There are several ways to get the audio input into a Mac. I got a Sony turntable (low-end model) on sale a while back, but it DID have USB output as well as the regular line level output. It did not say it was specifically Mac compatible and came with Windows-only software, but the USB connection worked fine with audio recording software I already had. Mac OS X recognized it as an audio input choice in System Preferences
Sound pane
Input tab.
If the turntable provides regular line level output (that does not require a pre-amp), you can convert from the pair of red/white RCA connectors to a stereo mini-jack using a commonly available adapter and connect it to the audio input port on the back of the iMac.
If the turntable provides output that requires a pre-amp, your can first connect it to something provides the pre-amp function (such as a stereo receiver), or connect it to a Griffin iMic.
http://www.griffintechnology.com/products/imic
Once you have the sound coming into the iMac, you can test-listen to it through the iMac's speakers using a simple program called LineIn
http://www.versiontracker.com/dyn/moreinfo/macosx/18987
which +plays through+ the audio input to the iMac's speakers.
To record the audio input, you will need to use an audio recording program. Once you are running an audio recording program, you no longer need to have LineIn running because the recording software should provide a play through function.
There's a free program called Audacity, but when I tried to use it before, I found the interface somewhat difficult.
iMic comes with a basic recording program called Final Vinyl
http://www.versiontracker.com/dyn/moreinfo/macosx/16442
It may be usable with any audio input, even if you don't have a Griffin audio input product.
Another one is WireTap from Ambrosia Software
http://www.ambrosiasw.com/utilities/wiretap/
I have an older version. It works well and is easy to understand.
You should be able to save the initial recording as AIFF, which is a lossless format.
