Looking at simple set-up for iMac to preamp

3.5 mm phone plug to stereo RCA cable. You can get this from Radio Shack (no affiliation whatsoever). Plug the 3.5 mm plug (like a mini headphone plug) into the back of the Imac, plug the RCA plugs into pre-amp or integrated, adjust gain at the iMac to approximate the same volume as cd, and voila'. You could probably pay for a fancy interconnect if you've got money to burn ;)

Does the iMac have a headphone jack? If so, and you said you wanted simple......

Most simple: get a mini to L/R rca plug (Radio Shack or?). Mini goes into the iMac headphone jack, the rca's go into an unused input jack of your preamp or integrated...Your done.

Next in line but better sound: The headphone jack on our Macbook Pro is both analog and optical. I would think it would be the same on the iMac. Use the headphone jack again but go optical out with a Toslink cable (mini adapter on one end) into a DAC and then into your pre.

Next and maybe the best: You didn't mention how far your iMac is from your pre. Near or far, Apple's Airport Express (around $100) could be the cleanest way. Check Apple's website for more info or maybe someone here can chime in.

I see Ths364 posted while I was typing mine. Ya got me :-)

The MAC optical outputs are great. You'd have to add a DAC, however.
IF the confuser is located away from the stereo, you could use an Apple TV or an Airport Express, either of which can be run in 'client' mode...that is, without a wireless network. I do that with my iMac and run the AE as a 'dead end'.
Minimum additional expense would be maybe 500$ for the AE and a DacMagic.

Get one of the HRT Streamer dacs. Simply hook the Imac via USB to dac and rcas from dac to pre.
I did the same just to listen to internet.