ePanorama.net - Audio Documents
Headphone amplifier output impedance
Typically headphone performance is affected very little by source impedance. The (new) international standard on audio interfaces, IEC 61938 (formerly IEC268-15) calls for an intermediate-impedance source of 120 ohms and a source voltage of 5 V rms, which provide reasonably satisfactory performance (sound level, frequency response, distortion) with headphones of any impedance over the range of 8 to 600 ohms at least. So 5 V rms maximum behind 120 ohms is the accepted international standard which modern equipments use.
Maximum sound level limiting
Because the sensitivities of headphones vary so greatly, it is not possible to provide an output on an amplifier that will ensure that high sound pressure levels cannot be obtained with any headphones. The only way to limit sound presure levels is to hard-wire a specific type of headphone of known sensitivity to the amplifier.