Can anyone confirm for me whether I need an audio interface to get sound into my Mac from a regular dynamic mic (SM58)? I've connected the mic to the 1/8 input using an adapter plug, set up Garageband to connect to the built-in line input, checked this in the Preferences, checked that the Mac's Sound menu is reading the correct devices, but I still see no signal at all on Garageband's level meters... none. I was under the impression I could record an audio signal directly without any outboard a/d conversion.