Microphone preamp measurements: How to get the signal level right

[Rja4000]

A look at distortion vs. gain at either maximum output level or something more realistic (like +22 dBu or +10 dBu) may prove interesting.

That's indeed what I want to do next.
To measure above +24dBu, I have to lower level with the Autoranger II, just like I would for a power amp.
+24dBu input range is pretty common for Pro ADC, I think. And you need to still have some room to avoid clipping the ADC with peaks.
So measuring THD and IMD for each gain setting just below +24dBu output seems the way to go.
Measuring max input for, say, 0.1% distortion is also interesting (but way more time consuming)
Distortion starts to raise above +27dBu...
That's a good 18V rms...

 

[AnalogSteph]

Any mistake here, in your opinion ?

Nothing blatantly obvious, unfortunately (though I'd look for a better window function like Kaisel-Bessel and can't vouch for Virtins in terms of accuracy and would probably double-check that M/S mode works as expected). Still, if the manufacturer of a high-dollar pro device like the HV-3C with high input and low output impedance bothers to spec the gain to 0.5 dB and you find a constant ~1 dB offset, that sounds fishy to me. Got any other DUT to throw on there?

 

[Rja4000]

Nothing blatantly obvious, unfortunately (though I'd look for a better window function like Kaisel-Bessel and can't vouch for Virtins in terms of accuracy and would probably double-check that M/S mode works as expected). Still, if the manufacturer of a high-dollar pro device like the HV-3C with high input and low output impedance bothers to spec the gain to 0.5 dB and you find a constant ~1 dB offset, that sounds fishy to me. Got any other DUT to throw on there?

Thanks for taking the time to answer

I've read in different places that the noise is supposed to be measured with Rectangle window, which is said to be the most accurate for level measurements.
Virtins and REW measurements do match.
I use Virtins because it offers more flexibility and I'm able to automate it.
M/S is basically doing it, which means lower ADC noise by a few dB by averaging it.
But the noise we measure here is way above the RME ADC noise (since it's amplified).
I measured in non-M/S mode with same results.
So I keep the M/S method since that's how I perform my measurements.

The 1dB gain difference is strange, indeed, so I measured it several times.
I see that the source impedance makes a difference.
When using the passive attenuator with high attenuation, I get even lower gain, around 57.9dB with PAD and 59.6dB without PAD.
If I use the RME output directly, I get 60.4dB.
When any combination with the transformer-based JDI at the end of the chain (with or without passive attenuator first), I get 60.5dB.
I repeated that several times for both channels, which repeated exactly the same figures.

In any case, I compare to direct RME loopback, so that should compensate for RME mismatches.

I can't measure the 10mV-level input signal directly with anything like 1dB accuracy, since I don't own a high accuracy multimeter. And to get that kind of accuracy in true RMS, you need an expensive one.

By the way, that gives us a clear idea how impressive the RME performance is.

So, even if I don't give too much trust to the EIN raw figure, its still interesting to look at the trend and to compare with other devices, using the exact same setting.

Also, looking at Millennia specs, they spec it for
Minimum Gain 8.5 dB
Maximum Gain (1.5 dB per step, 36 steps) 60 dB (up to 70 dB on request)
Which is pretty much matching my measurements.
For EIN above, I've used gain figures I got by feeding the Preamp directly from RME, so respect. 8.67dB and 60.43dB.

 

[Rja4000]

I can't measure the 10mV-level input signal directly with anything like 1dB accuracy, since I don't own a high accuracy multimeter.

Well, now I do.

And the nice thing is that I'll be able to automate multiple measurements.
Combined with the RME and the Virtins software, that's a powerful combo.

EIN for the Milllennia HV-3C is 129.0dBu(A) for 150ohm and 60.59dB gain, by the way.

 

[AnalogSteph]

EIN for the Milllennia HV-3C is 129.0dBu(A) for 150ohm and 60.59dB gain, by the way.

Which translates to a dynamic range of over 95 dB(A) even at highest gain, since it will still comfortably handle a +27 dBu output (-33.5 dBu input). (With a 150 ohm, -59 dBV/Pa mic, that gets you up to +117.5 dB SPL with a noise floor at 22 dB SPL(A), roughly 4 dB above mic self-noise which is 20 dB unweighted @ 20 kHz.)

This amp one might actually consider getting an output attenuator for, e.g. if you are stuck with a lowly RME ADI-2 FS and its measly +19 dBu maximum input level. Not like you're losing much more than ~1 dB of EIN if you give up 10 dB of gain, mind you. (But that could give you ~104 dB(A) of dynamic range instead.)

For comparison, a lowly Behringer UMC202HD will also achieve -129.1 dBu(A) of EIN. Of course, at this point it will reach 0 dBFS at -51 dBu, and you rather want to stay away from anything above -5 dBFS if memory serves, so the practical limit would be around -56 dBFS. That's about 73 dB(A). The input only gets to around 100 dB of dynamic range at the best of times.

I wonder what would happen if you were to attach your favorite Fetliftercloudhead of choice (or even a Klark Teknik cheapie) to the Millennia? I suppose distortion performance up top would suffer, but you should be able to wring out anywhere from 2-4 dB less input noise.

 

[Rja4000]

RME ADI-2 FS and its measly +19 dBu maximum input level

RME max input range is +24dBu.
And yes, to measure the max level out of the Millennia, I use the Autoranger II to lower the level by 18dB.

Typically, distortion quickly raises above 27.5 to 28dBu output. Which means above -32 to -32.5dBu at the input.
Yes, that's over 96dB(A) dynamic range at 60.5dB gain.

 

[AnalogSteph]

RME max input range is +24dBu.

Just in the ADI-2 Pro FS with all the DSP bells and whistles, not the pleb spec ADI-2 FS that costs less than half as much (or its discontinued non-FS predecessor from back in the day).

 

[Rja4000]

Just in the ADI-2 Pro FS with all the DSP bells and whistles, not the pleb spec ADI-2 FS that costs less than half as much (or its discontinued non-FS predecessor from back in the day).

Sorry. My mistake. Indeed.