I did not open the unit, so I can't say anything about the build quality inside. Overall build quality is good, only H/L gain switch is a bit loose, but you do not need to switch it often. Volume knob is smooth and nice to use. 3.5mm cable that came with amp is very poor quality, so prepare to buy good cable. Battery charging works and full charge is displayed with green led, while red is on while charging. Sound quality is very good. I have DT990 250 ohm headphones and Fiio K10 DAC. Fiio alone is not powerful enough to drive DT990, but I use line output on the Fiio and this as an amp. I like the sound signature and have not noticed anything to complain. Bass is tight and controlled and vocals are loud and clear, highs play just as from Fioo's own headphone output. I have also very good sound card with headphone amp on my desktop PC and compared to that this amp performs well.
thanks