just bought a pair of real 650's with a scarlet 2i2 solo and it sounds pretty low. I read on the internet that I'd need to amp it. But my question is, I bought an audio interface because with Daws that's what it takes, to deal with the latencies of the vst and stuff like that, so I'll need to plug the amp into the audio interface using the audio output of the amp right ?
Well, this is not an electronics forum, but I am an audio electronics engineer as well as a musician. I'll try to keep technical jargon to a minimum.
The HD650 is a great pair of cans, but it is quite high impedance - 300 ohms. That basically means it needs lots of voltage swing to get volume. (You can get headphones in a range of different impedances, 8, 16, 32 ... and other. 300 is almost as high as they go.)
The problem with USB-powered interfaces is that USB only gives 5V power. Unless there is an internal circuit to boost the voltage (called a DC-DC converter) that basically puts a hard limit on how much voltage swing is available. This is confirmed by Focusrite's own specs, which say that the maximum headphone output is +7dBu, which is 4.9V pk-pk.
(Compare this with what you would have on a pro mixing console, which would be about +20dBu, about 30V pk-pk. Much higher. And this is the volume of the absolute loudest peak that can get out of your system - your average level should be quite a bit lower.)
So basically you have a combination of factors which limits your maximum volume.
What can you do? well there are two possibilites:
1) buy lower impedance headphones, or ..
2) use an external headphone amp that is powered by more than 5V - one that will give you a lot more output voltage swing. Even a CMOY amp (powered by a 9V battery) would help. You could run the headphone out of the 2i2 into this.
Nothing changes in the way you use the Scarlett with your computer and DAW program. All you need to do is amplify the Scarlett's audio output a bit. As others have said, the 5 volts available to a USB-powered device will only pump just so much power into 300 ohm headphones.
If you buy an amp specifically designed for boosting headphones, you'll doubtless need to take the headphone output of the Scarlett into the input of the amp.
If you use a general-purpose amp, maybe one designed to run a hi-fi system, maybe you'll take the Line Outputs of the Scarlett into a pair of Line In or Aux In sockets.
I choosed to refund the scarlett t2i and buy a clarrett 2pre instead. as said here: https://support.focusrite.com/hc/en-gb/articles/210708269-What-impedance-headphones-should-I-use-with-my-audio-interface-
Clarett serie can support up to 600Ohm headphones. So instead of buying an amp and connect to something again I'll just upgrade my interface.
Thank you so much guys for all your answers, I learned so much, really appreciated !