monnay wrote:Headphone spec tells me 108 dB SPL, i assumed its maximum is that
Probably depends on what the manufacturer thought the customer would want.
Headphones can vary in impedance by an order of magnitude, amplifiers can also vary in power out of the headphone socket by a similar amount, and people's ears probably vary by twice that.
Then the efficiency of the transducer could be anywhere from 10% to 90%, better-sounding ones being less efficient
H&S regulations put a limit on sound levels for the general public, so headphone manufacturers will probably design them to approach (but not exceed) that level, for some amount of distortion, when some undefined signal is applied. Putting a larger signal in may destroy your hearing, may clip (and sound horrible) or may destroy the voicecoils. Or may not.
TL:DR there's almost no correlation between what you put in and what you hear.
The Pi can put out about 1v from 100Ω, which isn't very much. If your headphones are efficient and have between 32Ω and 200Ω, and the room's quiet, you may hear something.
Otherwise you'll need an amplifier.
My own amplifier has an output stage driven from a ±25v supply.
That's enough to have blown my speaker coils, but only when I turned it right up - but then the volume control is a five-turn pot. Turn it up to 11? mine goes to 55!