The JDSLabs O2+ODAC looks brilliant. It has a machined knob, which makes it look quite a bit more stylish, instead of the normal plastic knob you get on O2 entry level kits. Everything else on the front plate is as NwAvGuy describes.
The ODAC is integrated into the same enclosure as the O2, which leaves no room for batteries, so this combination forgoes the opportunity to be used on the go. The ODAC is designed around the same philosophy as the O2, and it boasts some very impressive stats for a $99 DAC. JDSLabs implementation makes the line-in for the O2 a line-out for the ODAC when the headphone amplifier is switched off.
The soldering quality looks very good and all leads have been trimmed nicely. The grounding for the chassis seems good as well. The test version we had for review was hand assembled, the final version will be machine assembled and wave soldered which will give it an even cleaner look.
The back side of the O2+ODAC just has one mini USB connector; it feeds the DAC. The DAC is completely self-powered, so you only have to hook up a USB cable to make it come to life.
This picture shows how the ODAC is wired up to the input/output. The final gain is configured via the resistors mounted next to the volume pot. You can get custom gain with the JDSLabs O2, which is a great feature. The standard gain is a normal 2.5x and 6.5x high. To put this into perspective: the most demanding headphones we have on stock, the HiFiMAN HE-6s, achieve plenty of volume with 2.5x gain. If you are going to use this headphone amplifier with in-ears, getting a 1x gain option is recommended. You can also modify the amp to 1x gain by simply removing one set of the resistors next to the volume pot.
Although all the components on the O2 are quite cheap, it looks and feels alright. The front layout is very compact since the entire faceplate is just 109 mm wide and 32 mm tall. The switches work well, but you do not get the same feel as with some of the pricier units. There are no luxury items on the JDSLabs O2 amplifier, aside for the knob and a good chassis. Every other selection is based on performance and cost. This no nonsense approach allows JDSLabs to sell the combo for just $285 - a bargain. The amplifier can of course be DIYed, since a basic kit costs around $100. The ODAC is, due to many SMD components, pre-made and sells for $99 which, performance considered, is quite alright. Some of the FETs in the design are quite sensitive to electrostatic shock, but the build is otherwise pretty straight forward. The process takes quite a bit of time for a rookie like myself, but it is totally feasible for anyone with a little soldering experience and some skill.
The front layout is alright for a portable unit, but we would like to see a less cluttered front for a desktop unit; the in and output jacks could be on the back of the unit. Only having to mill the front plate keeps costs down, but a few more convenience features and a desktop-orientated front plate would be nice. NwAvGuy, being the smart guy that he is, is already working on a pure desktop design called the Objective Desktop Amplifier (ODA for short). When ODA will be available is still unknown, but we are looking forward to it!
The build quality is just better with the JDSLabs version than what I was able to achieve. You can probably match the quality if you are great with a soldering iron.
In this design, the JRC 4556 is king, as demonstrated by NwAvGuy's blog.