As a relative newbie to full-size headphones, I'm not very clear on the wattage/voltage/impedance relationships and how to evaluate different amps for my HE-400s. I currently feed my cans from an Emotiva XDA-2 DAC/digital preamp, which has the following output from its headphone jack:
1.9 VRMS max into 10 Ohms @
< 0.02% THD (360 mW)
2.25 VRMS max into 47 Ohms @
< 0.015% THD (100 mW)
2.25 VRMS max into 100 Ohms @
< 0.01% THD (50 mW)
I'm assuming the output at the 35 ohm impedance of the HE-400 is going to be somewhere around 2 or 2.1 vrms. I played with a spreadsheet from apexhifi.com and tried to make the relationships between VRMS, p-p voltage, impedance, and wattage all make sense. The Schiit Lyr, for example, is rated at 40V P-P at 32 ohms, and my calculator says that corresponds to 6200mw - right about the 6 watts claimed by Schiit, so I think I'm not screwing this calculation up too badly. Using that calculator, it seems that the output of the XDA-2 (2.1 vrms at 35 ohms) is only 126mw. That's twice what I need to drive the HE-400 to 110dB (not that I want to listen at 110, I'm just using 110 as point of reference). I want to compare that to the Asgard. If the Asgard is putting out 20V P-P at 32 ohm (the Schiit site doesn't say 32 ohms, but that is what they quote for the Lyr voltage measurement), that's 1562mw - more than ten times the current output of the XDA-2! I've heard class A amps output about 10% of the power draw. The Asgard draws up to 35 watts, so 1.5 is only 4-5%, but seems in the right ballpark. Does this sound right? The XDA-2 is reputed to have a decent hp output (and I'm not saying it doesn't!), but I'm surprised that it would have only 10% the output of the Schiit.
I have no idea if that difference in current matters or not, but I'm hoping someone can help confirm that these calculations sound reasonable. Unfortunately, posting this in the HE-400 thread didn't result in any response, but that thread is so crazy I'm assuming this question wasn't read by many.
Thanks in advance for any help!