Pics added! Sources: FLAC > Foobar WASAPI > SPDIF > Cambridge DacMagic Ipod with 320CBR mp3's Headphones: AKG K702, Grado SR60 (bowl pads), Audio-Technica ATH-AD700, HiFiMan RE262, Altec Lansing UHP336 (Rebadged Super.Fi 3 - will be referred to as Super.Fi 3 from now on) Past Amplifier Experiences: Mini^3, Schiit Asgard, AV123 X-Head, Benchmark DAC (demoed), NuForce uDAC, Fiio E5 & E3, various Little Dot amps (demoed friends units), Audio-GD Sparrow Music (FLAC): The National – Boxer, Spoon – Transference, Nine Inch Nails – With Teeth, White Stripes – White Blood Cells, Daft Punk – Alive 2007 --All music is replaygained to ~89dB— Notes about the Amplifier: I expect this review is going to be a point of contention for a lot of people currently, considering this amp is designed by a banned member of Head-Fi. Nonetheless, since I’m one of the first owners of this amp I’ve decided to make a review to help others that are currently considering it. This amplifier was made with a gain of two and six respectively to ensure safety against clipping from sources up to 3.4V while using AC while still offering a fair amount of gain from portable sources (2v when using headphone out from ipod with gain of two, 3v out when using an ipod LOD and gain of six). For more information, please read the DIY thread regarding this amp to understand the maximum input of this amp. Considering my impressions, I believe this compromise was an excellent way of maximizing performance and compatibility of an excellent amplifier. Price: Approximately $100 DIY or $150 built by MySlim at DIY Audio/ABI (Anything But Ipod) forums for a fully finished unit. For those in England keep your eyes on Epiphany Acoustics as they too will be offering it (currently in pre-order stage). The Impressions: I have to say that no holds barred I was impressed with this amplifier. The authority bass was reproduced with was excellent; it provided the K702 with more than enough voltage and current to really shine. Details were rendered impeccably, allowing the music to come through and not the sound of the amplifier. Treble is definitely present, but not exaggerated or shrill in the least. Everything sounded very straight forward and effortless with power to spare for what many at head-fi consider very power hungry headphones. I wish I had my K601 to test with since they require a bit more power, but unfortunately they’re out for repair. Nonetheless, with the headroom I got powering the K702 I have no doubt the O2 will power them just as well in all honesty – I may update this review once they’ve been returned to me from AKG. When used with the various less demanding headphones, I still was continuously impressed. The Grado’s had their bright house sound, but impactful lower end they’re known for when powered properly, the ATH-AD700’s had their nice full midrange with upper range “sparkle”, and the Super.Fi’s had their relative neutral sound about them. In terms of hiss the amplifier was absolutely dead quiet; even with a gain of six, plugged in to AC, with a Super.Fi 3 at max volume (note – this is dangerous and I would NOT recommend you try it!). Crosstalk is negligible really emphasizing the separation of left and right channels compared to the Mini^3, which when tested I found issues with potentially audible crosstalk (using RMAA and listening to test signals). In fact, using RMAA I almost had identical test results to NwAvGuy. The O2 scored -73dB on crosstalk. In comparison, the Mini^3 using the exact same load scored -39dB. The volume control usually sat around 10 o’clock showing excellent potential for headroom (it can almost turn to 6 o’clock from past the 10). I have yet to test a quietly mastered track (recommendations appreciated!), but I have no qualms saying this amp should be able to reproduce them loud and full with almost any dynamic headphone around. This was also using an almost redbook standard DAC (2.1V RMS). Those producing higher voltages (~3V) should benefit even more with a gain of 2. When used with batteries and an ipod I was able to drive the RE262’s with ease noting once again how balanced it sounded, more than likely due to the amount of available headroom. I was curious and also tried the K702 off battery – to my surprise they were able to get more than loud enough with little to no distortion, but an obvious loss in the degree of headroom. Obviously this won’t be common usage, but I found it quite impressive. I only have a couple issues with this amplifier that I will address now. First, there’s an on and off “click”. It’s been noted this is a small on/off transient, and in line with what I heard powering on say the Mini^3 – not to the extent of a pre-relay Asgard that was potentially dangerous to many headphones (from the mouth of AKG). Some may be a bit concerned upon hearing it, but it is very small in reality. Even with the Super.Fi 3’s, while distinct, it surely doesn’t strike me as really bad. The largest issue however is the pocketability of the amplifier. It’s clear to me that this is a transportable amplifier more than a portable. It will be great for stays at a hotel while traveling for example, but in terms of actually using it as a portable on one’s self I find it awkward at best. I find the Mini^3, Cmoy, and O2 fit in this category of amplifiers I really wouldn’t want to carry on me. If you’re serious about portable usage I recommend sticking with an E5 or a well performing slimmer amplifier. Aesthetics: The recommended case looks very basic yet handsome, but the biggest downfall is probably the fact all cables plug into the front including power. Depending on the usage one can work out a different enclosure, but that may ruin the easiness of transporting the amp. On the plus side though, the default enclosure is cheap and available in a wide range of colors to match whatever look you really want. Personally I went with silver. Conclusion: Did this amplifier get across its point? I think so. I won’t say it’s the do everything amplifier that the designer intended, because I really don’t see it being used in someone’s pocket on the go. If you’re the type to carry an amplifier in a bag and leave it in there while routing out your headphones then batteries may still be worth including. As a transportable or even desktop amplifier it certainly is punching way above what the price would suggest, measurements show solid performance and my impressions though sighted are the same. I would recommend it above any solid state amplifier I’ve had on hands experience with thus far, even if you have the funds available for some of the higher priced offerings. I probably would not recommend it to tube user that is most likely used to a different sound thanks to tubes' higher output impedance, but if they want to see what their headphones sound like from a really accurate amplifier this would be the one I'd suggest. If you want a reference grade solid state amplifier on the cheap, this would be my go to recommendation right now. When my AV123 dies I will probably get a second O2, or consider the future desktop version that’s currently in the works. PS: If you plan on building this amp, there is currently a group buy for PCBs at DIY Audio. The cut off is going to happen soon from my understanding, so if you’re on the fence I really suggest you go for it. Hopefully this was helpful, and I am open to recommendations of quietly mastered tracks with lots of dynamic range and other recommendations as to make this review as complete as possible. Pictures! : Got the FPE panel installed, let's take a look shall we? Without flash. With Flash Size compare 1 Size compare 2 EDIT : On batteries, it seems mine may have been lacking charge. After letting them fully charge (at least they should be) I noticed no difference in average listening volume regardless of potentiometer position. Apparently the battery section will not get as loud at the end of the pot travel, but it should be a max of ~3dB or so. As for quiet tracks, mentioned it later but there's plenty of power to bring quiet tracks up to a very high level. Plenty of ability for tracks with large amounts of dynamic range IMO, but I don't have any seriously challenging headphones rated below 100dB (K702 and K601 are rated above, still need to try K601 when it arrives from repair this week presumably). Pics of amp in the enclosure to come sometime mid-next week. May try doing something interesting to the box if I have some spare time . . . we'll see ^_- EDIT 2: Pictures added, you can see it's a rather minimalist design, but looks quite good overall (do know that the left port is for the AC wire though - maybe taller feet can allow routing the wire. Experience with K601 same with K702- the get plenty loud with no distortion and plenty of "oomph" to say the least. Even mildly efficient cans should be powered very well, and that on mine outputting a "measly" 4V with my standard setup (the 6V using the ipod sounds just as good).