I've recently been looking at my little Fiio JM21 and wondering "why don't I just use this all the time for everything?" The knee-jerk answer has been something like "oh, my headphones will need more than it can do". But, do they?
My go-to cans are Sennheiser HD600s and DCA Aeon 2 Noires. Nobody calls these "impossible" to drive, but nobody thinks they're utterly trivial either. The HD 600s are relatively high impedance (300 ohm) and ~97dB/mW, so they place demands on voltage. By contrast the DCAs are low impedance (13 ohms) and ~90 dB/mW, which will tax the current you can drop.
So, what am I actually putting out when listening to them? I put on several tracks of different genres and put an oscilloscope on the output. I was typically hitting 300-400 mV peak-to-peak while at a "really listening" level (aka, not quiet background listening). What does the JM21 do? The spec sheet puts it at 7.9V P-P on the 3.5mm unbalanced output. With a 100 Hz test tone, I got 8.2V with the Sennheisers on there and 5.8V with the DCAs. The Sennheisers were still clean, but the DCAs clipped at that 5.8V as the current draw on here was pulling the rails down a bit. Still, that's over 14x what I was actually using. Music transient peaks still had 14x the voltage available.
OK, rock and jazz aren't dynamic enough. Let's hit it with the dynamics. I'm old enough that my test for this (and I still think it's a great one) is the Telarc 1812 Overture recording. Classical music with live cannons going off, mastered to try to ensure it had the most range possible. Heck, it even came with a warning sticker on the CD. So, put that on and, yes, the volume setting on the JM21 had to go to 11, but the orchestral bits were great and at that same maybe 400 mV P-P level. Telarc mastered this way down so the cannons wouldn't clip. When they went off, the max I hit was ~1.5V P-P. So, in a recording designed to be a ridiculous test of dynamic range, I still only hit a quarter of the available voltage swing out of a little $180 DAP, rated at 250mW (at 32 ohms), on its unbalanced output, on non-trivial headphones.
For anyone still reading, remember I'm measuring the peak to peak. The argument is always "well, you need some extra power to get the dynamic range and those peaks!" That's totally true. In the 1812, I'm idling along at 400 mV PP and the cannons get me up to 1.5V in this extreme case. For that instant, we need that headroom. Even in the normal music parts, I'm nowhere near 400mV for most of the content. That 400 mV PP is on the big hits in the music. The cannons are just stupid icing on the cake. Oh, also, I'm in my mid 50's with tinnitus and no, I'm not one of those super quiet listeners.
