TL;DR
Should I get one? Probably not for 300 Ohm headphones.
An Analysis of Impedance
Below is a conversation I had with a friend who asked me if I used a DAC for my HD 650 mixing headphones.
I do have a DAC (Apogee One) and was experimenting between using it and my MacBook/iMac’s output. Kinda can’t hear a differece yet in audio quality.
The output impedance (of the computer/audio jack) must always be lower than the consumer’s (headphone’s) impedance. This priciple also true in live sound: when plugging a mic into a mixer, the mic’s impedance must be lower than the mixer’s receiving impedance—otherwise you use a DI box.
The rule of thumb for this ratio is a factor of 8. Most consumer headphones have an impedance of 16-32 ohms, so the output impedance of the MBP’s/iMac’s jack is probably 2-4 ohms.
If the ratio is not safe—suppose the MBP jack had an output impedance of 10 ohms—then the frequency response will be altered in the headphones. This could lead to a different spatial response/altered EQ/etc. and depends on the actual headphone design.
In comparison, the HD 650 has a nominal impedance of 300 Ohms — so it’s ratio is 100x! Quite safe from distortion there.
What you/I will notice is that for the same volume setting on the laptop, the volume will be sofer on the HD650s — because of the nearly 100x impedance ratio. It requires more power to drive it. But as long as it’s loud enough, I think you will be fine.
Hi end mixing headphones have high impedances for the same reason high quality guitar mics have high impedances — they have more coils in the wire, allowing for a better high frequency response. Having the high impedance also makes them conistent—no matter what they’re plugged into, they easily meet the rule of thumb of 8x, so the sound will be identical with no distortion.
Headphone amps work the other way: they basically create a near zero output impedance so that no matter what headphones you use, there will be no distortion.