To Start… There are a lot of amps out there that use exotic topologies and expensive parts. Even “budget” amplifiers try to get in on this game. Too often, that practice fails and you end up with a “transparent” amp that is anything but. Or an amp is made that trades reliability for a perceived gain in sound quality. Is there anything out there that can faithfully reproduce an audio signal without jeopardizing your headphones? This is a review of a DIY-product, the Objective2 amplifier (O2 for short), that attempts (and succeeds IMO) to realize those goals. So, if you are looking for an amp that can be perfectly transparent, can power a number of hard-to-drive headphones, and that costs less than $200, read on. If you are looking for the next big tube amp, or an amp to help offset whatever coloration you are looking to offset, then the O2 amp probably isn’t for you. Details, sir: [NOTE: The O2 was designed to be a portable and a desktop headphone amplifier. As such there is a difference in the maximum gain that can be realized on AC power versus on Battery power. This is due to the differing rail voltages connected and disconnected from the grid. The numbers for maximum gain are: 7/Vin for AC power, and 4.5/Vin for Battery power (both RMS voltages). This will cover all charge states from the batteries.] This amp was a loaner from NwAvGuy (aka Lord Voldemoort). It is completely “to spec” according to his design, using the suggested enclosure, board configuration (all board mounted jacks and buttons, for example), wall wart, and front panel. The DIY version can be had for ~$100 (for the entire amp) or for $150 via Mr. Slim on diyaudio. The gains are set to 2.5x/6x. Headphones used: Ultrasone Pro 900, Shure E3C, Shure SE 535-V, ATH M50, Beyer DT-880 Pro 600ohm, and Senn PMX-680i (yes, I tried working out with this thing; more on that later). Sources: diy-MOD 5.5g, Sansa Clip+ (both rockboxed), Cambridge Audio DAC Magic, Emotiva XDA-1, and an Astro Mix-amp. Music: Danzig, Static-X, Norah Jones, Pendulum, TCM, CCR, Miles Davis, John Coltrane, Charlie Parker, Pheonix, Aesop Rock, Atmosphere, Clutch, Xx, Tristeza, Mars Volta, Maurizio Pollini etc. In short, just about everything, all FLAC/CBR MP3 @320kbps. Aesthetics & Build Quality: The B2-080 enclosure is a plain, though good looking enclosure. Box Enclosures gives you several different choices for colors (mine is bronze). The case itself is solid, and if you wanted, you could use it to punch holes in walls. Sorry, no demonstration of that will be forthcoming. Even though the case itself is solid, the controls on the front side stick out enough that if you were to drop it on one of those, expect something to break. Some minor Quibbles (non-sound related): Got to mention the size here. It is probably too big to fit comfortably in most people’s pockets for active use. It fit in my cargo shorts just fine, so walking around with it at the airport, etc. probably wouldn’t be too big of an issue for me. But most people probably would want to throw it in a backpag/bag or just use a smaller amp for this purpose. When I tried working out with it, I had a hard time keeping it in my pocket for a good portion of the workout. So, to echo what others have said, this amp is more of a transportable amp than a true portable amp. Another minor thing is the fact that all of the controls/jacks are on one side of the amp. This caused problems for my fat fingers when I tried operating some of the controls. Since this is a DIY design, there is the possibility of mounting the board backwards within the case and remote mounting some of the jacks on opposite sides of the board as you see fit. I would recommend that for those people with coordination issues (such as me). Battery replacement could be a little problematic. You have to unscrew and remove the back panel first. Then the batteries are mounted flush with the board. Removing and then reinserting them could be a hassle without stressing the board-mounted terminals overmuch. The other option is to remove the board entirely and then replace the batteries. Either one is a bit cumbersome. However, if you use the recommended NiMH 9-V batteries, this shouldn’t be an issue for a good long while. Sound Quality: What is there to say about an amp that literally adds nothing of its own to the sound signature? Any variations I heard in sonic characteristics were most certainly due to my headphones and not the amp. The O2 produces sound that is faithful to the original recording, without producing colorations or edge on the music. You can hear punchy, forceful bass, detailed midrange, or whatever else is in the playback chain. I compared this amp to the ALO Rx Mk II and the Headamp Pico (amp only). I was unable to discern any major differences between the amps, although I thought that the O2 had a slightly more laid back sound than the ALO amp (wouldn’t swear to this on a DBT though). Much fuss was made on the gain structure of the amp. Placing the volume pot after the gain stage was said to make the amp prone to input overloading. With careful selection of your gain setting with respect to your source and your headphones, this is not a problem. (Much more on this has been said on NwAvGuy’s site if you are looking for more information). With ReplayGain set to 85dB, the O2 was able to drive even the Beyers to acceptable levels without clipping noticeably, even on battery power. This is something that neither of my other portable amps can even come close to doing. Summary: In conclusion, overall what do I have to say about this amp? Everything I listened to was crystal clear. No undue colorations or “grain/edge/harshness” to note. Also, it was subjectively very close to a couple of amps many times its price. In short, the Objective2 is what it intends to be, a transparent, noise free amplifier that brings back the fidelity in hi-fidelity and sets the bar ridiculously high in terms of price/performance ratio. For further reading, check out NwAvGuy’s website, especially his O2 Summary article.