Long time, first time!
I have a modest headphone stack with a DAC that has an I²S input, so I ordered the U2pro because I wanted to see what it sounded like using I²S.
I had a big problem with the U2pro that made it useless for my purposes, but I eventually solved the problem so I'll definitely be keeping the U2pro! But the main purpose of this post is to describe the problem and the solution just in case anyone else runs into this, because the problem is truly a deal-breaker if not addressed.
There are also two other more minor issues with the U2pro that I will also describe.
Here was my stack before adding the U2pro:
Headless Mac mini Server (MC936LL/A, late 2011, Intel Quad Core i7 2nd Gen. 2.0 GHz, 16 GB RAM) running Audirvana Studio 1.12.0
USB out from Mac mini
S.M.S.L. DO200 takes USB input from Mac mini
Balanced analog XLR out from S.M.S.L. DO200
Schiit Jotunheim 2 takes XLR input from DO200
Balanced 4-pin XLR headphone out from Jotunheim into
HiFiMan Sundara (2020 edition). I also have an old pair of
Etymotic ER-4S IEM's, but I don't use those very much these days for some reason.
Then I added the U2pro, which obviously gets USB from the Mac mini and outputs I²S over HDMI to the DO200. I used
the shortest HDMI cable I could find that reaches between the two components without putting any stress on the cable or torque on the HDMI connectors.
I wanted to power the U2pro with 5v via the included USB to barrel cable, but the cord is quite short and could not reach the wall outlet area. I wasn't especially keen on using an AC extension cord to get a USB wall wart closer to the stack, so I decided to just power the U2pro off a USB port on a
Jackery Explorer 160 power bank that I had lying around. Perhaps I'll get a longer cable later, but I'm in no rush because it looks like the Jackery can run for a couple of weeks on a single charge while powering the U2pro 24/7.
When I connected the U2pro to the Mac mini, it was immediately and automatically recognized. No other drivers or setup was required as far as the Mac mini was concerned.
Audirvana also automatically recognized the U2pro and I was able to play music straight away without changing any settings other than selecting the U2pro.
At first, everything was working great. But every so often I would hear a
VERY loud
POP at the very beginning of some songs. And when I say loud I mean
VERY VERY LOUD, and it sounded extremely nasty. Definitely not something that anyone could ever learn to ignore or live with because that pop nearly takes your head off. It is literally the worst sound I have ever heard in my life, except possibly accidentally landing on
The View when flipping channels.
I noticed that when playing a song at a particular bit depth/sample rate, the next song would not have the loud pop as long as the bit depth/sample rates were identical.
So if I was playing 16/44.1 and the next song was also 16/44.1, there was no pop. Same thing for other sample rates - if one track was 24/196 and the next track was also 24/196 there was no pop.
However, if the next song was different in any way (bit depth, sample rate, or both), I would get that really loud nasty pop at the very beginning of the song, every time.
I then started going through the audio settings in Audirvana, changing every setting one at a time to try to find a way to stop that nasty pop. I also changed each I²S setting on the DAC using the DO200 OSD menu, but none of that worked either.
Eventually I found that if I changed the setting for Maximum sample rate (in the "For bridge devices connected to a DAC" section) down to 192 kHz, the nasty pop was gone!
Douk Audio says that the U2pro can support up to 384 kHz for PCM audio. And while it will indeed play at that sample rate, the nasty pop is there when the next song to play is at a different bit depth/sample rate. Limiting the U2pro to 192 kHz removed the nasty pop completely. I don't understand why, but that's the way it works, at least in my system.
There are two other issues with the U2pro, and I'll get to them in a moment.
Right now I'm going to post all of my Audirvana settings below because that way if anyone else is having this same issue, they may have success by trying some of my settings. Or maybe this is just an issue between the U2pro and the DO200, who knows!
You can
click here to view a text file generated by Audirvana that displays all of my settings and system information.
And below are screenshots of all of my Audirvana settings:
A quick note about the Audio Units plugins I am using, in case anyone was interested.
I use the Apple AUNBandEQ plugin to EQ the Sundaras using a 9 band parametric EQ. I am using
oratory1990's Harman target EQ settings with some tweaks to the low bass.
