Leckerton Audio UHA-6S MKII

This review describes version with OPA209, but other ones should perform on par or better and probably without GSM noise issue.
The amplifier is powerful enough to drive power hungry headphones such as orthodynamics, but also has very low impedance for balanced armature IEMs.
 
DAC, while very good, is not the absolute benchmark - but that is to be expected in an integrated device. Opamps can be replaced - they're SO-8 mono versions attached via a DIP-8 adapter.
The only actual drawbacks of this amp are related to ergonomics. Especially the very sensitive potentiometer can be problematic.

While I usually use an iPod or iPhone with a Fostex HP-P1 and my Symphones Magnums (modified Grados with far better drivers) in my transportable rig, I've found that the amp in the Fostex isn't as good as it could be and that best results were with a Triad Audio L3 or ALO RX3-B. While I've seriously considered the latter after owning the former, given the good reputation of the UHA-6II I wanted to see if I could get great results from a single-ended amp first.
 
The UHA-6 has certainly delivered, with a clean, clear sound and a lack of hiss. While I chose the stock opamps when I bought it, I swapped in a pair of AD797B opamps as I don't need to spare batter life. The difference is very minor, sounding very subtly smoother in the treble, but as I bought the amp to improve on that in the HP-P1, every little difference helps.
 
The sound from the built-in DAC isn't quite as awesome as that of the amp. I gather it was designed to work over USB from even an iPad Camera Connection Kit, so uses the standard BB PCM2706 receiver chip, which is now quite poor relative to newer chips. The S/PDIF input is also fairly lacklustre, but the DAC is listenable, if not up to the benchmark set by the ODAC in clarity.
 
What is most impressive about the design is the quality of the case and components, along with the thoughtful functionality. The case screws together perfectly with a tight fit. Since the USB input doubles as the charging input, a switch is included allowing charging to be turned off. The UHA-6 also auto-selects the input, so you don't end up ever wondering why there is no music playing. The only negative point with the aesthetics is that the micro USB port is slightly recessed so you have to use the USB cable that came with it which has a connector that protrudes slightly more than average.
 
Overall it's a lovely little amp that is great value for all the tech packed into the beautiful case and the sound quality of the headphone amp.

Right when I thought my gr07's couldn't get any better. This little amp brought my listening experience to a whole new level. Before with just the j3 and the gr07, the sound could get congested at times with not enough breathing space, but with this amp, it opened things up, it allowed everything to flow naturally. This amp is also super transparent (with the opa209 op amp) it shows you everything cleanly and effortlessly. If you haven't already gotten a portable amp for your dap GET ONE NOW and thank me later. Also, this amp works wonderfully for iems, there is absolutely NO noise or hiss at all.