Measuring Microphone Preamp Perf in Audio Interfaces

[Timcognito]

For those of us on a learning curve and interested in the recording end of audio I found this.

 

[amirm]

OK, after spending the last couple of evenings on this, I think I have this right. Here is the EIN computed as dBV:

Bandwidth is 20 to 20 kHz. No filtering.

I was not happy with the above as it doesn't relate to audibility. A-weighting was suggested but that just inflates the above number and doesn't have proper backing with respect to audibility/annoyance of noise. CCIR-2K/CCIR/ARM remedies that. It shaves about 2 dB from all the numbers:

Max gain was lower for all the interfaces in my measurements vs advertised. AP has a non-zero output impedance which impacts the gain especially with Topping. Here the values I computed:

Topping E2x2: 50.21 dB
Focusrite 2i2 (3rd gen): 49.0 dB
RME Babyface Pro FS: 53.4 dB

Given that it is not much more than 40 dB, I am still inclined to get rid of Max gain measurements. It will make presentation easier than having three sets of data as this list grows.

What do you think?

 

[thecheapseats]

for context on mic pre gain levels - the one 'unknown' is always the mic itself... almost every time I've dialed-in a mic-pre's gain close to its highest level - I've found they (usually) no longer impart anything flattering to the signal at those excursions - and time to switch to a different pre...

I hate it when this happens and am married to a mic's character in that moment - but it's simply an operational fact of life and happens more often with ribbons and dynamics -but some condensers as well.

for simple voice-announce/narration work today - there are reasonable digital gain-staging workarounds as @Julian Krause referred to above - but for those few times when recording a music performance this occurs more often than one would expect...

 

[Blumlein 88]

What was the source impedance? I've gotten a little better EIN results on my BabyFace. I used a 150 ohm metal film resistor for the source.

 

[amirm]

What was the source impedance? I've gotten a little better EIN results on my BabyFace. I used a 150 ohm metal film resistor for the source.

It is a 150 ohm termination Topping sent me. Note that these are dBV values. For dBU, you need to add 2.2 dB.

 

[KSTR]

AP has a non-zero output impedance which impacts the gain especially with Topping.

You could build a simple passive -30dB...-50dB attenuator, using the built-in output resistance and a shunt resistor, to bring down output resistance to below 10Ohms or so. Optionally then add new series resistors from that point to get back the nominal 150Ohms when measuring gain, FR and distortion at high gain settings because total source noise will be dominated by resistor noise.

 

[Blumlein 88]

It is a 150 ohm termination Topping sent me. Note that these are dBV values. For dBU, you need to add 2.2 dB.

I'm also suspicious the gain of the Babyface is more than 11 db less than spec. That isn't a slight miss.

 

[KSTR]

@Blumlein 88, max gain often depends on potentiometer parasitics, wiper contact and end resistances. When those are approaching several Ohms and the required gain resistance is only about 10Ohms (a typical value for 60dB gain for, say, a SSM2019-based circuit) this often gives lower numbers.

 

[amirm]

I'm also suspicious the gain of the Babyface is more than 11 db less than spec. That isn't a slight miss.

Yeh, across all three I am getting lower gain.

 

[amirm]

You could build a simple passive -30dB...-50dB attenuator, using the built-in output resistance and a shunt resistor, to bring down output resistance to below 10Ohms or so. Optionally then add new series resistors from that point to get back the nominal 150Ohms when measuring gain, FR and distortion at high gain settings because total source noise will be dominated by resistor noise.

Since the final test load is 150 ohm anyway, I decided to use that as the output of AP. This way, they match. If I measure with 10 ohm and then plug in the 150 ohm source load, then the gains won't be the same.

 

[Rja4000]

Max gain was lower for all the interfaces in my measurements vs advertised. AP has a non-zero output impedance which impacts the gain especially with Topping. Here the values I computed:

Topping E2x2: 50.21 dB
Focusrite 2i2 (3rd gen): 49.0 dB
RME Babyface Pro FS: 53.4 dB

How exactly are you measuring this ?
Is that the gain range, meaning max gain - Min gain ?
Or what do you consider as 0 gain ?

 

[Rja4000]

these are dBV values. For dBU, you need to add 2.2 dB

EIN is usually expressed in dBu.

 

[Rja4000]

Given that it is not much more than 40 dB, I am still inclined to get rid of Max gain measurements. It will make presentation easier than having three sets of data as this list grows.

What do you think?

As I said above:
Either you measure all interfaces for the gain giving 0dBFS for a given level (say 10mV) WHATEVER THE GAIN MARKING, either you measure all at their best, ie at max gain.

