The Magnetic Fields keep their ambitions high Even by the ambitious standard set by their 1999 masterpiece, the triple album “69 Love Songs,” the latest offering by the Magnetic Fields is an awe-inspiring piece of work: “50 Song Memoir” contains a song for each of singer-songwriter Stephin Merritt’s first 50 years on Earth. The collection, which is 2½ hours long, puts the frontman’s quirks on full display as it jumps through various genres and nostalgic touchstones — from the paean to his favorite dive (“Be True to Your Bar”), revisiting the joys of the disco era (“Hustle 76”) or taking an arch glance back to the height of the AIDS epidemic (“The Ghosts of the Marathon Dancers”).