so im planning to buy portable headphone amplifier like this Fiio A3 (E11K) or else but i dont know are these 270mW means much cuz mine sony headphones can go up to 1000mW @ 32 ohms, and what is standart output power on smartphones 
like if its 150mW or 100mW then these 270mW wont be that much
like if its 150mW or 100mW then these 270mW wont be that much You can do some elementary mathematics and answer your own question.
The short answer is that 270 mW into virtually any headphone, especially popular modern headphones designed to work with portable devices, is a whole lot of power. You will never, ever drive modern headphones (except maybe exotics like Hifiman) with anywhere near 270 mW while wearing them unless you are deaf. If you aren't deaf before doing this stunt, you most certainly will be after.
Look at your headphones specifications. See the efficiency spec? With that spec, tell me how loud your headphones would be if driven with 270 mW. SPL increases 3 dB with every doubling of power. Now get out your slide rule and answer your own question.
The short answer is that 270 mW into virtually any headphone, especially popular modern headphones designed to work with portable devices, is a whole lot of power. You will never, ever drive modern headphones (except maybe exotics like Hifiman) with anywhere near 270 mW while wearing them unless you are deaf. If you aren't deaf before doing this stunt, you most certainly will be after.
Look at your headphones specifications. See the efficiency spec? With that spec, tell me how loud your headphones would be if driven with 270 mW. SPL increases 3 dB with every doubling of power. Now get out your slide rule and answer your own question.
If the phone works off a 3.3V supply, that works out to around 42mW max.
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With 100 dB/mW efficiency (a typical specification for modern headphones designed to work with portable devices) this works out to 126 spl max, both channels driven, theoretically. Reality might be around 120 dB or so; in other words, as loud or louder than a Who concert.
It's easy to see that you don't need a lot of power to drive most headphones. Headphones today are different than the headphones of yesteryear; modern phones are low impedance to work from battery powered sources, and "vintage" headphones are typically 600 ohms and require more voltage (but not necessarily more power).
You're right. I calculated for voltage gain. 100+20(log15)
Still my point stands. 112 dB is mighty loud, and 120 dB is as loud as you should ever be exposed to for very brief periods. A system capable of 112 dB SPL is loud enough.
270mW at 32ohms and 450mW at 16
Well, you're part way there. Now all you need to do is get the sensitivity rating and impedance of your headphones, combine the numbers, do the arithmetic, and out pops your answer.
If you need more of a how-to and why, NwAvGuy's got you covered, just scroll down to the mathy bit.
one of them is Sennheiser HD 201 - Over Ear Headphones and another of them is http://www.sony.co.uk/electronics/headband-headphones/mdr-zx310-zx310ap/specifications i think thats why needed more volume cuz of 98db vs 108db
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