Hi, sorry for english, but it seems only forum about modding of dr.dac nano. First for all thanks how to fix low-freq issue, however I registered to tell my investigations results. Most of remaining distortions caused by ceramic (likely Y5V or X7R) capacitors used in after-DAC low-pass filter. They designated as C48 and C49. Remove them and put instead of them WIMA MKS 2.2nF (2200pF) or NP0 ceramic (I didn't find it with such capacitance). It possible to put them in more comfortable place - between one of legs of C45/C46 and signal ground. However if C45/C46 also Y5V/X7R ceramic (in my model tantalum) - they strongly needs to be removed from board and either replaced by electrolytic either shorted if DC offset on DAC output is small enough (in my case it was less then 1mV). Doing that instantly lowered THD level to < 0.002% that is effectively below noise coming from USB! That noise however also can be reduced a bit by adding more capacitance in parallel to some tantalum power filtering caps, but difference is very minor. Note that C40/C52 also from that ugly Y5V/X7R kind, so all that results assumed they also removed.
Also there is another thing to do if DAC is used with external AMP and builtin headphone preamp not neccessary. In this case U4 (switching voltage converter that makes negative rail for opamp) and U7 (opamp) can be simple removed. Also remove R35/R29/C45/C46 and connect post-DAC filter output to jack directly using thin wire connecting R35-C46 and R29-C46, see photo: http://savepic.su/7299882.png
This not only improved things by removing opamp from tract, but also reduced power rail noise since U4 is switching supply. On photo you may see that I doubled capacitance of C44/C56 by putting in parallel caps taken from C45/C46 (they were tantalum in my model). I also removed C39 and to access ground on its placeholder pin. C39 is negative rail filtering cap and since I removed negative voltage converter and opamp its not neccesary anymore. And you may see some additional caps I added to achieve better USB power filtering:)
