Okay, re-did the measurement on a lower load (previously 47ohm, now half at 23.5ohm), noticed a huge drop in voltage in the same setting to suggest obvious clipping / limiting. So I lower the volume from max and confirm the voltage drop is gone which leads to two immediate observations: 1) recalculated the output impedance to around 0.4ohm under a smaller volume, which is more inline with what iBasso has claimed. The previous number is higher due to bigger voltage drop between load and no-load under max volume condition, where in both cases iBasso DX50 fails to reach the claimed 3.1V(rms?). It managed only 2.55Vrms @ 47ohm and 1.66Vrms @ 23.5ohm. Current output for DX50 dropped slightly to around 73mA (at max volume). Same 23.5ohm test to X5: voltage is holding at 2.98Vrms (from previous 3Vrms) and current output drops a bit to 288mA (from 300mA). So to the same 23.5ohm load, X5 is driving 7 times more power than DX50 (from the previous 4x @ 47ohm). There is so much power in X5's output that the dummy load actually got warmer during testing, which means X5 is capable of blowing out your headphone in the maxed-out setting, not that I'll suggest you should ever try.
Anyway, despite the claimed number I think it is obvious DX50 doesn't quite deliver those number under actual test. X5 on the other hand does (and quite nicely so). Your calculated number is right but those would only be true in theoretical or very ideal condition, such as a very high impedance load which will be much easy for DX50 to drive. With real world 16ohm / 32ohm headphone, it can never achieve those calculated spec. Just go to show that you can't always trust what manufacturer have said (or not said, in this case).
Fiio has published the max. current output of their DAP, X5 is rated at >150 mA, X3 at > 250 mA, so X3 has significantly better driving power when compare to X5?
Coming from a traditional HiFi setup background, I certainly appreciate the amount of current that an amplifier can put out determine its driving power, I guess the same will hold the same in HeadFi DAP design. The obvious index in traditional HiFi game is whether an amplifier can double its output when the impedance drop by half (8ohm vs 4 ohm vs 2 ohm), I wonder is there an easy way out to speculate the current capacity of a DAP or headphone amplifier based on published specifications.
So DX50 high gain is hardly 3.1V as published? You didn't specify the gain setting of X5 and DX50 in your measurements, I assume both units are set in high gain from the figures. In fact, even Fiio website didn't specify the gain setting regarding their DAP output, I have asked here before with no luck, wonder if this is too much trouble for you to measure the X5 output in both LO and Hi gain setting, much appreciated in advance.
By the way, I have the ER4s with me, and I can tell you quite definitely that I prefer X5 over DX50 with ER4s at 100 ohm.
Edited by Andykong - 1/31/14 at 7:06pm