A realtime kernel is basiquely a vanilla kernel with Ugo Molnar's patch and the realtime-lsm module.
You must have some scipts too, in order to fix the realtime priorities when booting. The rtc (real time clock) must have the higger hardware priority, followed by the sound cards, one by one, followed by the other hardwares, one by one.
After, you must define the software priorities. For that, you must be in the audio group, and this group must have an higger priority as the other groups. So all the programs you will use will have an higger priority as the other, and that as a normal user. So, it is not necessary or recommanded to use jack as root.
A distro that have all that is Agnula-Demudi.
Otherwise, you can take a look at
Be aware at it can be problems with a realtime kernel and buggy hardwares as VIA chipset or even SIS chipset.