HRT music streamer & Airport Extreme

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Buff - I think you should check the specifications of the Airport Express.

The USB port on it is for printer output only, the only audio output available is from 1/8 stereo mini jack, either optical via a Toslink cable or analog with a 1/8 mini plug.
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Magfan, I'm sure you're right about the measured jitter from the Airport Express but with a good DAC the AE is sonically excellent and in situations where it provides convenience and accessibility to music it works really well.

The USB port on the Airport Extreme and the Airport Express is the same. On both of them, the USB connection is for a printer or USB drive only. There is no audio output.

Here is the language from the specifications page for the Airport Extreme on the Apple Web site, "USB 2.0 port for connecting a USB printer or USB external hard drive."