It wasn’t bad sounding at all when used as car audio input or with medium impedance headphones (32ohm-150ohm), but for portable earphones and high end portable headphones it was a nightmare. All kinds of impedance mis-matching, sensitivity issues, chip oscillations, etc. Good news is that most of this could be fixed!
1. They used horrible SMD electrolytic capacitors that were noisy, placed with small through hole since my SMD capacitors that were the right capacity were too wide. Plus, these were individually selected as to what they were near to, cap that were good for power were placed in power and signal grade were placed near the signal.The capacitors were routed and when appropriate, wima film capacitors added to near-signal electrolytics).
2. DECOUPLE! None of the chips were decoupled which is a big reason for oscillations and noise added to the signal. Decoupled the DAC and Headphone amplifier sections (BTW – the headphone amp was just a simple OP-AMP taking differential output from the DAC…so not really much different from a normal MP3, just a nicer chip).
3. Battery was fine, but did not put out enough current to drive the unit appropriately. Replaced with a Sony Li-Po police radio battery. It can output almost double the current when drawn upon as the previous batter and was much more durable.
4. Adjust the gain of the amplifier, Gain was much too high on the amplifier and toned down on the DAC, Basically, you just make the DAC not loose so much gain and get rid of the R/C filter (no reason for two stages of capacitors for the output when you have an I-out DAC) datasheets and op-amp gain calculations help a lot here, but there are numerous examples online.
The overall simple fixes, not major engineering overhaul, made it into a much nicer and genuinely hi-fi piece of equipment. Battery life was an insane 30hours too! Sold it on eBay as I ended up having no good use for it.
Sorry, these pictures are about 2-years old and yes, the manufacturer did take some of my advice and integrated it to the next two versions of this product!