TL;DR: The digital transport makes a difference in the sound of your system, and Unison USB is not the “great equalizer” making even your shitty PC sound as good as a multi-kilobuck purpose-built digital audio transport devices (it may get close enough for many people however). I’m not advocating spending multi-kilobucks on a transport, but don’t expect your 3 year old Chromebook to do as well as a device that’s specifically designed to feed audio to your DAC (Unison USB or otherwise). Intro: Not all digital transports are created equal. Bits aren’t bits. We’ve seen this through the evolution of CD players as well as USB as an audio interface. The quality of the transport matters, even with modern computer-based digital audio solutions that claim to have vanishingly low jitter. Many are eagerly anticipating the launch of Unison USB for Schiit’s upgradable DACs, wondering “does it really beat SPDIF?”, “is it better than AES”? I’ve had a Unison USB Beta board in my Yggdrasil A2 for over a year now and have been extremely impressed with it from the start. It has handily beaten any other USB implementation I’ve heard (granted I’ve never heard the TOTL stuff like the Berkeley Alpha USB or the dCS Network Bridge). @Hands has said recently however that he prefers SPDIF (his DigiOne Sig) over Unison on the new Bifrost 2. How can this be? Didn’t Mike say Unison was better than SPDIF or AES? Does @Hands have better ears than Mike? He must be smoking something right? Or maybe there’s just something more at play here than just SPDIF vs USB. I have owned a number of USB and SPDIF transports including the Cambridge Audio Stream Magic, microRendu, UltraRendu, Sotm SMS-200, SMS-200Ultra, and recently the SMS-200Ultra Neo and the Pi 2 Designs Pi2AES (I’ve also got an Arcam CD73 player I use as a CD transport). And of course I’ve also used my laptop (Dell XPS 13) and hooked it direct through USB to my DACs at various times. And what I’ve come to find so far is this: It’s not just about how good the input is on the DAC, it’s also what’s feeding it! Unison and Adapticlock and all this other DAC input technology is intended to raise the level of (and potentially equalize the level of) the digital transport. But it only goes so far - the transport can still have an appreciable effect on the sound. We can see this even with the very mature SPDIF technology and the DigiOne Signature - many agree the DO Sig was an appreciable step up above other low cost SPDIF transports even through Yggdrasil’s Adapticlock input. Now for the “bits are bits” crowd, of course they will find this mostly bullshit - once everything is “up to spec” you will have perfection in a digital audio world. If you want to live in that world, fine. Try another thread. And many people have tried different transports and say they can’t hear the differences between them. Ok, but there’s a few things to remember: The transport has an even lesser effect on the overall sound than the DAC. DACs can actually do a lot to make up for a shitty transport, back to the “as long as it’s in spec” part. And I think this is what Unison shows us - it can do a pretty damn good job even with a noisy/jittery source like a Windows laptop USB. Also because of #1, you have to have a system that is resolving enough to hear the differences clearly. I’m not saying you need TOTL stuff, but gear that masks detail and doesn’t do pretty well in most areas won’t likely display much of a difference between transports. And maybe this is the most important point - the differences between digital transports is not in the tonality, but in the technicalities (mainly). People who can’t hear the differences might be looking for frequency response differences, because this is what you mainly listen for with all the other gear in your chain, and where you tend to hear the most immediate diffs - is it brighter/darker, is it warmer/colder, does it have more bass or midbass, etc? You can sometimes find a bit of this between transports, but I find the majority of the diffs between transports include: cleanliness/sibilance control/digititus noise floor/blackground instrument/vocals separation soundstage width/height depth and layering transients/macrodynamics microdetail, plankton, atmosphere Ok so enough with the intro, let’s get to the meat. The shootout: Intel NUC (USB) vs. Pi2AES (AES) vs. Sotm SMS-200ultra Neo (USB) on a Yggdrasil A2 using Unison USB beta board input as well as the AES input for the Pi2AES. Rest of the chain is: Schiit Freya, 2 Schiit Vidar Monoblocks, and Graham Audio LS6 speakers. Also note that I made a power supply change on the Pi2AES part way through to see if I could discern a difference. I swapped the transports back and forth on each track at least a few times. Most tracks were rock/alternative, jazz and folk, with stuff that is at least decently recorded. Roon/RoonBridge was used with all 3 transports to keep everything as equal as possible. The Pi2AES used the Ropieee OS. Nuc had the Roon ROCK OS. Pictures or it didn’t happen: Note: This is mostly stream of consciousness thoughts and immediate impressions - I don’t have the time to pretty things up. Also note that the degree of differences I’m mentioning are nothing compared to the differences between transducers or even DACs from different manufacturers. Pi2AES (AES) with stock power supply