I was wondering if portable music players are limmited in their frequency responce. I have looked at the specs of several and saw "Frequency Responce 20Hz - 20 KHz".

Is this the Portable music player frequency responce or perhaps just the headphones it is bundled with?

If it is the former why did I buy headphones with 6Hz - 23 Khz responce if I cannot even find a player with 20Hz - 22 Khz reproduction?

Headphone specifications are mostly bogus. There is no way the specs hold with any reasonable thresholds (ie. +/- 3dB across the band). HeadRoom (http://www.headphone.com) has measurements of some headphones if you are interested.

Frequency response of most HDD players is fine, but some players suffer from roll-off at the low end (the bass frequencies) with low impedance headphones.

This is like saying that my car can accelerate from 0-70kmh in 2 seconds - it's true but I neglected to say you have to push it off a cliff to achieve it, hardly useful! The useful frequency response of your headphones is unlikely to be 6hz to 23kHz and even if it was you wouldn't hear the upper and lower frequencies because the human ear is (rarely) that good. You need response figures to be qualified with +/- decibel figures to have any meaning.

The frequency response of any resonable quality portable player exceeds that of the human ear, generally speaking. Notwithstanding that they often get supplied with inferior or ,at best, mediocre earphones...and then there's the codec and its settings to consider too.

As with any audio system though, best listen and let your ears be the judge...if it sounds good to you then do the figures actually matter?

This is caused by a small capacitor in series with the output, am i right?

That's right. Some players have a codec/headphone driver chip which allows usage without capacitors (iRiver H1xx and probably iPod Shuffle).

I may be stating the obvious here, or misunderstanding the question.
The frequency range of the device relies on the DAC and support components, not the storage media, be it HD, flash, or CDROM.

I realise it doesn't matter what the responce of the headpohones is as long as it exceeds 20Hz and 22KHz by at least half an octave either way for a smooth frequency roll off. However I was wondering if portable music players cannot reproduce high frequency well as I have seen only 20KHz. Any good pair of ears can hear a wee bit past 20 Khz.

But you can't.

Maybe you should test your own hearing limits. KikeG found this pretty unique sample that most people can ABX up to 17kHz. If you can hear a 19kHz brick wall lowpass on ony music then it will likely be on this sample. Yes, an artificial frequency limit is a worry, but it's less of a worry than you think - especially because you are likely to be listening to your portable in noisy environments. A -3dB point of 20kHz is pretty good performance for an amplifier that small.

Remember also that the frequency response of the amplifier in a portable is likely to be very dependent on how loud it's being played. Generally amplifiers in this sort of device are slew rate limited rather than frequency limited.

If you are still worried about it then buy one of the tiny portable headphone amps that are widely available. The high input impedence of an amplifier will cause the portable to behave better (especially if it has capacitor coupled outputs) and extend their frequency response downwards. You can get tiny portable amps that can drive nearly any set of phones extremely competently.

Interesting, thanks for the input.