This looks like a duplicate of my question here Global precision setting . Since I formulated it in a slightly different way, coming from Maple it might however be worth quoting the answer given by Mr. Wizard here. It worked for me in Mathematica 9 if there are no arbitary precision numbers in the notebook (defined as e.g. 2.`16):
$PreRead = (# /.
StringMatchQ[s, NumberString] &&
Precision@ToExpression@s == MachinePrecision :> s <> "`50." &);
3/1.5 + Pi/7
This will act on all later commands before the actual command is executed and set the precision of contained numerical values to 50 digits.