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Measurements show that one of the problems is lack of bass using normal 30 Ohm headphones (e.g. like the one supplied with the iPod !!!!). Because the line output seems to have a decent quality without these problems, the harsh sound is obviously caused by the headphones output. I'm wondering how much this has to do with the new electronics of the
Wolfson chip, or whether a too low quality/low value capacitor has been applied in the output (220uF as shown by
our measurements, and as
adviced by Wofson (pdf)). The low value capacitor output could easily be compensated by an equalizer setting (even matched to typical impedances of headphones), but the iPod doesn't allow to download custom equalizer settings, and Apple doesn't provide the right one, which is a missed opportunity. I've posted a
message on an iPodlounge forum, asking people
to report about the issue, so that Apple takes the problem seriously, and solves it by an obvious patch in a new version of their Software Updater.
Another problem could be bad design of power supplies, crosstalk due to PCB design, or use of bad components. This might induce intermodulation distortion, which is hard to prevent.
Matching the iPod with a pair of suitable (read warmer sounding) headphones compensates a bit for this phenomenon, but doesn't solve it."