Measuring for an arbitrary (strongly marketing related) "gain" figure is misleading, IMO.

Or I miss something and you're able to define a meaningful "gain" figure, independently of the fact that each interface uses a different ADC, which may all have a different input voltage range.

 

[Blumlein 88]

The more I think about this the more I agree with Rja4000. An input sensitivity test is what really is useful to the end user more than gain. Feed in 10 mV, see what gain setting gives 0 dbFS. Perhaps even better feed in 1 mV at max gain, and record the dbFS level of the interface. Many lower priced interfaces take a max input of 8 dbu. In such a case 66 db of gain on 1 mV would give 0 dbFS.

Then do EIN at max gain. Only two tests both of which are useful for comparing mic preamps.

 

[Blumlein 88]

I know you know all this, but I like the way gain is presented by Julian Krause in this video. He's posted already, maybe he has more to say.
The pertinent parts are from 5:54 to 7:21 in this video.

 

[Rja4000]

The more I think about this the more I agree with Rja4000.

Thanks.
I feel less alone, all of a sudden
(Well, @JohnYang1997 implicitly agreed too)

An input sensitivity test is what really is useful to the end user more than gain. Feed in 10 mV, see what gain setting gives 0 dbFS. Perhaps even better feed in 1 mV at max gain, and record the dbFS level of the interface.

10mV at full scale is already quite low.
I doubt all interfaces are able to reach 0dBFS with 1mV.

That would be reaching -60dBV on @Julian Krause 's gain plot.

 

[Rja4000]

In french, we say "Un dessin vaut mieux qu'un long discours"

So here we go

I measured my Millennia HV-3C first
This is an analog preamp.
I measured it through the ADC of my RME ADI-2/4 Pro SE.

First for Max gain (60.6dB), with the RME on minimum ADC range (+1dBu)
(Changing the ADC range doesn't change the result here: Preamp's noise is well above the RME noise. What matters is only the Preamp gain.)

Then for a gain that gives me just below 0dBFS with a 10mVrms 1kHz signal

I kept the same ADC range first

Then I switched to 19dBu range.
To get 0dBFS on the ADC with the same signal, we need to increase the gain by 18dB.
(if I selected the maximum range, which is 24dBu, I couldn't get enough gain to reach 0dBFS with 10mV)

As one can see, we get pretty different results.
And we see that for best EIN results, it's better to have more gain and a higher analog output level on the preamp.
(By the way, it's the same if you want to cascade 2 preamps: You'd better maximize the gain on the first stage to get best results)


OK, here it is an analog preamp, and I can play with the ADC range, which allows this flexibility.

Now what happens with digital interface, where the ADC range is fixed ?

Let's measure with the RME 12Mic-D

We don't know the gain, but we may measure the value for 0dBFS in dBu
(Actually, RME is quite clear with the gain: 0dB gain = 18dBu for 0dBFS. Here, the gain is set to 56dB marking, since we target a level of -38dBu for 0dBFS)

We can then compare this figure with the previous one:
-127.97dBu un-weigthed vs -127.02 dBu for those 2 high end interfaces/preamps.
(Note that if we had selected a higher level for 0dBFS, like 20mV, as an example, the Millennia would have been able to take benefits of its higher output level - up to 28dBu before saturation - to close the gap with the RME)

Now, at max gain, the RME 12Mic-D gives us this

Not dramatically different.

For both examples, the EIN difference between max gain and 10mV range gain is around 0.1dBu.

So, so far, that gives some sense to the idea of measuring all interfaces at max "gain" for noise, if you don't want to go through the pain of explaining this "gain" topic to the readers.
Even if I still think that aligning sensitivities or "gain" is a more scientific way of doing it.

Similarly, measuring dynamic range at lowest "gain" (on mic input, without any pad inserted) probably makes sense.

But we'd still need our "Dashboard" and a set of all usual measurements, to be done at a standardized level AND sensitivity or "gain".
So, say, 4V at -6dBFS (or -3dBFS, if 8V is too big of a range for cheaper interfaces) as an example.



Next, I tried something seriously less good.

Here is the BlackmagicDesign ATEM Extreme ISO video switcher's Microphone input.
"Gain" is set to -1.5dB, for 10mV at -0.2dBFS

Interesting to see that the CCIR-2k value here is almost identical to the un-weighted value.
The good thing is that we may now really compare this one with the other ones.
Not really a contest, as expected, of course...
We are virtually at max gain here, so I didn't measure it any further.

