EDIT: May 22, 2011
First a little background. I have been heavily involved in high-end audio for decades, but I am a relative newcomer to mobile audio. I have only been using it for around 5 years, and have only recently come up with a system that I can call high-end, due in no small part to the excellent information to be found on this site.
I have long had the suspicion that one of reasons it is difficult to achieve excellent sound with a portable setup is the low power DACs that most of these units employ. So I felt it was imperative to pick a unit with a S/PDIF input.
I don't have the opportunity to try out a number of units, so I rely on technical information. It is helpful that I come from an EE background and have some professional and audio amateur circuit design experience. Leckerton Audio is more forthcoming in design details than some of their competitors - I suspect that this is more due to most companies not thinking the information is of interest than it is to secretiveness. Leckerton has good detail on their website and I was able to get more detail from them via email. One of the ways the Leckerton is different from the competition is in the power supply. This is an area that seems to be an afterthought in many models but it is critical to performance. Many (though by no means all) designs do with a single supply rail. The Leckerton uses three supplies, a dedicated 5V rail for the DAC, and a bipolar +- 6V supply for the DAC output filter stage and the DAC stage. Why does this matter? First the noise generated by the DAC is isolated from the analog stage. More important in my opinion is that the analog section can be all DC coupled. Using a single supply rail means that the output will be referenced not to ground but to approximately 1/2 of the supply voltage. I don't know of any way to get that signal into a usable state other than using a very large coupling capacitor. How large? A 20Hz f3 with a 16 ohm load requires approximately 500uF and indeed the two amps I was able to get circuit data on uses 470uF capacitors in the output. These are often non-polarized electrolytics which are notoriously non-linear. I found one design that uses tantalums bypassed by film caps - an improvement but still the best output capacitor is none at all. Most manufacturers don't publish their power supply details, but there are ways to get an idea. If a unit has a DAC but still uses a 12V charging port it is likely to be single rail. If you are using a DC to DC converter there seems little point in not using the USB power for charging. If a unit uses a pair of 9V batteries (which I have seen) it is almost certainly bipolar. Other than that, rather than asking the vendor, you can look at the circuit and see if there are large value coupling capacitors. As far the other technical details, I will readily admit I am not up on current op-amp choices - but the Leckerton's are socketed if you want to roll your own. The DAC is as I have alluded to earlier, a home audio unit, the highly regarded Cirrus Logic CS4398. This is a higher quality DAC than used in my Marantz pre-amp/processor. And unlike one competing product that uses home type DACs the Leckerton does not have a steep high frequency roll-off. The Leckerton gets 15 hours battery run time when using TOSLINK compared to 35 using line in, but still good enough for an international flight. And you can power it from a portable USB supply if push comes to shove, something you can't do with a 12V charged unit. In keeping with the philosophy of providing deep technical information, Leckerton has published a full distortion test with a comparison to a competitor: http://www.leckertonaudio.com/2011/01/uha-6s-harmonic-distortion
On to the listening impressions. I first compared the Leckerton to my previous amp, the Total AirHead, using the Cowon D2 line out. This is a fine amp, excellent value, and I would still recommend it in its price range. This is a single supply amp BTW with NP electrolytics output caps. The Leckerton bettered it in every respect.
Where I was really interested in the results was with a TOSLINK input so I was eager to try out the Leckerton with my "new" iRiver H140 (Rockboxed) which I purchased from a head-fi'er. I use a mixture of FLAC and 256k MP3 when running portable. Not having another high-end portable rig to compare it to, I tend to reference my home theater system (see .sig). The Leckerton really doesn't have much of a sonic signature. It is extremely neutral, as a well designed piece of electronics should be. It does not suffer at all in comparison to my Sunfire amp - in fact the best thing I can say is that the rig does not sound like I have come to expect from portables - I come up with words like "rich" and "full". Attacks are not at all compressed, which indicates that there is no slew limiting going on. I have heard details in this setup that I haven't even caught with my home theater (there are some advantages to headphone listening). The Leckerton did an awesome job of keeping bass taught and well defined (I suspect a low output impedance). There is one area though in which the Leckerton really goes above and beyond, and that is in sound stage. The sound stage with this rig is huge. You have to hear it to believe it. The unit has plenty of drive capability. Orthodynamics have a reputation of being difficult to drive, but my Thunderpants sound fantastic with this amplifier. At no point do I get any feeling that the amplifier is limiting the sound quality of the rig. I prefer the high gain setting with these headphones, though the amp has enough output to drive them with the low gain setting.
Construction is solid, and controls are pleasant to use with no artifacts in operation.
Lastly, I like to support domestic industry. I believe this is the only US made DAC/Amp with an S/PDIF input (I will likely get corrected by someone - I don't know all that is out there). However I can state that the primary US made competition (fine units I am sure) run around double what the Leckerton does. I don't know how they are able to do it.
First rate design, excellent construction, world class sound quality, domestically produced at a comparitively bargain price and great customer support. I can't recommend the Leckerton UHA-6S too highly. And if you don't need the TOSLINK input, the UHA-6 is available for $20 less.
Here is a picture of the whole rig.
Edited by sbradley02 - 5/22/11 at 9:13am