vs. Intel Nuc (Unison USB) Nuc USB out (Unison): Good/Pleasant but a bit boring sounding - lacking dynamics Fairly flat dimensionally, lacking depth Stage mostly caught between the speakers Decent seperation, but a touch of greyness and a bit blobby - instruments can blend together especially in busy passages A bit mushy in bass overall, not the best with slam and impact cymbals and high hats a bit tizzy sounding, not clean. Pretty smooth and not much in the way of digititis or fatigue, which is an improvement almost all other USB implementations I’ve heard. But a touch soft and lacking scale, separation and dimensionality Pi2AES (AES) with stock power supply: Better blackground Immediately more 3D soundstage- better depth and layering, a bit wider stage as well - less sound trapped between the speakers More dynamic, less soft, less blurred Tighter bass with more slam Better transients and macrodynamics overall with all instruments Cymbals cleaner, more shimmery, better decay More extended sounding esp in the highs. The NUC almost seems dark in comparison. Better instrument seperation and vocal intelligibility More open and transparent sounding overall - much better atmosphere and more plankton than the NUC Maybe slightly more sibilance, but this could be due to better detail and transients After this initial comparison, I decided to try putting a better power supply on the Pi2AES, so I tried my Sbooster 10.5V linear power supply (which I usually use with the Sotm) and compared it to the stock switching power supply. Pi2AES (AES) with Sbooster power supply vs. stock supply: Smoother. A bit blacker background. Cleaner sounding. Even more depth (if the recoding has it) A bit less edge, less fatiguing, better sibilance control A bit less dynamic however and slower transients than the stock switching supply Cymbals a bit more clear with better decay Slightly wider, taller and more open soundstage Slightly better seperation of instruments Maybe a touch warmer/darker sounding? Overall I prefer the sound with the Sbooster, but it may lose a slight amount of dynamics and skew a touch warmer sounding. The differences are pretty slight though and the stock supply does a very good job. Sotm SMS-200ultra Neo (USB) vs Pi2AES (AES) - both using the Sbooster power supply: WOW depth forever and huge soundstage. No boundaries in my room anymore. A bit tighter sounding, a bit better transients Yet even more separation of instruments and better layering between them. Even more open and more transparent sounding More atmosphere, more plankton. I’m at the recording session! Absolutely as clean as a whistle - no edges or etchiness of any kind. ZERO digititus. Less warm, more neutral sounding than the Pi2AES with the Sbooster, and more dynamic. More like the Pi2AES with the stock switching supply, but way cleaner and more open sounding. After having been listening to the Pi2AES for a number of weeks exclusively, I was actually shocked going back to the SOtM with Sbooster - it sounds so non-digital and has so much detail and scale. Just beautiful. It should be noted however that without a direct comparison like this, I’ve actually really been satisfied with the Pi2AES. As a last comparison, I decided to go back to compare the Sotm/Sbooster with the Intel Nuc. Both of course are using Unison USB and the same USB cable. Nuc vs Sotm Intel Nuc: Bass bloated and soft. Softer sounding overall sounding. Greyer, noisy, edgier/dirtier sounding but not terrible. Increase in sibilance but not fatiguing. Way flatter dimensionally - huge loss of depth Loss of atmosphere and microdynamics Narrower stage and not as tall More congealed, less open, muddled in busy passages Sotm is far more relaxed sounding, but more dynamic and detailed at the same time. Bass more balanced. Everything cleaner and smoother and more separated. Larger scale. Less grey. Vocals much clearer and more articulate and real sounding - more “organic” and less reproduced. So that’s it for now. My conclusions at this point are: Unison makes shitty USB transports (i.e. a standard computer USB output) sound pretty decent - acceptable to a lot of people I think and a worthwhile upgrade over Gen 5 USB definitely. Pi2AES IMO is going to be better than a standard computer USB out going into Unison, and you can improve things even further with a better power supply (but the stock supply is still pretty good). You gain in dimensionality and soundstage and atmosphere/plankton, and it sounds more dynamic and less soft/rounded. Considering the price of it with the stock supply, it is absolutely an awesome value. I consider it better than the microRendu and Cambridge Stream Magic that each cost much more. The Sotm with Sbooster can take Unison USB clearly above the level of the Pi2AES. It’s better in almost every respect IMO, aside perhaps from macrodynamics (where they are pretty close). It’s also many times the price of the Pi2AES, and if other parts of your chain need improvement, you’re probably better off spending your money there, since you’ll likely gain much more obvious differences. I’ll probably continue to bring in more transports to try them out and report back (and mess a bit with power supllies as well, although I feel that the Sbooster probably gets me mostly as far as I need to go - I’ve owned a few others as well) Hope this was a useful exercise.