At 72 Ilja Hurnik is the grand old man of Czech music. Though little known outside his country, he's taught for a long time at the Prague Conservatory, where his rather conservative taste and relatively tonal style put him in the good graces of the communist cultural commissars. As a concert pianist, he studied with Janacek, a fellow Moravian who shared his passion for folk music, and piquant folk rhythms dot much of Hurnik's more distinctive work, including film scores and operas. His Sonata de Camera (1954), premiered here a few seasons ago by the musically astute Rembrandt Chamber Players, is a fine example of an earnestly folksy Eastern European romp. Its popularity with the audience prompted the Rembrandt to commission a work from Hurnik for the group's unusual instrumental mix (Sandra Morgan, flute; Robert Morgan, oboe; Sharon Polifrone, violin; Keith Conant, viola; Barbara Haffner, cello; and David Schrader, piano). Variations on a Theme by Pergolesi, the 25-minute piece the group received in the mail last September, revels in folk melodies and Baroque conventions. They'll perform it for the first time this weekend, along with Quintet for Flute, Oboe, Violin, Viola, and Cello by Mozart wannabe Franz Sussmayr and the Piano Quartet in D Major by the father of Czech romanticism, Dvorak. Pianist Frank Weinstock, a Cincinnati Conservatory professor, subs for Schrader, who's on sabbatical. Sunday, 3 PM, concert hall, DePaul University, 800 W. Belden. Monday, 7:30 PM, Lutkin Hall, Northwestern University, 700 University. 708-328-2492.