Purists were uneasy at last night's concert by the Brandenburg Ensemble, which filled the Terrace Theater with conductor Alexander Schneider's rich-toned and romantically textured interpretations of Bach and Handel. Period instrument performances have become so much the rule these days, particularly on records, that when the ensemble launched into the Larghetto affettuoso first movement of Handel's Concerto Grosso Op. 6, No. 6, the sleek fullness of its sound came as a shock. But even the most style-conscious ear adjusts quickly when performances are as natural and expressive as Schneider's.

The Handel work, weighted down with slow-motion tempos and thick sonorities, was the work on the program that suffered most from this approach. But the three remaining pieces, all J.S. Bach favorites, were exhilaratingly successful.

The Bach portion of the program got under way with the D minor Concerto for Two Violins. Soloists Daniel and Todd Phillips balanced the almost florid manner of their approach to the central Largo movement with aggressively energetic playing in the fast outer movements, with exciting results. The same feeling of high spirits helped the performance of the B minor Suite for Flute and Strings to win out over balance problems.

In many of the tutti sections, flute soloist Marya Martin's fine, idiomatic playing was overwhelmed by the Mahlerian strings, and some of the contrapuntal writing in the first movement was so viscously toned that it gave the impression of being extruded rather than played. The concluding Brandenburg Concerto No. 3 was given such a spirited performance that the end of its third movement had to be repeated as an encore.