"ATP Systems makes a couple of headphone amplifiers. I bought the Xenos OHA-R mini portable headphone amp for use with my iPod along with the AC Adapter and the rechargable 9v battery. The portable amp is an amazing addition to any iPod increasing the clarity and volume, not that the iPod needs more volume, but the clarity is significant. I'm using the amp with the Etymotic 6 ear buds. More significant was the improvement that the OHA-R made on my Grado 80 headphones and my McIntosh system. It transformed the Grado's from relatively dull and lifeless, improved the volume, clarity, definition and depth immeasurably. I was amazed at how well an $80 (including shipping) portable headphone amp would work with a high end system like the McIntosh and the Grado's. I am now eager to check out their other headphone amp and make it the dedicated headphone amp for my McIntosh system. One minor issue with the portable amp is that the belt clip likes to pop off. Other than that, this is an acoustically GREAT little unit that no one has heard of from our Canadian friends at ATP Systems. One other note - shipping is a bit longer because these units come from Canada, so be patient, it's worth the wait. "