I'm a bit confused about differences between power output - mW vs. Vrms. For example, I'm looking at the Shanling M5s, and it lists the balanced power output at 300 mW. My current dap lists the balanced power output at 3.4 Vrms. Which is greater in this scenario? Is there some sort of way to convert these measurements?
It's been over a decade since I was in Electrical Engineering school (and failed at it by the way) so my electrical knowledge might be rusty. You'll forgive me if I give some information that is not entirely accurate, I'm sure more knowledgeable people can chime in and set things straight.
First of all mW, is a unit of power, meaning amount of work done in a given time, it's a unit that works both on electrical power and...physical power. The thing with electrical power, it varies according with the impedance of the headphone, in turn headphone impedance varies with the frequency. Some headphones have a fairly linear impedance, others have peaks at certain frequencies so the amplifier has to give more power to get the same sound pressure at other frequencies, or you lose some at that frequency. Which is why when looking at power output of headphone amps, always look for the Impedance at which that power is being given. I'll echo what another guy said here: 300 mW at 16 Ohms, is not that impressive. 300 mW at 600 Ohms, is very freaking impressive.
Second, Vrms. Is "Root Mean Square" Voltage, it is not a unit of power but a unit that measures how much work could be done, or how much work needs to be done to "move" against it. The RMS means that it's the comparable AC voltage generator (swinging up and down) would give, it if were DC (flat line). Vrms alone will not tell you how much power the amplifier is going to give. You need to know the load connected to it.
For most of us the power an average DAP has is more than adequate for a vast array of IEM and most headphones meant for mobile use. If you want to compare power of the two, look at the posts by megabigeye. Those two equations should be more than enough.
I'll add as well, any DAP manufacturer worth its salt should be giving more complete specifications and the conditions they apply to, if they are trying to appeal to the hi-fi crowd. Us audiophiles have built in frequency response graph generators. The most seasoned of us can determine the Total Harmonic Distortion, the Dynamic range up -123 dB and figure out at which frequency the distortion exceeds 0.5 dB, just by listening a single tone coming from an old earbub connected to what we are evaluating, some can even determine the Vrms by wetting their finger with saliva and touching the headphone jack, seriously. Any manufacturer must state all their proper specs at a moment's notice, less they'd be proven wrong by the next audiophile that walks into their store.
