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May 5, 2018
(Note: This review was dragged over here into Reviews from my first posting of it in the general discussions about this amp before they created a Reviews section. The review began during the run-in phase after acquisition.) (Note number 2: It's long, especially after adding more headphones.)
First, about those stars; I gave 5 because, at this price point, this amp does what it's supposed to do, and it does it better than the cost for the amp would suggest. It outperforms expectations for the price-point of the device.
THIS AMP IS A LITTLE POWERHOUSE!!! Listening with my Beyerdynamics DT880 600ohm cans.... Even on LOW gain setting, it runs them well, with no distortion or dropout, on medium volume setting. Thus far, BEAUTIFUL and detailed sound. (REMEMBER, adequacy of power from an amp isn't just about how loud, but also about how clean and correct!) It's well built and has a clean interface, solid but not overly heavy. Cables don't just drag it off the table. This is getting ready to kick the Schiit Fulla2 that I own to my living room......
First impressions; as already mentioned, I love the build quality of this powerful little amplifier. It's assembled VERY well, and the layout of the buttons and knobs etc. is very comfortable and easy to manage, even in the dark when reaching above one's head. It's GOOD that the gain button is adjacent to the volume knob, for instance. I PREFER MATTE FINISHES. I rather hate fingerprint magnets, the urge to compulsively polish shiny surfaces of smudges kind of messes with some of the joy, if you catch my drift. And then I hesitate to touch the thing again....okay enough about it, but, YOU KNOW. First cans, and I chose to review it vs. the Schiit Fulla, as the Fulla, I think, is representative of the amp class and in a similar cost bracket, with similar user purposing. Kind of Music chosen; (Yes, you've seen this list before, because it's THE SAME LIST I ALWAYS USE),Lots of classical with much strings, voice, Beethoven through Bernstein and Glass. Lots of electronic music a la Wollo, Boddy, Roach, Serrie, etc. Much Leo Kottke guitar happiness, with some Bluegrass and Appalachian schtuff tossed in. Classical piano. Silk Road Ensemble. Popular and Rock, from Joplin to Foo Fighters to Neil Young to classic Santana, to The Killers to Maroon 5 and so on. Jazz and Blues , Muddy Waters, early Severin, Puente. I don't really do Rap, Dub, or Hip-Hop, although I did run a little Black Eyed Peas and Outkast through it. My musics were chosen to represent the spectrum and check out what this Amp offers in the way of clarity, speed, presentation of details and ability to translate input from my iPad and iPhone. (NOT from a computer, as there is no DAC.) The Cans; I chose the Fostex X Massdrop THX00 mahogany and the Beyerdynamic DT880 600 ohms. I could actually stop with these, (but I don't want to), because this rather represents the range of drive power required, and two distinctly DIFFERENT sound signatures. Nearly OPPOSITE, although I did choose these Fostex because they tend to be a tad more neutral and the tweek a'la Massdrop was to tame the bass a bit. RIGHT AWAY- the Fostex were very pleasantly served by the O2 on the first day, the easy presentation and tonality we love in our Fostex was handily managed by the O2. Ran them on low gain setting, as high gain is a) definitely unnecessary, and b) was kind of blaring and harsh when the cans were overpowered. (EXPECTED) Day 2, mounted the 880's on it,(couldn't wait any longer), and was IMMEDIATELY impressed by how easily these 600 ohm cans were managed by the O2, EVEN ON LOW GAIN. Detail, which is what I love with BeyerD's, was absolutely present through the frequency ranges, vocals were CLEAR, strings, percussion, all lucidly presented without anything being noteable as harsh or strident. Lovely transparency! And on the 880's detail only became MORE with high gain setting, again, not presented harshly or inappropriately balanced in any way. I can honestly state, especially given my PERSONAL preferences in music listening, that this is producing a very transparent and authoritative sound, and that's before my 30-hour run-in had been managed. I'm not going to say anything silly about stage or headspace, etc. as I believe that's what the headphones long as the amplifier meets the power need of the cans. The Fostex were not overwhelmed, the 600 ohm 880's were not under-served, not even remotely. As I have already said, you can even power the 880's on low gain and lot lose really much. High gain only enhances clarity. Oh, and before I forget- let me mention SPEED, just a bit here. This is a FAST little amp. Something that i feel shines out with the sequencer music heard in pieces by Klaus Schultz, Ian Boddy, and the like. NEVER ONCE, did I feel like I was hearing