Hi Dody,
You don't give any details about what you want and why, so you have few replies.
1) What are you measuring and why doesn't it have its own regulation like an audio amplifier?
2) Line regulation?
3) Load regulation?
3) Ripple rejection?
Just a short piece of wire will drop more than only 2mV if it has any current in it.
A half-wave rectifier and a tiny filter cap will produce too much ripple voltage.