Are audio interfaces also headphone amps?

Say you have high impedance headphones that require a headphone amp. If you use an audio interface with a headphone output (which most, if not all, have), does that serve as a headphone amp?

For the sake of example take a basic interface like a Focusrite Scarlett 2i2, and a basic headphone amp like a FiiO E10K. If you have a 2ii2, would you also need the amp?
 
Well...basically anything that has a headphone output has some kind of a headphone amp, and interfaces are no different. The FiiO claims it's "suitable for headphones with an impedance from >16Ω", which means its output impedance should be around 2Ω - the rule of thumb is that the amp's output should be roughly the headphones' impedance/8. The 2i2's output is about 10Ω, so theoretically it's suitable for headphones with an impedance of >80Ω - but of course smaller impedance 'phones will work, they'll just be a bit quieter.

Another complication of this scenario is that the FiiO is USB only, which means it's really an output-only audio interface, and using it together with the 2i2 might prove to be problematic. You'd be using the 2i2 as the input device and the FiiO as the output device, but I doubt they'll work together (although on OSX you can create "aggregate devices" from multiple interfaces; dunno if something similar exists on the Windows side of things) if you want to use the 2i2's outputs for monitors, for example.
 
Thanks so much, that's helpful. Can you elaborate a little on this:

"the rule of thumb is that the amp's output should be roughly the headphones' impedance/8. The 2i2's output is about 10Ω, so theoretically it's suitable for headphones with an impedance of >80Ω"

So with a 10Ω headphone output, 10 X 8 = 80, so why do you say greater than 80Ω headphones would work? Say I want to power 250Ω headphones (something like Beyerdynamic DT880 250ohm) -- by the 8 rule wouldn't I need a headphone output impedance of around 30Ω?

I think I'm just confused by your "greater than" sign. :)
 
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