any drops of anything from musics that I am quite familiar with. Not at any range, not with any genre. Can hear finger squeaks on strings, for instance, intakes of breath. Nothing incorrectly attenuated, EVER, even on harder to drive headphones...always adequate headroom and detail. And we have to admit that 600 ohms is DEFINITELY in the harder to drive end of things. VS The Fulla; The Fulla, being quite nice it it's own right, is simply outshone by the O2. Not quite as fast, definitely not as powerful. The Fostex do not present quite as cleanly, the 880's do not sound as crisp. Volume adjustment when playing the 880's needs to be MUCH higher, nearly at 3/4 for everything to be audible that is audible with the O2 on low gain setting at 1/2. The O2 features a gain switcher, the Fulla does not. Fulla is input to a 3.5 jack, I went with the O2 desktop model which employs RCAs....which I believe always improves the input sound quality because it's just MORE electron flow available, less impedance at the input, less risk of crosstalk. So there's THAT....... In any case, I can say that I already prefer this amp to the Fulla, although the Fulla is definitely more portable with a slightly smaller footprint and a bit less finicky inputting. I wouldn't strap either of them to my iPhone. Coming in Part 2; A few more headphones. ..probably not necessary, but maybe helpful for those newer-to-the-audiophile world who have different cans and might want more information. Something to help with decisions or just understanding what we are talking about with specific equipments.
Hoping this helps ANYONE, or is, at least, a decent read.... cheers! PART 2 ADDENDUM, with a couple more headphones now included; So, I have been messing about with a couple of other sets of headphones on this happy little amplifier, and, as mentioned above, I will now add them to the original.
Let's start with a Massdrop offering, the Massdrop X Hifiman 4XX magnaplanar. As a magnaplanar headphone, while the impedance is fairly low, they can be rather demanding as far as power needed to drive them optimally. The 4XX cans have a rather nice V-shaped sound signature, so a tube amplifier isn't necessary to produce a warm sound from these,(but it doesn't always hurt, either), and a solid state amp can be just the thing. After listening to the 4XX on this amp, I was very impressed with how well they were powered by this little amp, even on the lower gain setting... the higher gain setting did not muddy the sound, just gave a more forward presentation of the lower end.The highs and mids are nicely revealed and clean sounding here. All-in-all, a happy presentation of the musics listened to. As a frame of reference, the HE560's also sound pretty good here, but my feeling was that they want just a tad more powerful an amp to really serve them correctly. They may well be the upper boundary of HiFiman cans with this amp. The Element by JDS Labs sounds better, IMO.
I don't own the Sennheiser 6XX Massdrop offering, but I DO own the Sennheiser HD600, which is one of my favourite headphones for listening to classical, jazz, and acoustic musics. They are a 300 ohm impedance set of cans, so will challenge any desktop amp to serve them correctly. (se above note in bold about how I define that.) I am pleased to say that my Senns sound very nice on this amp. The HD600 is known for its rather neutral, clean, nearly critical sound sig, yet retains intimacy and warmth because the bass is still present and cleanly represented. An appropriate amp will allow these properties to present themselves, even to shine...and the Objective 2 amplifier does its job well here. I won't say that one is at risk for overpowering these cans with it, yet, if it can power those Beyerdynamics 600ohm 'phones, it's not likely to fail the Senns....and it doesn't. I felt the intimacy and natural spaciousness of the Senns when kicked back for a listen- a special favourite here was the Stan Getz-Joao Gilberto collaboration*, which has plenty of sultry-soft vocals and breezy jazz instrumentals covering the full range. It's a must listen for audiophiles, and this amp justified the Senns nicely. I think the Massdrop-Sennheiser 6XX will probably do as well with the O2 amp.
Grado, for those of you who own them; You'll be FINE. I own multiple Grados, and they ALL sound good on this amp, very, well... Grado-y. Nothing robbed or inappropriately nuanced by this amp, yet I tend to run my Grados from my Schiit gear, as I kind of prefer the ss of that combo. I think it's purely subjective on my part.
Okay, that's it for my addendum PART 2 of the review. If anybody has a specific headphone they want remarks upon, and I have experience of it, reply and I'll do my best. :)

* will be added to my test list of listens for any gear I am reviewing.
May 5, 2018
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