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Motu 624 Audio Interface Review

amirm

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#1
This is a review and detailed measurements of the Motu 624 professional audio interface (DAC, ADC and headphone output). It is on kind loan from a member. The 624 costs US $795.

The MOTU has been around the pro audio industry for decades. The look of the unit goes with that industry in a professional and unique looking enclosure:

Motu 624 Pro Audio Interface ADC DAC Review.jpg

As can be expected, there are tons of inputs and outputs both on the front and the back:

Motu 624 Pro Audio Interface ADC DAC Back Panel Connectors Review.jpg

A very small switching power supply powers the unit (not much bigger than a phone charger).

Unusual with the previous products we have tested is the inclusion of Ethernet port which can be used to control, and gang multiple units together.

While I am not a fan of loading drivers and software, MOTU makes this as delightful as possible. A self-enclosed single compressed file installed not only a control panel but a very cool web interface. A high professional interface is shown in a browser tab, both when using the USB and Ethernet! This allows remote management and control of the unit from any browser on any device (although I only tested it on Windows). It all worked exceptionally well and reliably.

In addition to the web interface, you can also control a few of the functions using the front panel control. Because you don't have full control, these are easy to operate. But then again, you can't hardly change much using just that interface.

Let me take the reliable thing back a bit though. By default these pro devices attempt to have the smallest latency to make capturing audio in real-time in good sync with real life. That small latency caused my Audio Precision analyzer software to lose data every few seconds. I changed the latency/buffering from 128 to 1024 and that remedied that issue. You may want to do the same if you plan to use the 624 strictly for music playback. I should say that Audio Precision software is terrible in this manner so other software may not have any problem keeping up with it even with default latency.

Of all the professional interfaces I have tested so far, the MOTU 624 software is definitely ahead of the pack.

Don't laugh but I could not find a USB 3.0 cable to use with the thing. :) Had plenty of them in our old house but left them all there, not needing one until today. So I used the unit with USB 2.0 cable and speed. Fortunately two channel audio is fine with 2.0 interface.

DAC Audio Measurements
Let's measure the 624 with its DAC output form USB using Main Out:

Motu 624 Pro Audio Interface DAC Audio Measurements.png


As you see, I had to pull the output down 8 notches to get the output to be 4 volt as to make comparison with our audiophile DACs easier. We still match and beat the spec although one channel picked up more noise -- a problem that persisted later.

As is, the DAC performance is definitely in the competent bucket:
Best Pro Audio Interface Review.png


Setting the output to max gets us better performance:

Motu 624 Pro Audio Interface DAC 0 dB Audio Measurements.png


The weak channel is no longer and noise floor is pushed even lower. Distortion product land around -130 dB which is in the absolutely inaudible range. The SINAD of 114 to 117 dB is therefore definitely strictly due to noise. So let's measure that:
Motu 624 Pro Audio Interface DAC Dynamic Range Audio Measurements.png


At full level, you have dynamic range of 21 bits which is excellent. At 4 volt output it falls shorts by state-of-the-art performance of some of our consumer DACs, "limiting" us to 19 bits. That still matches our dynamic range of our hearing so no audible concern either way. Any noise you hear will be a function of the amplifier than this DAC.

Here is the filter response:

Motu 624 Pro Audio Interface DAC Filter Audio Measurements.png


We have the typical DAC chip default response which is a bit lazy, truncating at 24 kHz instead of 22.05 kHz. The attenuation though is excellent, going through the floor of my measurement here at -100 dB. Strange that we get filter choices in consumer products but not in pro.

Measuring both at 0 dB (pink) and -8 dB (green) we see the latter display the noise issue I spoke of earlier:

Motu 624 Pro Audio Interface DAC IMD Audio Measurements.png


At first I thought this was the classic "ESS IMD Hump" but it is not. I watched the spectrum (not shown) and there is a lose frequency noise that starts to overwhelm the signal some at those intermediate levels in one channel. The other is essentially quiet showing a tiny bit of IMD hump. Maybe this is an instrumentation issue. I an not sure but it is not good for a pro product. I hope MOTU sees this review and can investigate.

Jitter test is a bit less than perfect with some spurious tones:
Motu 624 Pro Audio Interface DAC Jitter Audio Measurements.png


Multitone test shows clean bill of health:
Motu 624 Pro Audio Interface DAC Multitone Audio Measurements.png


Linearity was perfect:
Motu 624 Pro Audio Interface DAC Linearity Audio Measurements.png


ADC Audio Measurements
Switching roles and feeding the 624 a very clean analog signal from my Audio Precision analyzer, and capturing digital content over USB gives us this dashboard:

Motu 624 Pro Audio Interface ADC Audio Measurements.png


Oh, this is a bit disappointing. The spec is with a-weighting I think so we may be compliant with that but SINAD of 107 dB is not that great. We see that second harmonic distortion now dominates to the tune of -110 dB. Here is how that ranks compared to other interfaces we have tested:
Best Professional Audio Interfaces Reviewed.png


That is fair bit of shortfall compared to Lynix Hilo and RME ADI-2 Pro.

