The vigorous, brusque Cello Sonata in D by Beethoven, the lyrically adventurous Violin Sonata in A of Schubert and above all the Schubert Piano Trio in B-flat -- one of the world's greatest pieces of chamber music -- all sounded as fresh and new as the day they were made in last night's performance by the Smithsonian Chamber Players in the Hall of Musical Instruments.

Substantial credit for this goes to cellist Kenneth Slowik and violinist Marilyn McDonald, who gave their usual thoughtful performances: energetic without overstatement and carefully styled without ever seeming fussy or pedantic.

But there was a new element in last night's program (which will be repeated tonight): a Graf fortepiano, on long-term loan to the Smithsonian, built in Vienna between 1825 and 1830 -- precisely the vintage of the music. Played by Malcolm Bilson, who knows his way around the fortepiano as well as any man living, it has qualities midway between the 18th-century instruments that are more often heard today and the modern concert grands on which this music is usually played. Problems of balance between the keyboard and strings are virtually nonexistent with this strong but modest instrument; it blends with and supports the sound of its partners without ever overwhelming them. It might not be quite as appropriate for Mozart as it is for Beethoven and Schubert, but for this occasion it was ideal.