Measuring Microphone Preamp Perf in Audio Interfaces

[amirm]

Other than in one occasion, I have not attempted to quantify the performance of microphone preamp in the context of testing audio interfaces (ADC). A good discussion erupted in the Topping E2x2 review thread on this. I like to have a dedicated thread for it so that the topic is better explored.

Note: please consider explaining things more than arguing with each other. You can disagree but let's make sure the task at hand is properly explained with the eventual goal of me adding a test to the reviews.

Thanks,

 

[Blumlein 88]

One obvious thing is actual max preamp gain. Usually it is close to spec, but sometimes not.

Another is EIN at various gain levels. I think the standard is to measure at 60 db gain, but at lower gain values EIN varies quite a bit among various interfaces.

Basic frequency response usually no big deal. But nice to confirm.

THD/IMD at various gain levels for near max output. Little such testing is done.

 

[SuicideSquid]

I don't have the technical expertise to know what would go into testing microphone preamps but I would be very interested in seeing this added to the testing suite for pro audio interfaces.

It would be really useful to me, as someone who runs a hobby recording studio, to have some objective numbers on preamp noise performance, gain, distortion, and frequency response.

 

[fpitas]

Maybe a test where the input is momentarily overdriven to observe the recovery.

 

[fpitas]

Check phantom power for voltage and available current.

 

[Blumlein 88]

One that can be important is input impedance. Again usually you can trust the specs to be close.

One I don't know if worth doing or not, much less the proper test, but maximum current for phantom power. Some people run into some microphones that are too tough for some low price microphone pre's though I personally have never had a problem with it. So maybe so rare not worth doing. fpitas beat me to it.

 

[amirm]

One I don't know if worth doing or not, much less the proper test, but maximum current for phantom power. Some people run into some microphones that are too tough for some low price microphone pre's though I personally have never had a problem with it. So maybe so rare not worth doing. fpitas beat me to it.

What is the max current that would be good to have and how do we define it? We would need to have a metric for voltage drop I suspect.

 

[fpitas]

Evidently there's an IEC standard of 10mA max per microphone. The Wikipedia article claims most modern mics only draw 240mW max:

Phantom power - Wikipedia

en.wikipedia.org

 

[Blumlein 88]

What is the max current that would be good to have and how do we define it? We would need to have a metric for voltage drop I suspect.

The IEC standard is max of 10 ma of current draw. Even this would droop the 48 volts some because of voltage drop across the usual 6800 ohm resistors phantom power goes thru. 7 ma is the max power you'll get with 48 volts. Most microphones don't exceed 4.5 ma though there are exceptions.

fpitas was quicker again.

 

[Blumlein 88]

Don't know how in the weeds you want to get with this. A spec Rane suggests is dynamic range at 15 db and 60 db(or max gain if lower than 60 db). Certainly some interfaces vary the in the available dynamic range at high and low gain which is somewhat related to the EIN which isn't constant over gain ranges.

 

[amirm]

Interfaces are already very time consumer to test especially if they also have a headphone amp. So ideally I would get in, get out in a couple of measurements.

 

[Labjr]

I've always thought a good microphone preamp with an audio interface would make a good phono preamp as well as a tape head preamp because the RIAA and NAB EQ can be done with DSP.

 

[Blumlein 88]

Interfaces are already very time consumer to test especially if they also have a headphone amp. So ideally I would get in, get out in a couple of measurements.

I think max gain, and the EIN at a few gain levels is most useful in terms of letting someone figure out if the microphone preamp basically works for their needs. There usually isn't a spec even listed for EIN levels at various gain settings. Doing the EIN test at 0 db gain and every 10 db or even 15 db would help.

So if you only do a couple, those are the two I would pick. Maybe some others can give their ideas on which is most helpful.

 

[DVDdoug]

I agree that maximum gain and noise are the most important things. Sometimes people complain that their interface doesn't have enough gain when using a dynamic mic.

And obviously distortion and frequency response but I'd expect acceptable results from almost any interface. (Acceptable to my tin ears, anyway!)

I've always thought a good microphone preamp with an audio interface would make a good phono preamp as well as a tape head preamp because the RIAA and NAB EQ can be done with DSP.

