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:
As can be expected, there are tons of inputs and outputs both on the front and the back:
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:
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:
Setting the output to max gets us better performance:
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:
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:
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:
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:
Multitone test shows clean bill of health:
Linearity was perfect:
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:
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:
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:
As does incoming spurious tones with our j-test 12 kHz signal:
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:
And it has lower noise too (in blue).
There is also some rise in distortion as frequencies go up:
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:
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/
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:
As can be expected, there are tons of inputs and outputs both on the front and the back:
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:
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:
Setting the output to max gets us better performance:
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:
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:
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:
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:
Multitone test shows clean bill of health:
Linearity was perfect:
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:
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:
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:
As does incoming spurious tones with our j-test 12 kHz signal:
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:
And it has lower noise too (in blue).
There is also some rise in distortion as frequencies go up:
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:
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/