Motu M4 - Tear down, bit of internals analysis and few in-house measurements

[trl]

I can not answer that just based on the lifespan rated, sorry. I am confident that both audio devices should last enough to keep us busy and happy.

 

[Interference]

I just got my M4 after selling the Clarett, hope I can have some more measurement fun (I still have some data to post about the Clarett).

Did someone understand whether the volume is controlled through an integrated stepped attenuator or a variable gain stage?

 

[trl]

Did someone understand whether the volume is controlled through an integrated stepped attenuator or a variable gain stage?

Isn't the first post detailed enough? Can you please be more explicit or even to quote some paragraph from the first post?

 

[Interference]

Isn't the first post detailed enough? Can you please be more explicit or even to quote some paragraph from the first post?

What I read is:

Six OPA1678 (https://www.ti.com/lit/ds/symlink/opa1678.pdf), marked as TI 02 1AW7, are used as output buffers for the four TRS and RCA outputs from the backside of the case. These are connected between the DAC-outputs and the interface balanced output plugs, so the TRS and RCA outputs are direct-coupled, without any capacitors in between.

But from this I do not undertand how the volume knob works. Maybe it controls the native ESS9016 digital volume?

 

[Interference]

From loopback measurements I can confirm the THD is excellent but the noise floor grants no more than ~95 dB SINAD with a 2 Vrms unbalanced signal on the line input. Apparently this is the price to pay for having such a large headroom.

The MIC input does not like to be driven so near to FS and provides significant distortion at such level (-3 dBFS).

A very neat piece of hardware otherwise.

 

[trl]

But from this I do not undertand how the volume knob works. Maybe it controls the native ESS9016 digital volume?

It does makes lot of sense for the ES9016 to handle the output volume, see https://ismosys.com/wp-content/uploads/2020/08/ES9016S-PB-v1.91.pdf which tells us that ES9016 does have built-in volume control inside; now it all depends if the manufacturer enabled or disabled this function, but given that I see no other chips able to perform volume control function, I really think that ES9016 is handling it.

P.S.: There's also https://panther.kapsi.fi/posts/2020-02-02_motu_m4 wrote by @Minigun, here his post about it here: https://www.audiosciencereview.com/...u-m4-loopback-measurements.10890/#post-321501. He did some very nice volume measurements there.

 

[Interference]

It does makes lot of sense for the ES9016 to handle the output volume, see https://ismosys.com/wp-content/uploads/2020/08/ES9016S-PB-v1.91.pdf which tells us that ES9016 does have built-in volume control inside; now it all depends if the manufacturer enabled or disabled this function, but given that I see no other chips able to perform volume control function, I really think that ES9016 is handling it.

P.S.: There's also https://panther.kapsi.fi/posts/2020-02-02_motu_m4 wrote by @Minigun, here his post about it here: https://www.audiosciencereview.com/...u-m4-loopback-measurements.10890/#post-321501. He did some very nice volume measurements there.

I think for interfaces and DACs in this price range the digital control through the DAC is the best way to achieve volume control. For more ambitious designs, it is possible that a variable gain line stage can reach higher specs.

 

[Interference]

From loopback measurements I can confirm the THD is excellent but the noise floor grants no more than ~95 dB SINAD with a 2 Vrms unbalanced signal on the line input. Apparently this is the price to pay for having such a large headroom.

The MIC input does not like to be driven so near to FS and provides significant distortion at such level (-3 dBFS).

A very neat piece of hardware otherwise.

I need to correct myself here. I come from a fresh installation of REW and noticed that 16 bit dithering was enabled on the generator by default. It's not the first time I make this mistake but I keep forgetting.

THD+N at 2 Vrms through unbalaced loopback stands at -101 dB, which is very very good. It drops below -106 dB in balanced, outperforming the Clarett 2Pre (at list in my findings, comparisons are always tricky and mistakes possible).

Very happy with my purchase and I finally found a nice swiss-knife to keep on my desktop and move around for the occasional measurements!

 

[SylphAudio]

Anyone who knows or measured the input impedance of line-in 3/4? No mention on it in user manual

 

[lizhuoyin]

Anyone who knows or measured the input impedance of line-in 3/4? No mention on it in user manual

Is it impedance load in manual?

 

[SylphAudio]

I'm looking for input impedance for the low Z input section (the opamp buffer for ADC part)

 

[lizhuoyin]

I'm looking for input impedance for the low Z input section (the opamp buffer for ADC part)

Input impedance = load impedance = TRS (input 3/4) impedance load in Motu M2/M4 manual
Check out bhphotos:https://www.bhphotovideo.com/c/product/1514483-REG/motu_3140_m4_4x4_usb_c_audio.html/specs

 

[SylphAudio]

I see so it's 2.65K ohms. I have a soundcard interface that can be connected to speaker outs of amplifiers for measuring THD+N, however my current implementation only has an SE output, a balanced output will make it a better match for M4. For size and cost considerations, I'm planning to reduce the value of the coupling cap from my interface to the M4's input. So I'm looking for M4's input impedance.

Thanks!

 

[lizhuoyin]

I see so it's 2.65K ohms. I have a soundcard interface that can be connected to speaker outs of amplifiers for measuring THD+N, however my current implementation only has an SE output, a balanced output will make it a better match for M4. For size and cost considerations, I'm planning to reduce the value of the coupling cap from my interface to the M4's input. So I'm looking for M4's input impedance.

Thanks!

So you want to measure your power amp's speaker outputs THD+N thru another soundcard interface which will be connected to M4? Just make sure your soundcard has better THD+N than the amp. Also you may consider connect your amp's speaker outputs to M4 thru some attenuators.
See my post: https://www.audiosciencereview.com/...amp-thd-n-etc-with-motu-m4.22910/#post-763700

 

[SylphAudio]

Yes that's why I bought a Motu M4 because I saw some loopback measurements where it can get a 103dB SINAD, my QA400 has a very limited THD+N.
Here's the sample of my TPA3255 module + the Interface + QA400, I hope I can get better measurements with my M4.

 

[lizhuoyin]

Yes that's why I bought a Motu M4 because I saw some loopback measurements where it can get a 103dB SINAD, my QA400 has a very limited THD+N.
Here's the sample of my TPA3255 module + the Interface + QA400, I hope I can get better measurements with my M4.

Unless you connect power amp directly to M4 as suggested in posts I referenced, you will be limited to THD+N 98 dB if going thru your QA400. (I cannot find it's specs but QA402's THD+N is around 98dB)

 

[Evgeniy]

thanks for good fotos and measurements.

 

[SylphAudio]

Got my TRS connectors, here's the loopback of MOTU M4 monitor out (single ended) to Line In 4

 

[lizhuoyin]

Got my TRS connectors, here's the loopback of MOTU M4 monitor out (single ended) to Line In 4

Is TRS balanced? Also what's your output signal? -10dBFS?

 

[SylphAudio]

No, the screenshot is for single ended loopback only.

Monitor out #2 RCA single ended to TRS line-in #4 with -IN tied to ground. Full SE output signal which is around 2.5Vrms