I am a hard-core audio enthusiast and have been through a lot of equipment both high end and consumer average. I service smartphones full time and study electrical engineering and audio processing. Unlike most Audio experts, I understand voltage, current, and impedance. I have not thoroughly tested the audio performance of all the Apple devices therefore I cannot speak for most iDevices.
I have had a new iPhone 5s for about 4 months now and I am very impressed with it's audio performance. I have a collection of headphones, dac's, and a few CD players and I must say what comes out of the 3.5mm phone jack on my 5s is not the BEST I have ever heard in my life, but pretty damn close. I have tested it on numerous headphones with familiar recordings along with some oscilloscopic tests and am thrilled with the results. I have to say it's the most musically accurate smartphone to date.
I have 56GB of my cd collection ripped to ALAC on the device and originally planned to use it for on-the-go only, however, I find myself using it in my living room as well.
The output is 1VRMS and has an impedance of 4ish ohms. It only drops about 7% with a 16ohm load connected (power hungry headphones) and can drive almost all of my headphones to tectonic levels with minimal distortion. My Sennheiser HD-600's are high impedance (based on higher voltage) and require more than 1VRMS to get them to a "rocking out" level. The iPhone 5s (based on low impedance headphones drawing more current) outputs enough voltage to drive them to a decent level with most music, which blows away almost every smartphone (and quite a few portable audio players) I have tested (except the HTC One M7 and M8) which are louder, but have higher distortion levels and a questionable frequency response.
The iPhone does down-sample and up-sample to 88200/24. Contrary to popular belief, the deficiencies of this reproduction is far from audible to the human ear. The dac chip in the device IS capable of 192000/24 and obviously 192000/24 is better, however the conversion to this requires more cpu usage in order to avoid conversion distortion than 44100/16 to 88200/24 since 88200/24 is exactly double 44100/16. This can be done with minimal buffers and calculations per second while still maintaining optimal sound and avoiding the deficiencies of 44100/16 being doubly present. The benefits of anything above this is pretty much inaudible.
In my opinion, the various deficiencies in analogue performance of different headphones and amplifiers, even those of high end ratings, should be questioned long before the listener should consider anything above 88200/24.
CD audio is a lot better than people think it is. It has an extremely low noise floor and spectacular frequency response. When played from a good CD player, the sound is audibly flawless.
Ripping a CD to ALAC (Apple Lossless) or FLAC (Free Lossless Audio Codec) is just like ripping to .WAV except uses the same compression technology as a ZIP file therefore has the same advantage of smaller file size at deeper digital compression level that does not affect the audio stream. Instead of us "extracting" or "un-ziping" these files and then playing them, the decoder/dac can "unzip" these files as it plays them resulting in the same PCM CD- Audio file we started with but a smaller file size as if it were a zip file. iPhone 5s 64GB with a **** load of CD-Audio anyone?? ..I'm smiling
The iDevice's DAC takes this PCM format and decodes it using twice the resolution (88200) and twice the bitrate (24 bit) This results in a transparent 44100/16 reproduction.
The problem we have in cases where Vinyl sounds better than CD (even though technically Vinyl had a wider frequency response) is where the CD version has been re-mastered. Most re-mastering is not done correctly and uses compression and dynamic range limitation to flatten out the response of the recording which results in a ridiculously loud reproduction and poor dynamic range and audible clipping. I believe this is to blame for the misconception of the quality of CD audio and why "music doesn't sound as good as it used to"
If I had 3 wishes, I would want $1,000,000 (duh), the best headphones of the year 2050 and a copy of "Night Visions" by Imagine Dragons - Mastered properly without the loudness war.
Ok, back on topic now........My conclusion is that there are portable audio devices out there that are superior to the iPhone/iPod. However, if you need a portable audio player with superb sound AND a built-in phone, the new iPhone 5s (and without a doubt, the 5 and 4s as well) are definitely the way to go!