I use the
Goodhertz CanOpener Studio plugin to add crossfeed. While this plugin can do quite a bit more, I use it to add crossfeed - and that's it, all other settings are disabled.
Another strange thing happens when using the U2pro: the display on the DO200 shows "no signal" when using the U2pro to feed audio to the DAC via I²S.
Normally, the DO200 will properly display the sample rate on the display. But when using the U2pro, it always says "no signal", despite the fact that there is definitely signal going to the DO200 because music plays through the headphones.
While this is strange, it's not a deal breaker for me. That said, it would be interesting to learn why the DAC says "no signal" when it so obviously is indeed receiving a digital signal from the U2pro and passing an analog signal to the Jotunheim.
And finally, there is an issue with Audirvana not allowing the volume control to be locked out when using the U2pro. When I only use my DO200, the Audirvana software volume control is locked out when I tell it to disable software volume control. However, with the U2pro, that is impossible because the Audirvana software volume control is always active no matter what you select as the setting for software volume control being on/off.
This is also not a deal-breaker because I just leave the software volume control at maximum all the time. But it really should just do what it is told and disable the software volume control when using the U2pro.
Perhaps these three issues could be fixed with a firmware upgrade, but I'm not sure if that would be possible for me to do because it appears that Douk Audio does not support upgrading the firmware on the U2pro in any way on an Apple computer (other than running some PC emulation software).
I would like to be wrong about that, but it really wouldn't surprise me if it was impossible to upgrade the firmware on the U2pro with a Mac because many Chinese hardware manufacturers are just fine with ignoring the needs of people who don't use PC or Linux.
My system sounds great with the U2pro feeding I²S to the DO200! I have not gone back and forth between the U2pro in the chain vs. no U2pro to compare sound quality. I might do that later but I'm quite happy with the way my system sounds now, so perhaps I'll just leave it at that for now.
Other than that, I have not tried the Coaxial or Optical outputs on the U2pro because I am not interested in using either. That said, the fact that the U2pro can quickly give most USB devices a Coaxial or Optical output is very useful from a utility perspective, and of course the I²S is a fun feature as well. Getting all three on a $66 box is quite a bargain imo so I'll be keeping the U2pro for good, regardless of whether or not it stays in my stack for the long term.
I have a few DSD samplers, so I played a few DSD tracks with the U2pro just to check if it works. The DSD files played without incident. That said, other than just seeing if the files would play, I have no other interest in DSD and I won't be using any DSD features in the future. The DSD format is pointless and ill-fitting for 2 channel consumer use because it takes two steps back for every one step forwards. I used to have a bunch of SACD's that I used in a multichannel system - and that was totally awesome! But for 2 channel, DSD is not a serious format. Has to be said.
Another nice thing about the U2pro is that I think it's going to save me a few bucks down the line, or at least make a new DDC purchase much easier down the road.
Before I heard about the U2pro, I was shopping around for a device that would let me send audio from the Mac mini to the DO200 using I²S or AES/EBU (either via USB or some networked solution). And as you probably already know, most of those devices that support I²S and/or AES/EBU are at a minimum 5x more expensive than the U2pro
if you go with a Raspberry Pi solution, or 20x+ more expensive if you go with
a more finished product from an established HiFi manufacturer.
I still plan on trying out some other I²S (and AES/EBU) compatible DDC's, but now I have a budget baseline to compare those products to. Big advantage!
So if (when) I order a new and more expensive DDC and it doesn't provide a significant and obvious improvement in sound quality, I'll send it back for a refund. If it beats the pants off the U2pro, that would justify the purchase and in that case I would keep the new DDC, whatever it may be.
Given that the U2pro can only be used at a maximum of 24/192 (in my system), it will be good to compare it to a more robust product that can take up to 32/768 because that is the maximum that my DAC will do. If a proper DDC can't soundly beat this $66 wonder with the deck stacked in its favor, back it goes!
Given the low price and Amazon's liberal return policy, there isn't much reason to not pick up a U2pro if you are looking to experiment with I²S.
Have fun!
Thanks 
