Has anyone gotten there's yet and compared the sound quality (especially with older models)
To my ears, the headphone out of the iPhone 4 is considerably better than that of the 3G. I never owned or listened to a 3GS.
One of my biggest complaints of the 3G's output was that it was pretty shrill and grainy up top - and the effect was not subtle. The 4 still has a "live" sound, but the treble is a lot less grainy and somewhat more subdued. Generally, there's a lot more blackness/space between notes, and it images better. There seems to be a lot more headroom power-wise, too. Through my Ety HF2s, it doesn't give up much to my desktop rig. It even drove my SR225s admirably. I have heard no objectionable hissing or interference yet. I'm very impressed.
Something they didn't fix is the large volume steps.
Certainly, but I rarely feel like lugging along two devices during my daily routine, no matter how small the second is. The entire point of the iPhone, to me anyway, is convergence. I can use the iPhone and the hf2s while walking from place to place, sitting on the bus, or while riding my motorcycle. I can just wrap my hf2s around my iPhone and drop them into my pocket when I'm not using them. And honestly, the iPhone headphone out sounds good enough without an amp to be serviceable while at work.
FWIW, I have a little D3. It's about as portable as anything out there, and it sounds good ... but it rarely sees use outside of long flights. I just can't be bothered to mess with an extra cable and an amp, however portable, more than twice the size of the iphone (volumetrically). Luckily, the iP4 output is decent enough that it isn't a huge sacrifice. I can deal with the volume stepping. It's not a big sacrifice in the name of convenience and traveling light. :)
I plugged my Fidelity Trips in to one today briefly for about 15min. and completely agree with Dinglehoser. I too am used to an iPhone 3G out.
What I would love to know is how much you would need to spend on a portable amp (line out or digital out from the iPhone 4) in order to get better sound quality.
We must also take in to account burn-in of the dac and headphone amp in the iPhone 4. no-one will have put more on it than about 1-48 hours on it so we don't know how it will sound after the roughly standard 300. time will tell. keep impressions coming though!
another comparison would be iPad out vs. iPhone 4 out - anyone able to do that?
Just a quick note as I've just started listening through it. Through sensitive IEMs it has a much much lower idle noise level in comparison to my 1g iPod nano, Macbook Air, and Thinkpad T61P. I had a 3GS last month but I don't remember what the idle noise level was like in comparison.
The iPhone 4 outputs more radiation and consumes more power just to accommodate its new antenna to prevent dropped calls. This probably would make it noisier. That and the glass front and back panel can't help w/ external interference.
To anyone of you who get the new iPhone 4 - bless you all!! I can't wait till Sept so me and the Mrs can get ours. And the fact that the SQ is improving (no more distortion while using the EQ? EXCELLENT!!!) is a godsend. Plus, the fantastic battery life really makes the smexy beast a real winner!!!!
Apparently the 4 uses some new Cirrus Logic integrated codec/amp I'm not familiar with - the same one as they use in the iPad. I think it's a newer version of the one in the 3GS. By contrast, the 3G uses a Wolfson device. It's not surprising that the characteristics of the 3G and 4 are so different, while the 4 and the 3GS seem to be sonically similar.