Equipment Used

Sources:

Headphones:

Packaging

O2 Amplifier

Two 9v Batteries

Power Supply

Instructions

Build Quality

Sound Quality

Frequency chart. Not my image, taken from here.

Channel Balance measurements. Not my image, taken from here.

Conclusion

I want to thank JDS Labs for the fantastic Objective2 (O2) Amp that they've hand-built and kept to NwAvGuy specifications. John is a very nice guy who sent the O2 much faster than he had quoted and also quickly responded to my emails. I highly recommend JDS Labs if you're considering an O2, but aren't too handy. With that said my review will be based off of the JDS Labs version, your results may vary if you buy from someone else or build it yourself.Macbook Pro->USB->iBasso D4->Line-Out->O2, iPod Classic->Line-Out->O2Superlux HD668b, Audio Technica Ad700, Audio Technica Ad900, Audio Technica AD2000, Sennheiser HD558, Brainwavz HM5The packaging of the O2 is no frills. My O2 came in a cardboard box that was packed to the brim with bubble wrap ensuring that the O2 and the power supply inside were safe from damage and the amp is enclosed in a static-proof bag for added protection. Included is an instruction guide and information on the O2. There's nothing more to say about the packaging, John takes the no-frills approach to his products but he ensures their safety with some of the best packing I've seen.Much like the packaging John shows pride in his work through the careful craftsmanship of the O2 from the inside out. The internals are housed in a two tone aluminum shell held together by four screws on each side. The faceplate is grey with brand and item information on the top while the rest of the body is black. One decision I'm not too fond of aesthetically is the choice to have the power input on the front, I understand that it's how the board is designed though and John built this to specifications. Still it throws off some flow aesthetically.Every thing on the O2 is on the front, from left to right you have: Power Input, power switch, 3.5mm headphone jack, red LED power indicator, volume potentiometer, gain switch, and 3.5mm line-in jack. Everything feels absolutely solid, the power and gain switch use the same silky smooth switch that clicks to let you know it's been pressed. The volume potentiometer has a nice amount of resistance when turning giving a nice solid feel to it, the double rubber rings are a nice touch aesthetically as well. The 3.5mm jacks are the only thing I have a few gripes about, firstly the light blue throws off the flow of the aesthetics again I wonder if NwAvGuy couldn't have found something black with just as good quality. I also don't like how snug the jacks are, it takes a little more force to plug-in and remove jacks.One thing that many people might notice missing is the lack of an RCA input option as well as no 1/4'' headphone jack. The reasoning, from my understanding, is due to NwAvGuy wanting this to be a portable option. In NwAvGuy's articles he does mention a case option that would allow for more space to add your own RCA input and 1/4'' jack making this a more viable desktop option. This gives you the option to add these yourself if you do build one, so it's not out of the question. As for portability usage, the O2 is rather big, in-comparison to my iPod the O2 is roughly the same height, slightly wider, and almost 3 times as beefier, depending on how you decide to compare anyway.Comparing build quality to other amps I've used the O2 feels better built than my iBasso D4, FiiO E7, and iBasso D-Zero. On the subject of build quality alone the O2 feels just as sturdy as the SPL Phonitor, without the beautiful aesthetics. When you buy an O2 from JDS Labs you're getting a high quality product at an affordable price.NwAvGuy claims that the O2 requires no burn-in time because there are no capacitors in the signal path, while I whole heartedly believe him I've given the O2 at least 100 hours of listening time before basing any opinions off of it.To make things simple, the O2 brings new life to my music. That sounds like a cheesy cop-out, I know, but it's the truth. The biggest contributor to this is the vastly increased soundstage coming from the iBasso D4 to the O2. Not only is the soundstage wider with more depth, but it's not cluttered and confusing as the D4 tends to get with more complex passages. The added width and depth give instruments better separation and positioning in the soundstage allowing them to be picked apart easier. I've noticed that sometimes with an increased soundstage that instruments tend to sound thin, or distant. This is not the case with the O2, while the instruments have more space their presence is not lost, in-fact everything I've thrown at is seems to have a more full sound.Clarity is the second biggest change I am hearing as I listen. To put it simple, it's as if a very thin blanket has been removed in comparison to the D4. The low distortion is the reason for this, looking at the measurements the highest THD at any point is 0.0023% which is incredibly low and certainly out of the audible range. What's even more impressive is that in comparison to the O2 has less noise at the same volume as the $1,600 Benchmark DAC1 PRE. What all this means though is that the signal is arriving cleanly meaning I only hear the music, not distortion.When I was looking to upgrade my amp situation the biggest thing for me was a neutral amp that provided a strong clean boost. Looking at the frequency response chart for the O2 it's clear to see that the O2 is exactly this. At worst the O2 has a -0.6 dB dip at 5 hz, which is absolutely phenomenal for someone wanting a neutral amp. Some people may want a warm amp or a brighter amp and that's fine, but I prefer to let my headphones be the only thing coloring my sound. With that it mind it's easy to see why I like the O2.Another thing that NwAvGuy touts is that his O2 is compatible with headphones from 18ohms to 600ohms. Unfortunately my most demanding headphone is the Sennheiser HD558 so I didn't get a chance to test just exactly how powerful the amp section is, but one thing that surprised me was how well the O2 worked with my IEMs. A lot of stronger amps seem to have problems powering IEMs properly usually leading to hiss. I can say that everything from my q-JAYS to my JVC Marshmallow sounded fantastic through the O2 with no hiss whatsoever. Using my full-sized headphone line-up I never had to turn the dial past 10pm. The problem with that in most amps is that channel imbalance tends to pop up at lower listening volumes, thankfully measuring less than 1 dB difference at -55 dB which is impressive.NwAvGuy has set the standard for what an affordable, and potentially portable, amp should be. I don't think it's possible to find a better amp under $500 and if there is one I'd love to hear it. The O2 provides everything you could ask for sound wise, though it leaves some to be desired for those wanting more options on outputs or inputs. Even with that flaw, I can't think of any reason to consider any other amp at this price point. All in all the O2 has passed not only the technical measurements with flying colors, but my listening tests. I highly recommend the O2.To see more pictures of the O2 come check my full album