 

[thecheapseats]

In french, we say "Un dessin vaut mieux qu'un long discours"

So here we go

I measured my Millennia HV-3C first
This is an analog preamp.
I measured it through the ADC of my RME ADI-2/4 Pro SE.

First for Max gain (60.6dB), with the RME on minimum ADC range (+1dBu)
(Changing the ADC range doesn't change the result here: Preamp's noise is well above the RME noise. What matters is only the Preamp gain.)


View attachment 324244


Then for a gain that gives me just below 0dBFS with a 10mVrms 1kHz signal

I kept the same ADC range first

View attachment 324246

Then I switched to 19dBu range.
To get 0dBFS on the ADC with the same signal, we need to increase the gain by 18dB.
(if I selected the maximum range, which is 24dBu, I couldn't get enough gain to reach 0dBFS with 10mV)

View attachment 324249

As one can see, we get pretty different results.
And we see that for best EIN results, it's better to have more gain and a higher analog output level on the preamp.
(By the way, it's the same if you want to cascade 2 preamps: You'd better maximize the gain on the first stage to get best results)


OK, here it is an analog preamp, and I can play with the ADC range, which allows this flexibility.

Now what happens with digital interface, where the ADC range is fixed ?

Let's measure with the RME 12Mic-D

We don't know the gain, but we may measure the value for 0dBFS in dBu
(Actually, RME is quite clear with the gain: 0dB gain = 18dBu for 0dBFS. Here, the gain is set to 56dB marking, since we target a level of -38dBu for 0dBFS)

View attachment 324250

We can then compare this figure with the previous one:
-127.97dBu un-weigthed vs -127.02 dBu for those 2 high end interfaces/preamps.

Now, at max gain, the RME 12Mic-D gives us this

View attachment 324251

Not dramatically different.

For both examples, the EIN difference between max gain and 10mV range gain is around 0.1dBu.

So, so far, that gives some sense to the idea of measuring all interfaces at max "gain" for noise, if you don't want to govthrough thecpain of explaining this "gain" topic to the readers.
Even if I still think that aligning sensitivities ir "gain" is a more scientific way of doing it.

Similarly, measuring dynamic range at lowest "gain" (on mic input, without any pad inserted) probably makes sense.

But we'd still need our "Dashboard" and a set of all usual measurements, to be done at a standardized level AND sensitivity or "gain".
So, say, 4V at -6dBFS (or -3dBFS, if 8V is too big of a range for cheaper interfaces) as an example.

Next, I'll try something seriously less good.

Here is the BlackmagicDesign ATEM Extreme ISO video switcher's Microphone input.
"Gain" is set to -1.5dB

View attachment 324484

Interesting to see that the CCIR-2k value here is almost identical to the un-weighted value.
The good thing is that we may now really compare this one with the other ones.
Not really a contest, as expected, of course...

Thank you - I enjoyed that immensely...

 

[Rja4000]

Since the final test load is 150 ohm anyway, I decided to use that as the output of AP. This way, they match. If I measure with 10 ohm and then plug in the 150 ohm source load, then the gains won't be the same.

Currently, I'm using this https://www.shure.com/en-US/products/accessories/a15as to lower level
(which is given for 150 ohm output impedance)

 

[Timcognito]

Currently, I'm using this https://www.shure.com/en-US/products/accessories/a15as to lower level
(which is given for 150 ohm output impedance)

The following is from, https://jazzprofiles.blogspot.com/2018/06/the-search-for-roy-dunann.html. In the late 50's, Contemporary Records made some of cleanest recordings with high dynamic range to this day.

Roy explains that "Lester wanted to set up the studio as cheap as possible, and make it sound as good as possible." Lester's expensive condenser mikes had high output because of the tube preamps built into their heads. When Lester took them into a recording studio (like, for example, Capitol's, which was set up for a variety of microphones, primarily dynamic), the signal coming off Lester's mikes had to be attenuated so that they did not overload the equipment.

"So," Roy continues, "it was my idea — why attenuate the microphones and then amplify the signal again? Why don't we just take the signal out of the microphones and run it through variable attenuators, and we wouldn't need any amplifiers? So that was the original console. Nothing to it. I probably had eight attenuators. That was before they had sliders, even. Couldn't find any decent sliders. Didn't even want one. We did all our mixing by turning knobs. We went from the attenuators right into the tape machine — no other equipment."

Forty-five years later, Bernie Grundman reflected that "Roy was making the best sound in the business by cutting corners. It was such a clean signal path. All the gain that was needed for the mixing function came right off the microphone preamps. Roy could mix like that, on the fly."