PerformanceOur test setup for this particular DAC and amplifier combination was quite simple: a Windows7 PC with Foobar 2K and source material in various bit-rates. As far as headphones go, we tried out quite a few including the HiFiMAN HE-500, HiFiMAN HE-300, HiFiMAN RE-262, Westone ES5, and JHAUDIO JH16|Pro. Since our test rig is in the EU, we could not get any of the NwAvGuy recommended power supplies, but we were able to source one that JDSLabs recommends for use on 230 V/ 50 Hz. It is a 15 V AC/AC 500 mA Wallwart that is quite powerful, but the O2 amplifier can be powered by a variety of different 14-20 V AC/AC power supplies since the conversion is done in the box. JDSLabs has a stock of Triad power supplies for the US market. Simply add one to your order for $10.99.
The test unit JDSLabs so kindly sent to us was setup with the stock gain of 2.5x and 6.5x, which should fit both low and high efficiency headphones. If you plan on using it solely with in-ears, you might want to set the gain set to 1x and 2.5x. To put it into perspective: this little amp has more than enough power to get the HiFiMAN HE-500s singing at 2.5x gain, and even the notoriously hard to drive HE-6s had plenty of volume available on the 2.5x gain setting. Due to the channel imbalance at very low volumes, you ideally want to set the volume pot to at least 20%.
Once the unit was hooked up, we took it for a spin with our reference HiFiMAN HE-500s. They are a tad on the fun side in terms of frequency response, but have a very fast presentation due to their planar magnetic driver design. The O2+ODAC combination stunned from the get go: the bass was fantastic and clean. Especially the sub bass was kept in check, which I have not had on anything except for a few very high-end solid-state amplifiers. The resolution on this thing is staggering. It is even better than the ASUS Xonar Essence One DAC/Amp. Even an upgraded ASUS Xonar Essence One will have a hard time following this little amplifier. This is truly remarkable considering that the O2+ODAC combination is less than half the price.
Everything from the bass to the treble is perfectly balanced and there are no oddities of note, even with the low impedance high efficiency JH16|Pros or the Westone ES5s. Even low efficiency medium impedance headphones like the HE-6s keep the same qualities with the JDSLabs O2+. It is a very versatile creation, which underlines the quality of the design. Amps such as the balanced RSA Protector cannot drive the HE-6s efficiently and it is $120 more expensive.
With a set of very sensitive in-ears, like the Westone ES5s and the JHAUDIO JH16|PROs, the 2.5x gain is a bit high, so the listening level will, in order to get past the channel imbalance at the low range of the volume pot, exceed a level I am comfortable with over extended periods of time. This will, of course, depend a bit on the recording. It will be too much for a modern pop recording that is excessively compressed but will be fine for well mastered or classical tracks. You could dial back the volume by turning the volume down through the player software, but that will reduce the sound quality - using 24-bit material will help push the boundaries a little bit, but such material is not widely available.
The channel imbalance is completely gone once you push a little bit past the first notch on the volume control. This is similar to the imbalance you get on a much more expensive ASUS Xonar Essence One.
It would be recommendable to get a different gain setting than stock if you plan on using it with in-ears and reasonably easy to drive headphones (up to HiFiMAN HE-500s). 1x / 2.5x seems ideal, since the amplifier will reach insane volume levels with the HE-500s at roughly twelve o'clock on the volume dial.
Powering the HE-500s for a day at gain 2.5x barely had the unit get hot. The power supply gets slightly hotter, but its heat generation is nothing to worry about.