LEON Kirchner is the Renaissance man of the Music Department. Pianist, teacher, composer, conductor, he seems to be in constant metamorphosis. As contemporary composer, Kirchner has been awarded a Pulitzer prize for a work that combines sounds produced electronically with those of a string quartet; as teacher of courses for music concentrators, he is tireless in his efforts to spread the gospel according to Schoenberg. Yet Friday night there he was, conducting the Cantata Singers and an ensemble of Boston-area professionals in a program consisting solely of music by J. S. Bach.

Kirchner conducted the three cantatas with his peculiar blend of romanticism and objectivity. His own music displays a predilection for big chords and thick, lush sonorities, and this love of sound for its own sake carried over into his interpretation of the Bach. Kirchner demanded a full-bodied sound from his small ensemble. Occasionally his insistence backfired, as in the final chorus of "ewiges Feuer" (BWV 34) where the sopranos had to force and went noticably sharp. Most of the choruses were full of dramatic dynamic contrasts, crescendi and decrescendi. And Kirchner had no qualms about taking expressive liberties with the tempo.

Kirchner's tempi were often brisk and never slovenly. He used a baton when precision was called for, conducting only with his hands in the sections where he wanted more flexibility.

The conductor was as capable of holding his ensemble back as bringing them out. The tenor aria in "Herr, gehe nicht ins Gericht" (BWV 105) contains many rapid sixteenth-note passages in the instrumental parts. Tenor Karl Dan Sorensen displayed a voice that was light, supple and unforced, but nonetheless somewhat diminutive--potentially something of a problem in Sanders Theatre. But Kirchner kept the instrumentalists down to a virtuosic pianissimo, and in spite of the busyness of the parts and his own brisk tempo, the aria was a model of balance and clarity.

NOT all the soloists were as gratifying as Sorensen. Regulars Marsha Vleck and Jane Struss gave creditable enough performances but had relatively little to do. Struss's solo work in "Gottes Zeit ist die allerbeste Zeit" (BWV 106) had an uneasy, unsettled quality, probably the result of a case of nerves. Bass Francis Hester revealed a rich and well-trained voice, but his murky German detracted from his performance.

More disappointing were the two guest soloists, perhaps because of the attention drawn to them by separate billing and fancier stage protocol. In BWV 105 soprano Carole Bogard began confidently but was evidently unsure of most of the aria beyond the opening phrases. As the movement progressed she became increasingly dependent on the score in her hand, and while her opening phrases had been nicely shaped the rest was little more than competent reading. Still, she obviously had a good ear, enviable accuracy of pitch and a fair amount of vocal agility. Alto Eunice Alberts sang with the inertia typical of her voice range. Her aria in BWV 34 was a minor battle for tempo, she pulling back, Kirchner trying to move things forward.

The Cantata Singers are a good example of how a small, well-trained chorus can have fuller body and greater impact than one larger but less precisely controlled. Like all human performers, the group had its moments of weakness: sloppy ensemble, faulty intonation, a tendency (especially among the men) to expend force at the beginning of a phrase and then be unable to carry it through to the end. But Bach's voice writing, though beautiful, is quite impossible to perform, and the group has to be admired for its accomplishment. The performance was neither too fussily "authentic" nor embarrassingly emotive. It was simply musicianly.