Preamp for a single XLR measurement mic

[john61ct]

I plan to use an Echo Audiofire 12 for REW and other measurement / filter design tools.

It is I/O only, includes neither HPamp nor mic preamps.

I have UMIK-2 but have decided to also get a calibrated XLR mic in order for both the signal output and the measurement precisely clock-sync'd by the one interface.

I have been looking at both standalone single-channel preamps with 48V phantim power

and separate portable mixer / interfaces that include one or more mic preamps

I believe the latter are better value, given they are multi-function.

Note that this latter will NOT be involved (as an interface to the PC) in the measurement / convolver process, only analog connection

mic --> preamp --> Audiofire12

Mackie VLZ series mixers include highly rated Onyx preamps, specifically 402-VLZ4 (402VLZ4) has two and available at affordable prices.

So, not only feedback wanted on any of the above, but suggestions on "better value" units, including singleton standalone units.

portability and DC-powered, even from a USB powerpak would be a plus.

Rolls is decent quality?

Rodyweil RO-04 and Polsen MP60 came up on the google...

 

[john61ct]

And I know off topic but while I gotcha

can anyone explain "J-BOX" units?

apparently works with REW, as well as something called justMLS...

 

[olds1959special]

I'm not sure if I'm using a flat pre-amp, but I plug my Beyerdynamic MM1 measurement mic into my Focusrite ISA One, because I can change the impedance to medium (2.4 k ohm) which is close to the recommended impedance for this mic (2.2+ k ohm)

 

[levimax]

I would get a 2 channel interface, Focusrite or Motu are both good. The reason you want 2 channels is so you can do "loopback timing" measurements which are more accurate and repeatable than messing around with acoustic timing. In some cases you have to use acoustic but loopback is so much nicer and easier that you don't want to give up that option getting a single channel interface.

 

[john61ct]

I would get a 2 channel interface

Sorry if I was not clear enough. I'm not looking for a single-channel interface.

I already have the measurements interface, very clean and capable of doing loopback timing, 12-channels in and 12-channels out.

It just needs a preamp for the XLR mic, that's the topic here.

 

[john61ct]

I'm not sure if I'm using a flat pre-amp, but I plug my Beyerdynamic MM1 measurement mic into my Focusrite ISA One, because I can change the impedance to medium (2.4 k ohm) which is close to the recommended impedance for this mic (2.2+ k ohm)

Thanks for bringing up that factor, I will keep that in mind.

On the Mackie mixer I asked about is not adjustable, but its Onyx preamp input has an impedance of 2.55 kΩ

That Focusrite unit is very interesting, but costs many times more, I think overkill for this use case.

 

[DVDdoug]

I can't answer all of your questions but I have some comments-

One advantage to USB measurement mics is the dB SPL level is calibrated to the digital dB level. With an analog mic you have make your own calibration (if you care about measuring SPL) because it depends on preamp gain and volume adjustment, etc. Plus, the whole setup is a lot easier.

With a condenser mic (phantom power) you should be OK, but many audio interfaces don't have enough gain to work well with dynamic mics. A lot of people end-up using a Cloudlifter or a FetHhead ("pre-preamp" or "booster"). A separate preamp or mixer might have more gain.

I'm not sure that the clock/timing are THAT critical. You are making short-time measurements, a few seconds at most, and measuring times related to the speed of sound. Moving the speaker or mic an inch will maker a bigger difference than a clock error which is usually measured in parts-per-million.

And there is ALWAYS latency (delay) through the computer (because of buffering). I believe the latency can be calibrated-out and it should be consistent so it shouldn't be a problem.


To me a "J Box" is a junction box, like where wires come into a building, ect...

 

[Roland68]

Sorry if I was not clear enough. I'm not looking for a single-channel interface.

I already have the measurements interface, very clean and capable of doing loopback timing, 12-channels in and 12-channels out.

It just needs a preamp for the XLR mic, that's the topic here.

This shop offers calibrated measurement microphones and a matching MPA-102 microphone preamplifier, which can also be calibrated as a set.
However, you can also buy the IMG Stageline MPA-102 individually everywhere.
https://shop.hifi-selbstbau.de/produkt/messtechnik-pack-mm-1c/
https://shop.hifi-selbstbau.de/produkt/messmikrofon-mm-1c/
https://www.thomann.de/de/img_stageline_mpa_102.htm

 

[levimax]

Sorry if I was not clear enough. I'm not looking for a single-channel interface.

