[imackler]

The designer of the O2 recommended additional wall warts than the often sold WAU12‐200. He said "If your line voltage is below 117 VAC or 235 VAC, and/or you plan to drive difficult low impedance headpones (i.e. HiFiMan planars), I would suggest a 14+ VAC transformer at 400+ mA. The best Mouser transformers are the WAU16-400, 412-218054 or WAU16-1000 CUI. But those are more expensive than the WAU12-200. The WAU20-200 also works for higher impedance headphones." 
 
Do you all think that the science behind it would justify getting different wall warts for different phones? Right now, I've got a 300ohm HD600 at the office and another at home; I'm using the WAU12-200 that was sold with the O2. I'm also considering getting a 50ohm Mad Dog (modded Fostex T50RP). Dan from Mr. Speakers recommended a different wall wart with the O2 for the T50RP. 
 
Wall warts really don't cost much so I'm willing to get whatever would be best. What wall wart would you recommend w/ the 300 ohm HD600 and what with the 50 ohm Mad Dog orthodynamic planar? Or should I just stick w/ the WAU12-200? 
 
Do you expect I would hear a difference? I'm curious if you think i would...

 

[mikeaj]

The transformers with higher ratings are to ensure that if you're drawing a lot of power from the amp and the wall voltage is a bit lower than nominal, that the voltage into the regulators won't drop to the point where they're running a bit out of spec (and not regulating very well), hence sending more noise into the power supply rails.
 
The only advantages of the smaller rating WAU12-200 are (1) lower cost, (2) smaller physical size, and (3) lower operating temperature for the regulators and thus less power losses and heat produced. (With respect to temperatures, they're plenty low enough even if you use a high input voltage, but I guess over the many-years long run you don't want those elevated operating temperatures because you have some electrolytic capacitors nearby.) You might just want a WAU16-400 and forget about anything else. It's a matter of the cheaper option being default because it's sufficient for most things usually, not that there are real tradeoffs and you should mix and match transformers and headphones. Compared to a lot of other cost optimizations, saving ~$5 per build by default by spec'ing the WAU12-200 seems to be a big deal for the designer.
 
Note that the power supply rejection ratios of the parts are pretty high, and to be drawing a lot of power you pretty much need to be listening very loudly, so it's dubious (but you might) that you'd actually hear a difference in practice. It certainly doesn't seem like good practice to operate the circuit out of spec in some sense by letting the input voltage sag, but it may not practically sound worse. I know some people have tried, but they don't report hearing anything different with different adapters. It might take using HE-6 and listening very loudly while simultaneously charging the batteries, or something like that. I don't really know and haven't done the calculations.