Due to space limit,i want to use as little less amount of devices as possible, use pre-regualtor and regualtor plus add on components will take too much spaces.

I would also use a preregulator, especially if you want extremely low noise! As noise increases with die temperature toasting your regulator with 0.625W heat will not result in extremely low noise.

By the way, you specified noise as lower than 50µVeff. Please tell us to what bandwidth this is refered.

If I remember correctly the lowest noise is achieved with good old LM723. A LM317 is not necessarily a good choice if low noise is wanted. I have observed several times, that noise level of LM317 is much higher than that of 78XX standard voltage regulator. In any case the use of this filter capacitor from adjust pin to ground is of special importance!

But also by the help of simple RC low pass filtering using a big electrolytic storage capacitor noise can be decreased dramatically. Another option, if the 'regulated' voltage may change some %, is to filter out noise by the help of an active transistor stage, which is often used with moving coil amplifiers. But here again a preregulator is of benefit.

Kai