The function between amp and dac is different.

Amp will provide power to amplify analog signature (especially for high impedance or low sensitivity headphone) and decorate the timbre. A good amp will let the sound become lively.

DAC means "digital/analog converter". It can convert digital signal (010101...) which come from computer or CD into analog signal (sound). A dac with good D/A quality will provide good music details, big sound stage, neutral sound signature...

There many kind of digital input, such as usb, optical, coaxial, i2s...
If you just want to connect dac to your computer, can buy a dac which has usb input, like Fiio E10, E17, X3, Anue T1...

Some old dac just has optical or coaxial output. They are designed to connect CD transporter.
If you want to connect these kind of dac to your computer, have to buy to DDC (digital to digital converter) which can transfer usb signal to optical/coaxial signal.

This post has been edited by liquidgal: Feb 20 2014, 01:19 AM