonmics.com/wp-content/uploads/2023/04/TRP3-Spec-Sheet.pdf

 

[FrenchFan]

Hello everyone .

I don't have a microphone interface , but
I have an E1DA scaler + E1DA adc .

I made a small interface for a "low cost" lavalier microphone and I found that the recording noise was really low compared to my zoom H1 .

My microphone has the following Amazon specifications.

Sensitivity ~32db (SPL 1Pa)
SNR >= 60dB
BW 20-16000
Impedance 2.2Kohms

The kind of microphone that you can find for 10 Euro on Amazon .

To have sufficient gain , the scaler having 2 channels ,
I put them in series , which gives us about 53dB of
gain , approximately the gain of the "Motu M2" .

The input impedance is ~ 100Kohms.

I measured the noise at max gain with short circuit with
the ADC on the 10V caliber
Noise brought back to the scaler input < 150dBFS
53.4 Scaler + 97.91 ADC.

I calculated approximately my max level at the
scaler input at -30 dBu before clipping the ADC caliber 10v.

I would like to characterize this configuration more precisely

I see that you are in full discussion on this subject.

Are you stopping on a pool of measurements, that I can follow ??? .

I have a Fluke 177 6000 points which is not too bad + oscilloscope
to try to have the best possible measurements.

 

[AnalogSteph]

I made a small interface for a "low cost" lavalier microphone and I found that the recording noise was really low compared to my zoom H1 .

Which, mind you, seems to have a bit of a reputation for being noisy with external mics. If the H2 is any indication, I can see why:

Recorder Tests - Avisoft Bioacoustics

Avisoft Bioacoustics Hardware and software for investigating animal sound communication: ultrasound microphones, integrated acoustic data acquisition systems, recording and sound analysis software optimized for applications in bioacoustics.

avisoft.com

-99 dBu A-wtd / -95 dBu unwtd, yikes. That's like 10 µV. This makes a TL071 preamp on a 9 V cell look like the epitome of high fidelity. I haven't checked for the availability of small external mic preamps lately, one of those would be a good idea though.

When it comes to consumer-level/video microphones with plug-in power, there are two things that can ruin your day:
1. The input amplifiers themselves (and other misc. factors).
2. The bias voltage, as roughly half of any noise on there is going to end up in the input (depending on mic and its effective source impedance).

For example, Realtek onboard audio tends to have decent enough input stages but their insistence of generating mic bias voltage all internally leads to it being on the noisy side, resulting in consistently mediocre noise performance with electret mics. At the same time their input stages still are quite noisy by the standards of dynamic mics (I once got an estimate of ca. 20 nV/√(Hz), or ~2.8 µV of input noise in 20 kHz), so it's not an ideal experience for anyone. Now an XLR condenser mic with an external phantom power adapter, that should work decently assuming you can record L-R.

My microphone has the following Amazon specifications.

Sensitivity ~32db (SPL 1Pa)
SNR >= 60dB
BW 20-16000
Impedance 2.2Kohms

The kind of microphone that you can find for 10 Euro on Amazon .

Looks like a Panasonic WM61-inspired capsule. Actually quite decent for noise by itself.

I measured the noise at max gain with short circuit with
the ADC on the 10V caliber
Noise brought back to the scaler input < 150dBFS
53.4 Scaler + 97.91 ADC.

Max gain on the scaler seems to be +26.078 dB --> +52.156 dB total or ~400.

So you are seeing an input noise level of -97.91 dBFS rms on the ADC in this setup?
That would be
+20 dBV (= 10 Vrms) - 52,15 dB - 97,91 dB = -130.06 dBV = -127.85 dBu (shorted).
Assuming a 20 kHz bandwidth, that translates to 2.22 nV/√(Hz) worth of input noise density, which seems plausible given that the OPA1612 used in the scaler is spec'd at 1.1 nV/√(Hz) and you'd need super low resistor values to get particularly close to this. The Scaler's "Residual noise" spec of "250nVrms(A) @ 26db gain" suggests about 1.77 nV/√(Hz), but that's A-weighted and those values tend to be around 2 dB lower. If you add 2 dB you get almost exactly your results.

A little electret mic is not going to require input noise as low as this by any stretch as their FETs generate plenty of noise by themselves. Something around 10 nV/√(Hz) tends to be perfectly fine. Chances are you could get by just fine with just one section of the Scaler, especially if you were to reduce the ADC's input voltage range as well. Even in the 10 V range, your effective ADC noise floor would already be at <0.5 µV (or <3.5 nV/√(Hz)) like that, more than good enough.