Signal to noise ratio falls a bit short short too:

Motu 624 Pro Audio Interface ADC Dynamic Range Audio Measurements.png


As does incoming spurious tones with our j-test 12 kHz signal:
Motu 624 Pro Audio Interface ADC Jitter Audio Measurements.png


We have two clear spikes and sources of interference. Their levels are quite low at -130 dB but more care should have been put in there. Note that the RME ADI-2 Pro (in blue) is the non-FS version. The FS version doesn't have that broadening of the shoulder. The Lynx Hilo does best (in red).

The RME ADI-2 Pro pulls ahead with more headroom than either Lynx or MOTU:

Motu 624 Pro Audio Interface ADC IMD Audio Measurements.png


And it has lower noise too (in blue).

There is also some rise in distortion as frequencies go up:
Motu 624 Pro Audio Interface ADC THD+N vs Frequency at 96000 Audio Measurements.png


The drop in distortion above 10 kHz is artificial due to low bandwidth of the test. When I increase that by changing the sample rate to 96 kHz, we see that the distortion keeps rising with frequency. Nothing major but shame to be losing to a consumer audio interface (EVGA Nu Audio in red).

The frequency response at 192 kHz sampling falls short of 96 kHz:
Motu 624 Pro Audio Interface ADC Frequency Response Audio Measurements.png


Not a big deal as it will otherwise just capture junk. But be careful in using the MOTU 624 for audio measurements.

Conclusions
The MOTU 64 was a delight to use with respect to its rich and flexible web interface. As expected, the interface is professionally designed and tested.

That said, the ADC is a weak link here, not able to keep up with the other interfaces I had tested. The DAC also falls short of state-of-the-art DACs we have in consumer space.

Many other factors come into selection of these professional tools so I am going to refrain from giving or not giving it a recommendation.

--------
As always, questions, comments, corrections, etc. are welcome.

You all have done it... See how there is no panther in the review picture? No, I am not getting too old to forget it. They have all gone on vacation and left me behind because I have not fed them steaks every night. I have been economizing and giving them canned tuna (and keeping the steaks for myself). They revolted and said they are going on a trip and will be feeding themselves. On my credit card no less! I fear what the bill will be. Please help me be prepared by donating using : https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 

pozz

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#2
Good to see another interface review. Professionals should be supplied with this kind of information from the outset.
 

amirm

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#3
Yeh, I am surprised there is not more outreach to have pro products tested. I thought they would be first inline to get such data as it impacts their day to day work. Distortions and noise accumulate in production scenarios so such metrics are not as academic in playback.
 
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#6
Hi,

Di you set -8dB with TRIM (ESS DIGITAL VOLUME) ? IN DEVICE MENU ? ( -24 dBu variable from +20dBu)

20 dBU - 6 db = 14 dBu to have 4Vrms with TRIM menu, not in MIXER
Capture d’écran 2019-11-11 à 00.37.19.png


or the DSP MAIN OUT MIXER volume ?

Capture d’écran 2019-11-11 à 00.42.25.png


I think result could be different at 4Vrms depend on -8dBd the DSP volume (MIXER) or the TRIM that use DIGITAL VOLUME of THE DAC.

Same for ADC :

Trim at 0 ?
Capture d’écran 2019-11-11 à 00.43.52.png
 
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Rja4000

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#7
Did you try mic inputs?
Ideally at max gain.
And headphones?

By the way, this is not really a 'pro' device. Rather meant for home studio...

And both the RME and the Lynx ADC are state of the art.
I have several Pro mic pre amp interfaces with ADC in the same 103-107dB SINAD range.
The individual sources are hardly more demanding.
For the output, it's a different story, though.
 
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Rja4000

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#8
Also, I guess the frequency can't be set from the USB, but has to be set on the interface itself (or from one digital input sync) ?
That was like that with the Motu 828mkIII, but this one was firewire only.
That makes it hard to use as a day-to-day DAC in your HiFi.
 

BDWoody

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#9
I have a few moto boxes, along with the newer pcie card, and have been pondering hooking up the 896mk3 (hybrid USB 2.0 and firewire) and see how it works as a standalone interface either as a DAC, or for usb->aes digital multichannel distribution.
15734369424124298366743470308514.jpg


Any thoughts would be welcome...trying to work out a multichannel home theater solution with these until a decent AVR comes along.
 

amirm

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#10
Hi,

Di you set -8dB with TRIM (ESS DIGITAL VOLUME) ? IN DEVICE MENU ? ( -24 dBu variable from +20dBu)
I used the front panel volume control (it toggles between headphone and main out). It was very similar to lowering the digital input sample level in the analyzer.
 