I already have the measurements interface, very clean and capable of doing loopback timing, 12-channels in and 12-channels out.

It just needs a preamp for the XLR mic, that's the topic here.

Oops need to read more carefully.

 

[john61ct]

This shop offers calibrated measurement microphones and a matching MPA-102 microphone preamplifier

Looks great.

But wouldn't full 48V phantom power be safer, future proof?

And boy I hate rolling the dice with tariffs ATM

 

[john61ct]

One advantage to USB measurement mics is the dB SPL level is calibrated to the digital dB level.

So I guess good I have the UMIK-2 as well

separate preamp or mixer might have more gain

Thanks I'll look for that. The Mackie Onyx mic preamps has up to 60dB gain.

> I'm not sure that the clock/timing are THAT critical

Maybe not but many members I trust seem to think it's pretty important, and timing/phase issues with my analog crossovers is the one area that may force me to go to DSP.


> there is ALWAYS latency (delay) through the computer (because of buffering). I believe the latency can be calibrated-out and it should be consistent so it shouldn't be a problem.

thanks I will look into that as well.

 

[DVDdoug]

But wouldn't full 48V phantom power be safer, future proof?

Virtually all microphone preamps, mixers, and audio interfaces have XLR mic inputs with phantom power. It shouldn't be a problem. (Regular soundcards and the mic input on a laptop are not compatible with stage/studio mics.)

There are other limitations with USB mics, especially in other applications. Sometimes (like with the UMIKs) there is no way to adjust the analog volume into the internal ADC (analog-to-digital converter). You can clip (overload) the ADC with very-loud sounds, or if the sound is too quiet you can get a weak signal.

You can usually only use one USB mic at a time, so no stereo recording (unless you have a stereo USB mic) and no multi-track recording.

You can't use a USB mic with a PA system or mixer.

It just needs a preamp for the XLR mic, that's the topic here.

Frequently, an audio interface or mixer (or USB mixer) is less expensive than a stand-alone mic preamp. It's a "specialty item". The actual electronics to build a preamp are cheap.

separate preamp or mixer might have more gain

With a condenser mic you shouldn't need "extra gain", Dynamic mics (like the SM58 or SM7B) and Ribbon mics usually have about -20db lower output.

 

[john61ct]

Virtually all microphone preamps, mixers, and audio interfaces have XLR mic inputs with phantom power.

I specifically bought an interface that does not, as stated in OP. Echo AudioFire 12 has 24 analog channels via 1/4" TRS inputs can be balanced or single-ended.

But line level only, switchable between +4dBu and -10dBV.

The two Mackie 402-VLZ4 mic preamps have gain control as well as an output control knob for the main 1/4" TRS balanced outputs

Thanks for the background info on USB mics.

WRT choosing this mic preamp, I think better to stick with ones that provide 48V pp rather than less so as not to limit my future choices of XLR mic's

 

[DVDdoug]

I think better to stick with ones that provide 48V pp rather than less so as not to limit my future choices of XLR mic's

Yes! But 48V is standard so it shouldn't be an issue. Some "cheap devices" may have lower voltage and some mics may not need the full 48V, but most phantom-powered mics are designed for 48V and you'll find that in the specs. (As you may know, dynamic mics don't have any internal active electronics so they don't need power.)

Standard condenser "computer mics" use unbalanced connections and they run from 5V (from the soundcard) which is not "phantom".

 

[john61ct]

But 48V is standard so it shouldn't be an issue.

Many of the preamps I've looked at only go up to say 24V

I am only looking at well-calibrated XLR mics with good reputation for measurement specifically with REW and other tools like Acourate.

 

[Roland68]

Looks great.

But wouldn't full 48V phantom power be safer, future proof?

And boy I hate rolling the dice with tariffs ATM

Most measurement microphones have a permanently polarized condenser capsule and can also be powered with 12-24V phantom power.
48V phantom power is only required for microphones with externally polarized condenser capsules, but I would advise against using such microphones for measurement purposes.

