Stars hire Rick Rubin for one thing: to achieve a purity of essence. When it comes to singers like Johnny Cash or songwriters like Neil Diamond -- even rock bands like Weezer -- Rubin’s approach feels natural, as he strips away the varnish that’s clouded over the years, but Josh Groban is an odd choice for the Rubin maneuver. Not a singer/songwriter in the classic sense but rather a classical singer who dabbles in pop, Groban doesn’t need to be stripped to his essence because bombast is a crucial part of the equation. Smart guy that he is, Rubin doesn’t eliminate surging melodrama from Groban’s bag of tricks but he does limit and mute the palette, making sure the strings don’t soar so high and that the singer doesn’t bellow. These subtle adjustments, which include hiring Semisonic’s Dan Wilson as a co-writing partner for Groban, wind up having a drastic effect on the flavor of Illuminations, turning it into something warm, burnished, and intimate, a spell only briefly broken by the cacophonic clatter of the Portuguese “Você Existe em Mim.” Its presence is a testament to Groban’s curiosity but Illuminations is best when he plays close to the vest, which means this is another instance where Rubin succeeds by illuminating his subject’s essence.