Less amazing is the sound quality of the songs after they have been digitized. You may not notice it when listening in the gym or on the bus, but if you hook your i-device into a quality stereo, the shortcomings show.

The digital-to-analog converters in most devices are pretty rudimentary. In phones and portable computers especially, there is a need for small-size chips that are stingy with battery power. But devices like the iStreamer up your iPod’s game because they can contain larger, better chips with nothing to do but handle music. They can afford to be power hogs because they plug into a wall socket.

We set up an iPhone on an iPod dock and ran an iPad through the iStreamer to the same receiver and speakers. The results were a little surprising.

Listing to rock tunes recorded at 192 kbps, the iStreamer provided a very noticeable improvement. "It is definitely fuller sounding," said Mr. Dorsey, while listening to "All Right Now" by Free. The unaided iPhone was "thinner with less bass," and the soundstage was more open, meaning the music seemed to come from a physically larger area.

Most high fidelity DACs I’ve seen run $500 or more, so the iStreamer is comparatively inexpensive. Still, it may not be for everyone. If you aren’t using a setup with reasonable fidelity, you might not hear the difference. And if you like your tunes on the dramatic side, in some cases the iStreamer may make your music seem understated, though accurate.

The maker of the iStreamer said my test may not have been the best trial of its capabilities. To really get the full advantage, I should have used an uncompressed digital recording and compared that to a CD. We’ll find out when I finish building a music server from an old computer