There are also modern microphones that are quite power-hungry, and not every 48V phantom power supply is sufficient for them; you have to check this on a case-by-case basis.

The MM-1C mentioned can be operated from 9-48V without any limitations. This also applies to most measurement microphones that are specified for 9, 12, or 24V.

 

[Roland68]

Many of the preamps I've looked at only go up to say 24V

I am only looking at well-calibrated XLR mics with good reputation for measurement specifically with REW and other tools like Acourate.

Don't let anyone tell you otherwise; any measurement microphone is perfectly adequate as long as it's calibrated, regardless of whether it costs €40 or €200.

The IMG Stageline MPA-102 has proven itself in measurements, but the next truly superior quality level costs many times more. It's simply not worth it for normal audio measurements and development.

 

[john61ct]

To me a "J Box" is a junction box, like where wires come into a building, ect...

J-Box III Speaker Measurement System Review

I've posted an evaluation of the J-Box III speaker measurement system if anyone is interested: https://www.htguide.com/forum/missi...ment-tools-techniques/957531-j-box-iii-review

www.audiosciencereview.com

Something like Dayton's DATS?

Supports REW as well as justMLS, part of LspCAD

 

[AnalogSteph]

Mackie VLZ series mixers include highly rated Onyx preamps, specifically 402-VLZ4 (402VLZ4) has two and available at affordable prices.

From my experience:
The non-bypassable tone stage means that you may not get the last tenth of a dB of FR flatness (take loopback measurements for soundcard compensation if you insist), distortion substantially below the -90 dB mark, or dynamic range much exceeding 105ish dB. Neither matters for acoustic measurements.
In return you do get a lot of voltage headroom and substantial input voltage handling due to the +/-15 V supplies standard in this class of mixer, in ways that lower-end audio interfaces on +/-5 V supplies may struggle to achieve.
The P48V supply, presumably a voltage doubler affair, isn't particularly hard to overload and drop out of regulation, though it took multiple mics if memory serves. (I think that's why the mains transformers for these mixers have relatively high secondary voltages, 2x 18.5V~ in this case.) I can't guarantee that my 402VLZ4 specimen is representative in that respect, of course, a marginal cap in the voltage doubler definitely would not be helping matters as you can imagine.
The bar graph level display is on the output, so if you turned down the main mix substantially for a good match of internal levels to an onboard chip that can take little over 1 Vrms, the indication won't go very high. (The mixer can take +16 dBu without breaking a sweat, so you need roughly -12 dB of output gain in such a case.) There are extra clipping LEDs at the 1/2 level pots to gauge internal level.

I would not be surprised if the preamps themselves were capable of excellent performance, the rest of the mixer arguably isn't quite up the the same standard. Still an upgrade over a similar Behringer though. Those of the soldering-iron-wielding persuasion may want to investigate the possibility of a tone stage bypass, though I'm not sure whether it could be done without endangering the highpass and stereo pan (mix 1/2 or keep them separate) functions which I do find genuinely useful. I haven't bothered to do so because mixers tend to be rather tedious to disassemble, and my application as a preamp for spoken voice does not demand measurement-grade performance.

 

[john61ct]

Once you go past the 802-VLZ4, all the Onyx preamp lines have TRS "channel insert" Sends, basically designed as loops for outboard signal processing, compressors etc

These can function as "direct out" signal paths if you simply leave out the Return path, bypassing all the internal mixer circuitry.

Going bigger also gets a separate "mute-as-routing ALT 3-4 bus", acts like a "sub-group". In case I do ever want to use it as a mixer.

From my experience:
The non-bypassable tone stage means that you may not get the last tenth of a dB of FR flatness (take loopback measurements for soundcard compensation if you insist), distortion substantially below the -90 dB mark, or dynamic range much exceeding 105ish dB. Neither matters for acoustic measurements.

> Those of the soldering-iron-wielding persuasion may want to investigate the possibility of a tone stage bypass, though I'm not sure whether it could be done without endangering the highpass and stereo pan (mix 1/2 or keep them separate) functions which I do find genuinely useful.

If I'm understanding my research correctly, all that was designed in once you go past 8-channels.

And for maybe $50 more, the whole thing costing less than one low-end calibrated measurement mic!