I calculated approximately my max level at the
scaler input at 30 dBu before clipping the ADC caliber 10v.

Are you sure? +30 dBu would be about 24.5 Vrms, and the scaler doesn't go below unity gain.

 

[FrenchFan]

Which, mind you, seems to have a bit of a reputation for being noisy with external mics. If the H2 is any indication, I can see why:

Recorder Tests - Avisoft Bioacoustics

Avisoft Bioacoustics Hardware and software for investigating animal sound communication: ultrasound microphones, integrated acoustic data acquisition systems, recording and sound analysis software optimized for applications in bioacoustics.

avisoft.com

-99 dBu A-wtd / -95 dBu unwtd, yikes. That's like 10 µV. This makes a TL071 preamp on a 9 V cell look like the epitome of high fidelity. I haven't checked for the availability of small external mic preamps lately, one of those would be a good idea though.

When it comes to consumer-level/video microphones with plug-in power, there are two things that can ruin your day:
1. The input amplifiers themselves (and other misc. factors).
2. The bias voltage, as roughly half of any noise on there is going to end up in the input (depending on mic and its effective source impedance).

For example, Realtek onboard audio tends to have decent enough input stages but their insistence of generating mic bias voltage all internally leads to it being on the noisy side, resulting in consistently mediocre noise performance with electret mics. At the same time their input stages still are quite noisy by the standards of dynamic mics (I once got an estimate of ca. 20 nV/√(Hz), or ~2.8 µV of input noise in 20 kHz), so it's not an ideal experience for anyone. Now an XLR condenser mic with an external phantom power adapter, that should work decently assuming you can record L-R.

Looks like a Panasonic WM61-inspired capsule. Actually quite decent for noise by itself.

Max gain on the scaler seems to be +26.078 dB --> +52.156 dB total or ~400.

So you are seeing an input noise level of -97.91 dBFS rms on the ADC in this setup?
That would be
+20 dBV (= 10 Vrms) - 52,15 dB - 97,91 dB = -130.06 dBV = -127.85 dBu (shorted).
Assuming a 20 kHz bandwidth, that translates to 2.22 nV/√(Hz) worth of input noise density, which seems plausible given that the OPA1612 used in the scaler is spec'd at 1.1 nV/√(Hz) and you'd need super low resistor values to get particularly close to this. The Scaler's "Residual noise" spec of "250nVrms(A) @ 26db gain" suggests about 1.77 nV/√(Hz), but that's A-weighted and those values tend to be around 2 dB lower. If you add 2 dB you get almost exactly your results.

A little electret mic is not going to require input noise as low as this by any stretch as their FETs generate plenty of noise by themselves. Something around 10 nV/√(Hz) tends to be perfectly fine. Chances are you could get by just fine with just one section of the Scaler, especially if you were to reduce the ADC's input voltage range as well. Even in the 10 V range, your effective ADC noise floor would already be at <0.5 µV (or <3.5 nV/√(Hz)) like that, more than good enough.

Are you sure? +30 dBu would be about 24.5 Vrms, and the scaler doesn't go below unity gain.

Hello AnalogSteph

Thanks for breaking down the calculation of my
configuration , it allows me to understand
a little better everything that can be measured
and calculated .

Thanks for the recorder table , it
allows us to know what is important in the
measurements .

"scaler input at -30 dBu before clipping the ADC caliber 10v."
not 30dBu , excuse me for the sign .

It is true that we do not need 53dB of gain to
record with this type of electret microphone , it is just
to align myself with the microphone interfaces that exist
at low prices .

OK the Zoom H1 is not a reference in the world of
recorders , it looks like as you say ; the Zoom H2
in your table .

So what I notice in the comparison table
of your recorders is that it displays the following parameters
which are sufficient to characterize an interface

------------------------------------------------------
*Equivalent Input Noise (EIN)
impedance: 150 ohms

*Input Clipping Level
corresponding to 0 dBFS

*Dynamic Range at max gain!

------------------------------------------------------

I notice that the unit "dBu" is always used in
the language of the recorders, to keep in order to be able to make
quick comparisons with manufacturer specifications.

I will start on the measurement of the 3 parameters indicated above
by measuring a little more precisely the gain of the scaler and the
conversion factor of the ADC.

Thank you very much

 

[AnalogSteph]

"scaler input at -30 dBu before clipping the ADC caliber 10v."
not 30dBu , excuse me for the sign .

-30 dBu + 52.156 dB = +22.156 dB or 9.93 Vrms. So you have confirmed to a good degree of accuracy that it's in fact the 10 Vrms range you're using.