Audio In socket on Macbook question

Hi

I have a question: I tried to connect the keyboard's audio out (L/R Mono - Korg) to the Audio in on the Macbook. When I pushed hard, I have sound but a nasty and loud ground noise.(Garageband Audio track)
I unplugged it a little (1/8 " mini plug) and it worked: is that the way the Audio in slot is meant to work or is it a faulty one? This was the FIRST time I've attempted to plug into the Audio slot.

Thanks

oculus42 said:

Audio In is Line In, not mic in. You might need something like the Griffin iMic, which I use.

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Hi,

I didn't mention a mic. For my use, the interface you suggested doesn't have Phantom power for serious recordings. (condenser mics)

I understand that it's a Line In on the Macbook, that's why I'm plugging a Korg synth on it.

Thank you anyway.