iPod measurements

This is really interesting, thanks for making these available.

So it seems that the well known bass roll-off from the iPod headphone amp only occurs with low-impedance headphones and can be minimized by using higher impedance phones.

Although it is disappointing to see that even with an 80 ohm headphone like the HD25, that the roll-off is still so significant.

Interesting. What're the impedences of those 'phones?

Would testing it against incremental loads be useful? (E.g., 30 ohms, 50, 75, 100, 125, 150, etc.)

so how about this: if you hook up the headphone out to a portable amp (something like a cmoy) with a short cable, does this provide load on the ipod headphone out? i dont know much about how amps work, but i was just thinking, if you were to use it with an amp like this, the bass roll of will not exist... true?

a cheap cmoy, or a diy one then can be used to solve this issue for those who are bothered by the bass rolloff.

What is even more disturbing to me than the bass roll off on low-z cans is the terrible THD and IMD results for those, not to mention channel seperation. Funny, considering the most commonly used cans with the iPod are low-Z.

Yes, this is something that I truly don't understand. I really doubt that the Apple people were picturing their "hip", "trendy" customers going around with HD600s in the San Francisco subway

I wonder if this low-z problem is found in other DAPs?

At the same time, I'm surprised that with such an apparent bass roll-off there aren't more canalphone users advocating the use of an amp w/ the iPod. It's becoming more and more common to read the type of comment that goes: "amping the iPod doesn't make a big enough difference for me, so now I use it ampless for more portability". And then they talk about "Team Minimalism" and stuff like that...

Pretty much all DAPs use 220 ohm coupling capacitors and have the same bass roll off. Due to space issues, engineers are limited in the size of the capacitors they can use. However, it's not as severe perceptually as the measurements seem to indicate.

One of the reasons Etymotic designed the ER-6i was to counteract this rolloff by building in a natural bass emphasis.

By the way, here are Stereophile's measurements of the iPod. They're unloaded, but much more accurate as they're done with Audio Precision gear:
http://www.stereophile.com/budgetcom...34/index5.html

(As an aside, you can compare these measurements against the RMAA results to see just how poor RMAA is. It misses most of the interesting stuff. It's a shame there aren't better free software analysis tools out there.)

You need to compare oranges and oranges, not apples and oranges. Stereophile should have done measurements under simulated or standardized load(s), as they do with power amplifiers, but their results are much more accurate for the unloaded case.

RMAA is indeed poor (though tolerable for the price). Where is the DAC's rising noise floor with frequency in the RMAA measurements? The RMAA noise floor actually decreases with frequency! It's totally inaccurate. RMAA's THD+N graph in the unloaded case misses the second harmonic by nearly 60dB! This is very poor accuracy.

It's not just this one test. As Mr_Radar's RMAA tests on his Toshiba's digital out showed in another thread, RMAA shows a flat frequency response, when in fact there's an IIR filter running causing a significant dip in his Toshiba's frequency response.

I wish RMAA was a better tool, but it will never improve if people don't point out its weaknesses. Your claim that "I've got my RME Digi96/8 PAD calibrated by 'real' measurement equipment and I know how far I can trust the results produced by RMAA" is just pure BS. You don't have any idea how far you can trust RMAA. It's a black box which generates unpredictable measurements.

As for your remarks about "huge LF roll-off" and "enormous distortion", people need to remember you're in the business of selling headphone amplifiers. The LF roll-off, with the worst case cans, is down -3dB at 32 Hz. While this is significant in some cases, it will be hard to detect with Grados and 32 ohm earbuds since they have practically no low bass anyway. As for "enormous distortion", give me a break. Worst case, at 20kHz, the distortion is -80dB down. Enormous? That's very decent measured performance for an op-amp output stage, on par with what you'd expect from the RA-1.

By the way, Antonik, please follow the rules of this forum and contact Jude so that your profile will be identified as a commercial vendor. If you're going to go around being alarmist and misrepresenting a few measurements, people should be able to clearly see you're in the business of marketing headphone amps.

Did a quick test with H120 using the lowly ADC of chaintech av710. I guess the card is ok for testing frequency response (results quite flat response from the sound card's DAC which has been measured quite flat with better ADCs). Seems that iRiver doesn't suffer from bass roll-off with headphone load.

Looking at the Crosstalk figure, it is now confirmed that my previous finding on iPod poor performance on headphone output.

I did posted before at HERE and HERE (5th Post).