Excellent explanations by Bombaywalla.
A couple of additional points:
1)Another possible reason for the recommendation by your integrated amp manufacturer to use headphones having impedances of 50 ohms or more is that while its headphone amplifier circuit might have no trouble providing the voltage, current, or power required by low impedance headphones, its output impedance may be too high, and vary too much as a function of frequency, to provide good sonics when driving low impedances. That same problem sometimes arises in interfacing preamp outputs to power amp inputs.
If the headphone amp within the integrated amplifier has an output impedance that is a substantial fraction of the impedance of the headphones, and if that output impedance varies significantly over the audible frequency range (as may often be the case), the resulting frequency response will be significantly non-flat, as a result of the voltage divider effect. The voltage that the headphone amp "tries" to put out being divided between the headphone's impedance and its own output impedance. For a given output impedance, the higher the impedance of the headphones the less significant that effect will become.
2)Keep in mind that to produce a given volume level at the listener's ears, the power required by a set of headphones is VASTLY less than the amount of power required by a pair of speakers. Perhaps 100,000 times less, or thereabouts. Headphones commonly produce sound pressure levels of around 100 db in response to an input of 1 milliwatt (0.001 watts). Speakers having typical efficiencies that are listened to at typical distances will require, as a rough order of magnitude, something like 100 watts to produce that same spl.
So a hypothetical 32 ohm speaker, were such a thing to exist, would require vastly more current, voltage, and power to produce a given volume at the listener's ears than a 32 ohm headphone.
Regards,
-- Al
A couple of additional points:
1)Another possible reason for the recommendation by your integrated amp manufacturer to use headphones having impedances of 50 ohms or more is that while its headphone amplifier circuit might have no trouble providing the voltage, current, or power required by low impedance headphones, its output impedance may be too high, and vary too much as a function of frequency, to provide good sonics when driving low impedances. That same problem sometimes arises in interfacing preamp outputs to power amp inputs.
If the headphone amp within the integrated amplifier has an output impedance that is a substantial fraction of the impedance of the headphones, and if that output impedance varies significantly over the audible frequency range (as may often be the case), the resulting frequency response will be significantly non-flat, as a result of the voltage divider effect. The voltage that the headphone amp "tries" to put out being divided between the headphone's impedance and its own output impedance. For a given output impedance, the higher the impedance of the headphones the less significant that effect will become.
2)Keep in mind that to produce a given volume level at the listener's ears, the power required by a set of headphones is VASTLY less than the amount of power required by a pair of speakers. Perhaps 100,000 times less, or thereabouts. Headphones commonly produce sound pressure levels of around 100 db in response to an input of 1 milliwatt (0.001 watts). Speakers having typical efficiencies that are listened to at typical distances will require, as a rough order of magnitude, something like 100 watts to produce that same spl.
So a hypothetical 32 ohm speaker, were such a thing to exist, would require vastly more current, voltage, and power to produce a given volume at the listener's ears than a 32 ohm headphone.
Regards,
-- Al