Portable USB headphone amps

New model Android smartphones have USB audio onboard.
Connect a USB DAC/Headphone amp if the onboard audio of the smartphone is not up to your taste.

Likewise if sound quality of the onboard audio of your laptop leaves something to be desired for.

 

Small PCM and DSD capable USB DAC

63.5mm (2.5in) long, by 31.7mm (1.25in) wide and 19mm (0.75in) high.