Mic preamps typically have an input impedance of around 1K and the standard for (moving magnet) phono cartridges is 47K (plus some small capacitance).

It might be OK for a tape head but I've never seen a tape machine without a built-in preamp.

...A million years ago I bought a surplus cassette deck chassis with no electronics and I built a preamp. I "accidently" included the tape head in the op-amp's feedback-gain circuit and the head inductance magically took care of most (or maybe all) of the NAB EQ!

 

[AnalogSteph]

I think max gain, and the EIN at a few gain levels is most useful in terms of letting someone figure out if the microphone preamp basically works for their needs. There usually isn't a spec even listed for EIN levels at various gain settings. Doing the EIN test at 0 db gain and every 10 db or even 15 db would help.

Also "gain" in this context should rather be input sensitivity (dBu or dBV per 0 dBFS) or its inverse, system gain. So maybe data points for max gain (whatever it may be) alongside standardized 10 mV and 100 mV or so.

Note, the AP has a 600 ohm output impedance and can't accurately measure EIN even at 150 ohms for that reason alone; external passive attenuation is advised. Having an additional 50 or near-0 ohm value for better differentiation on the low end would be neat as well. With two such data points you can already get a fairly accurate estimate for input voltage noise. (Not sure any interface has issues with current noise at 600 ohms.) I think I went into some detail on the math somewhere.... ah, there we go.

Sooo many questions about preamp Equivalent Input Noise (EIN) measurement. please help!!!

I encountered a significant setback while studying the process of measuring the Equivalent Input Noise (EIN) of a microphone preamp. There are several tutorials on EIN measurement available online, but I have many areas of confusion, which has resulted in me spending over a week without fully...

www.audiosciencereview.com

 

[jerryfreak]

re: Rja4000's original post in the topping thread

im interested, i have a topping D90SE and a pair of 50 dB attenuators like these https://www.adorama.com/sesil50db.html

i have a bunch of pres/ads i could test including a few of the zoom 32-bit F series, benchmark ADC1, centrance mixerface, motu M2, as well as some standalone preamps from grace, benchmark, etc. that i could test in front of the best ADC

i also have a UCX II to hopefully replicate your results

would i necessarily need the pro version of the software?

some measurements ive done of various pre/ADs with the topping D90SE set to -44dB output using Rightmark Audio analyzer. some are included in that list above but i have gear i didnt have (or didnt exist) when i did those measurements in late 2020

including:
zoom F3
sound devices mix pre I
sound devices mix pre II
Grace Design Lunatec V3>Benchmark AD2402-96 ("AD2K")
Tascam DR100 MKiii
motu M4

i have tests of these interfaces at higher input levels as well.

as expected,the 32-bit devices add little in dynamic range between their EIN and max input level

 

[amirm]

Note, the AP has a 600 ohm output impedance and can't accurately measure EIN even at 150 ohms for that reason alone; external passive attenuation is advised.

AP has selectable output impedance ranging from 40 ohm to 600. The error at 40 ohm is pretty minimal even for low impedance interfaces. Is this your concern?

 

[thecheapseats]

...please consider explaining things more than arguing with each other....

seems most everyone has made the correct points - I only scanned the comments... impedance, phantom current, max gain to accommodate ribbons and the usual suspects of 'challenged' dynamics mics - noise of course - high-pass knee freq (if an hpf is included)... overall and recently, julian krause has always been rather thorough...

if you only had to 'pick' a few to include - well I'm glad I'm not you... : )

and just to be the odd person in the conversation - is this a user base that uses or cares about mic pre(s)?...

 

[jerryfreak]

seems most everyone has made the correct points - I only scanned the comments... impedance, phantom current, max gain to accommodate ribbons and the usual suspects of 'challenged' dynamics mics - noise of course - high-pass knee freq (if an hpf is included)... overall and recently, julian krause has always been rather thorough...

if you only had to 'pick' a few to include - well I'm glad I'm not you... : )

and just to be the odd person in the conversation - is this a user base that uses or cares about mic pre(s)?...

yes

 

[amirm]

and just to be the odd person in the conversation - is this a user base that uses or cares about mic pre(s)?...

You would be surprised at the number of professionals (in music production) that read our forum. That is why I test audio interfaces to begin with.