Connect an amplifier (Rotel RA 12) to the mac mini with USB plug?

Question

Is it possible to connect the mac mini with an amplifier via usb?

Answer 1

Hillbille
I have a Mac Mini and a Rotel RA12.  As you have found out, you can use the optical out from the 3.5 mm output jack to act as an optical input on the RA12.   This option works fairly well to give you a pretty clean sound.  However, there are other options available to you.
One option is to use the headphone out as an analog signal using a tradtional 3.5 mm to RCA (split) cable, and running it to the analog inputs on the back of the RA12.  This will provide sound, but not quite up to the quality you will be getting using the optical out.  The reason being that the music signal is being processed by the internal DAC on the Mac instead of the better DAC in the Rotel.  The other obvious reason is the quality of cable is typically not up to snuff using that route (although, you can get some high end 3.5 mm to RCA cables).
Another option would be to use USB output from the Mac Mini and hooking it up to the USB input on the front of the RA12.  This is how I play music from my iPad mini through my Rotel and it works well.  Again, however, you are using the DAC of the Mac equipment instead of the Rotel and are limited as to what type of fileyou can play. For example, you can't play FLAC or 24 bit 88kHz files. 
A third option, as someone already mentioned, is to get a USB/SPDIF converter, then run a COAX cable to the back of the Rotel RA12.  Depending on the quality of the converter, this is your BEST option for audio quality.   Using something like an M2Tech Hiface 2 converter will provide a high quality, jitter free signal between the Mac and the RA12. 
Within that third option are two "sub-options" for music playback.  You can use iTunes to play back the music through the USB/SPDIF converter directly, or you can use a third party application that will bypass the volume control of the MAC to give you a much purer signal.  An example would be Amarra or Audirvana. 
If you choose to use iTunes only, the sound will still be "okay", but you will not be able to play FLAC files or SACD ISO files.   You also won't have much control over the signal processing.   Either of thrid part applications I mentioned will give you the option to play other audiophile grade files, and give you a far better sonic experience compared to iTunes, and they can also be integrated with your iTunes library.  They will also allow you to utilize Direct Integer mode, upsampling, etc ... so that you can have a 44.1kHz file upsample to 192kHz at the Rotel, or have it upsampled to 192kHz in the Mac (using the 3rd party software) before it is sent out to the Rotel.  It just give you much more control over the signal. 
The last option option above will give you an audiophile grade signal that will allow your Mac Mini to be used with very high end audio equipment with no comprise to the sound.  It will rival the very best of CD or SACD players on the market.  It also sounds incredible and you can use it in conjuction with the Apple remote app to control your music through an iPad.