Here is a really well done article (the best that I have seen) objectively testing portable music players. It is a very in depth analysis of the Sansa Clip+ and the much more expensive iPod Touch. The Clip+ starts at about $39.99. It is an extremely long article so here is a quick summary.

FINAL SCORE: I count 4 wins for the iPod, 3 for the Clip+, and 2 ties. That makes it a fairly even battle--impressive considering the huge price difference. But given many of the iPod's wins are likely an inaudible advantage, and the opposite is true for the Clip+ (the lower output impedance, higher output at less distortion, and better square wave performance), one could argue the Clip+ is more likely to sound better in real world use. This will especially be true with balanced armature type headphones.

SUMMARY: As stated near the beginning of this review the Clip+ turned in excellent performance for just about any portable player, let alone a really tiny one under $40. While the iPod has the performance advantage in several areas I'm not sure any of them will result in audibly better sound quality. I am, however, fairly confident the lower output impedance and higher low distortion output power of the Clip+ are an audible advantage over the iPod with many headphones--especially balanced armature types such as most of the Etymotic IEMs, Shure IEMs, Ultimate Ear IEM's etc. These all tend to have wider impedance swings which, combined with the iPod's roughly 7 ohm output impedance, will create audible frequency response variations compared to the Clip+ (see the earlier graph). The flip side of this is some might actually prefer the "less flat" frequency response they get with the iPod. And some argue certain headphone makers "voice" their affordable portable products for use with iPods.

http://nwavguy.blogspot.com/2011/02/sansa-clip-measured.html

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