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I believe the RCA outputs and line-out (3.5mm) jack function the same and people do plug headphones into the line-out jacks all the time and not have headphones short circuit.
No, they do not function the same. It's not the headphones that short circuit, it's the line out that is shorting.
The RCA outs are
line level outputs. They will have large output impedance and they are designed to drive a high impedance load, namely the inputs of a line level input. These are expecting to see something with an impedance on the order of kOhms where as headphones typically have an impedance a thousand time smaller. Specifically, the
M50 has an impedance that is 38 ohms which is a heck of a lot smaller than the 10000 ohms typical of line level inputs.
Do not drive headphones from a line level output. Even if it doesn't fry your card right away, it won't sound the way it should because of large output impedance in the output (tends to make the bass very very thin), and because it will draw more current from the output than it was designed for, which will shorten it's lifespan.
To sum up:
- Headphones need current and power that line level devices are not designed to provide.
- Most headphones have impedances that are hundreds or even thousands of times smaller than what the output was designed to see---effectively shorting the output.
- The line level outputs have high output impedance that results in poor sound quality
Don't connect headphones to a line out.
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But I still think getting something designed to power headphones (like a USB/DAC/amp) is a better choice.
This is solid advice! Connect them to an output designed to drive them!
Cheers!