In most chamber orchestras that claim to do without a leader the first violinists are in effect conductors. Not the Orpheus Chamber Orchestra, which doesn't even have permanent section leaders, democratically selecting the concertmasters and principal players for each work. Founded in 1972, this superb ensemble has remained a modern chamber orchestra, bucking the trend toward period instruments and performance style as it plays everything from Bach and Handel to Charles Wuorinen and Elliott Carter. It has also commissioned many new works and made almost 70 recordings, including a couple Grammy winners. For this concert the orchestra is continuing the Mozart birthday festivities with the overture to Cosi Fan Tutte and two wonderful piano concerti, the majestic 25th and the deliciously sweet 17th. The pianist will be Emanuel Ax. His recent solo playing of romantic music has been disappointing, but then he's always best when collaborating and when playing classical-era works--his Haydn sonata recordings are outstanding. The program ends with Mozart's Symphony no. 35, the Haffner, a youthful work that's gripping from the first notes. a 8 PM, Orchestra Hall, Symphony Center, 220 S. Michigan, 312-294-3000 or 800-223-7114, $21-$50.