Saturday-Sunday:  The Minnesota Bach Ensemble, which is offering its debut concerts this weekend, has long been a gleam in conductor Andrew Altenbach's eye. Devoted to the incomparable Johann Sebastian and his coevals, the group comprises 18 of the Twin Cities' finest musicians, most of them members of locked-out orchestras. Their enticing inaugural program embraces a sinfonia by C.P.E. Bach, solo motets by Antonio Vivaldi (with soprano Linh Kauffman) and two evergreens by J.S. himself: the C-minor Concerto for violin and oboe (with Jonathan Magness and John Snow) and the "Brandenburg" Concerto No. 1. (8 p.m. Sat. and 2 p.m. Sun. Sundin Music Hall, Hamline University, 1531 Hewitt Av., St. Paul, $30, students $25, 651-523-2459, or tickets@mnbach.org.)