Egy elektrotechnikával is foglalkozó angol zenész kollégámat idézem:
It beats me why people get so confused about headphone impedence.
The loudness of any transducer (headphone or speaker) for a given input is called it's efficiency. It's measured in Decibels per Watt Meter or dB(W/m). It most definitely isn't measured in Ohms.
The impedence of a transducer is measured in Ohms. This tells you almost nothing about the efficiency of the transducer.
If you take two pairs of headphones both with an efficiency of 90 dB(W/m) one having an impedence of 100 Ohms, the other with an impedence of 10,000 ohms and drive them with 1 watt they will produce equal levels of loudness.
The only reason that headphone manufacturers give an impedence rating is that some cheap equipment has a problem with low impedence loads. As the impedence of the headphone drops the amplifier finds it more difficult to supply enough current.
Even the kind of £10 MP3 players they give away with the cornflakes can supply enough current to drive a 10,000 ohm headphone. Plug in a 100 ohm set and the MP3 player will probably go pop.
I've got a pair of Senn HD600's (300 Ohm). An ultra cheapo MP3 player drives them to literally deafening levels. The HD650's (also 300 Ohm) are considerably more efficient due to a neodymium magnet. Sennheiser's tech specs give the efficiency of the 600's at 97 dB and the 650's at 103db. It doesn't say for what input or at what distance but that's marketing departments for you.
The bottom line is, Daven, that the HD650's don't need a dedicated amp to drive them loud because they're efficient enough.
They don't need a dedicated amp because the have a high enough impedence not to stress the driving amp.
They might well need a dedicated amp because if you're paying for high definition headphones you may as well know that they're being diven by a low distortion amplifier.
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