Hey everyone. I wanted to share my findings in performing failure analysis on my failed DX3pro in the hope that others may find this information useful.
I purchased a DX3pro (SN 1812594045) from an eBay seller and have been using it primarily with Shure SRH1540 headphones (46 ohm) for the past month at work with the stock wall-wart PSU. I never unplugged the unit during this time, but let it automatically transition to and from standby based on whether or not an audio stream was being sent from my computer. Last week, I returned to my desk from a meeting to discover that it had suffered a failure similar to those others have been describing: the right channel of my headphones was producing only a hissing noise, and upon removing the headphone plug from the DX3pro's jack, there was a loud 'pop' and the DX3pro went into standby mode.
In the case of my particular unit, it appears the right channel of the headphone amplifier has failed and is outputting approximately -9.5V DC on the right channel of the headphone jack. The left channel appears to be operating normally. I traced the headphone amplifier circuit and it is a fairly straightforward op-amp based design with its output boosted by a single complementary BJT stage (2SB649/2SD669). Heres's an example of an extremely similar circuit:
http://sound.whsites.net/project113.htm
On my unit, it appears the PNP transistor to the -10V rail on the right channel has failed with a short from collector to emitter, resulting in the DC level present at the headphone output. I don't have good root-cause on why this transistor failed, but I suspect either a design issue (such as excessive bias current resulting in excessive heat and consequent damage) or a part quality issue. I'll keep working on this tomorrow to see if I can identify any issues.
Also FWIW all other functions of my unit still appear to be working normally (user interface, entering/exiting standby, etc.).