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Actually, very low impedance will stress the little amps excessively. This is because most small portables and soundcards have an extremely low current delivery due to their tiny parts. In fact, small amps deliver far less current than even tube amps, let alone huge mains-powered SS amps. And the sound quality from such small amps will deteriorate rapidly when the transducer impedance drops below a certain point (this is due to the often woefully inadequate current capacity of their tiny power supplies). Combined with an inadequate voltage swing, the small amps are ill-suited for use with low-sensitivity headphones regardless of their impedance. (The 300-Ohm Sennheisers which are at the top of their corded dynamic headphone lines are of medium sensitivity, in my book.)

On the other hand, it is much easier to manufacture a high-sensitivity headphone with low impedance than it is to manufacture a high-sensitivity headphone with high impedance. Thus, when dealing with the tiny portables and soundcards, it is recommended to ignore the impedance rating, and concentrate on voltage sensitivity (in dB @ 1V) when choosing a headphone (this is because one headphone rated at 106 dB @ 1mW sounds far quieter than another headphone with the same 106 dB @ 1mW power sensitivity rating even though both of those headphones have the same impedance rating - one headphone might emphasize the upper mids and lower treble just to make the sound appear much louder than the other at the same volume setting).

I think we're overthinking this.

Let's say for Arguments sake that we have headphone A @50 ohms
and it takes 20% of the Amps total power to run them at a given volume.

Then headphone Headphone B @ 250 ohms. would require 100% of the Amps total power to achieve the same volume.

That's running the Amp 5 times as hard.
The current for the lesser HF's is faster but less of it is needed to achieve the same results



Not precisely but close.

Simple Electrical current principles.

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In this equation, then yes, the 50-Ohm headphone has a 7 dB higher power sensitivity than the 250-Ohm headphone. The result will be that the 50-Ohm headphone will have a much, much higher voltage sensitivity (14 dB) than the 250-Ohm headphone. Even so, the 50-Ohm headphone will actually draw the same amount of current than the 250-Ohm headphone at the same perceived loudness level (in a "perfect" amp).

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Only if the output impedance of the amp is very low and the phones have to have identical sensitivities.

Understanding Impedance All you need to know about impedance (exclusive section about headphones)

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Thanks for the link!

Interesting thread as I've just "rediscovered" my old K240 sextets. With my Mini3 amp I have to turn the volume up higher than I did with my Ultrasone HFI-780s. But I seem to be getting better battery life. So even though my K240s are less sensitive then the HFI-780s, the K240s are "easier" for the amp to drive. Is this basically correct?

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That is true - but only if the portable amp is constantly operating near its maximum current output capacity (which is constrained by the amp's tiny power supply) at normal listening levels when used with low-impedance headphones (such as the HFI-780). Otherwise, when a portable amp operates well below its maximum output current capacity, there is little difference in the battery life of an amp (no more than a few minutes out of over 20 hours) between low- and high-impedance headphones.

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Thanks for the great link.
And welcome to Head-Fi