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#11
No panther? It’s possible that there isn’t as much demand from the pro community because they can’t afford to go without their tools for extended periods of time. I’m sure the measurements would be welcome, but the impact of not having your primary interface outweighs the impact of potential performance issues at the near inaudible levels we’ve become accustomed to.
 

amirm

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#12
That's a good point. I hope that when they think of their next upgrade, they drop ship the new one to me for testing. :)
 

Blumlein 88

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#13
With these multi-channel devices, I've found differences in noise between inputs and outputs. Usually not too much, but sometimes a handfull of db. It is possible one of the other combinations of line in and outs would have measured more like quieter one instead of the noisier one.

In any case, thanks for doing this. Glad to see more of these measured. Considering what all you can do with it, much better deal than most consumer DACs, though obviously all those extra features are clutter for simple music playback to most people.

My Zen Tour seems just the reverse with a clearly better ADC vs its DAC. For recording you'd like the ADC to be best if you can't have both. I doubt the ADC in this one is much of an impediment to getting good recordings however.

If Motu gets something similar to this out of their newly announced M2 and M4 they'll become the goto unit instead of things like the Focusrite 2i2, 2i4 or Steinberg UR22 mkII.
 
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#14
No panther? An outrage of inconsistency. I also had to endure a U of Colorado football game on Saturday with no Ralphie (if uneducated, please look her up). Please feed them accordingly. How many are in your zoo, by the way?
 

amirm

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#15

Tks

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#16
How peculiar. I was surprised to see the amount of (lack of) standards in the audio consumer market. Which is somewhat believable due to the foundations set by ages of mythology proliferation. But in every other industry when the professional market is concerned, most companies pay quite a bit for support, and for top tier performance.

It would be odd to have something like a photo/magazine company that edits images on a consumer LCD television.. Or any web company run servers using HEDT CPUs...

I remember Amir saying for some reviews of devices, that they should be in the studio's and sound engineers' desks, and that they should work to having products of top tier caliber.

Those sentiments not only sound proper, but anything otherwise seems like a contradiction to rationality..

What's up with this industry? And to be honest, I think device is probably one of the best - I feel if we put more pro devices on Amir's bench, we'd get quite a bit of a concerning showing..

As @pozz has said.. it's very peculiar this level of verification isn't something every pro company doesn't request scientific validation of this sort..(if not more, because it would be a massive insult that a hobbyist conducts tests to a higher standard than the demands of a pro-side of the industry).
 
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#17
I'm thinking of getting an Ultralite Mk4 with the ESS DACs for my active crossover rig. In particular the half rack width form factor with 10 balanced outputs seems really appealing. Maybe I'll drop ship it to Amir..
 

Blumlein 88

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#18
How peculiar. I was surprised to see the amount of (lack of) standards in the audio consumer market. Which is somewhat believable due to the foundations set by ages of mythology proliferation. But in every other industry when the professional market is concerned, most companies pay quite a bit for support, and for top tier performance.

It would be odd to have something like a photo/magazine company that edits images on a consumer LCD television.. Or any web company run servers using HEDT CPUs...

I remember Amir saying for some reviews of devices, that they should be in the studio's and sound engineers' desks, and that they should work to having products of top tier caliber.

Those sentiments not only sound proper, but anything otherwise seems like a contradiction to rationality..

What's up with this industry? And to be honest, I think device is probably one of the best - I feel if we put more pro devices on Amir's bench, we'd get quite a bit of a concerning showing..

As @pozz has said.. it's very peculiar this level of verification isn't something every pro company doesn't request scientific validation of this sort..(if not more, because it would be a massive insult that a hobbyist conducts tests to a higher standard than the demands of a pro-side of the industry).
There are AES-17 standards for measuring devices like this. Some manufacturers provide results according to those standards and some don't. Some do, but not for all the tests. Some do on some of their product and not on others. Even if they list AES 17 it is good to have 3rd party verification.

BTW, Amir only scratched the surface of the things this device can be used to do. Here is a 100 page user's guide.
https://s3.amazonaws.com/motu-www-data/manuals/avb/624_User_Guide.pdf
Specs listed in it match Amir's results or Amir's results exceed the claims by MOTU.
 

amirm

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#19
Did you try mic inputs?
Ideally at max gain.
And headphones?
No to all. I have the thing packed to send back. Speak if you must have those measurements and if so, please be specific. :)
 
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