Originally Posted by elfary
If accurate measurements can not explain the quality of an electrical signal. What else can? Gifted ears may be?
Excuse me but pointing out that a player with rolled off bass, middle of the road resolution and 5 ohms of output impedance (iPod 5.5) sounds better than a linear source with less than 1 ohm output impedance (iPhone 4) would not hold in front of any professional mastering engineer.
But what do i know.
I think you are mistaking some of the measurements presented.
For one, the "frequency response" graph is made by sweeping a tone from low frequency to high frequency. While this generally shows how a headphone or source responds to a linear tone, it doesn't really show what would happen when there are chaotic frequency shifts, like with most songs. Unless you enjoy listening to white noise and tone with linearly increasing frequency, I think frequency response graphs should be taken as a general idea of how a source, or headphone, compare to another... and not necessarily in how good its sound quality is.
A DAP may measure completely flat on the response graph, but then when you measure its power specs, it'll still fall short and won't be able to help make more power-hungry headphones and earphones shine. That's where you'll hear bad sound quality from an iPhone 4. Otherwise, why would portable amplifiers like the Fiio E5, E11 or Digizoid ZO improve the sound quality of the iPhone 4 so much?
And I also think the output impedance situation is blown out of proportion. While it's true that higher output impedance is undesirable, especially for matching with low impedance headphones, it has been found, in general, that an output impedance lower than 1/8 of the headphone impedance should not have any significant impact on transient response... since the voltage drop at that level would at most result in a <1dB drop in hearing level. Or in other words, any distortion caused by high output impedance should not be audible if your headphone has an impedance 8 times bigger than the output impedance.
As such, 5 Ohm output impedance should cope with 40 Ohm headphones just fine. The rest then depends on how well the internal DAC and amplifier handle your songs. And I'm sure you can ask around and see if the iPhone 4 or 4S would win any award against the iPod Video when plugged in to a 40 Ohm headphone. I'm sure you'll find that most will prefer the iPod Video 5.5G.
And for the iPhone 5 comparison I made earlier, I didn't use 8 Ohm IEMs to listen to it. I used my Audio Technica ATH-ES10, with 42 Ohm impedance. It should rule out any problem with output impedance. Unless the iPhone 5 output impedance is higher than 5 Ohm... but I sincerely doubt it. Not to mention headphone impedance may swing up higher than the rated value depending on the frequency, so the damping factor would still be higher even if the output source has high output impedance.
Also different headphones react to high output impedance differently, so some may not be affected by high output impedance at all. Despite that, the 1/8 rule is a good one to adhere to because it leaves no assumption, and you can rest assured knowing that if there is any shift, it wouldn't be audible.
Edited by Bill-P - 10/4/12 at 7:57am