Classical guitars and big concert halls have an uneasy relationship. Play a guitar unamplified and you risk inaudibility. Amplify it, and the tone becomes unnaturally bloated.

The fine guitarist, Sharon Isbin, wrestled with both problems in a pair of baroque concertos she performed with the Zurich Chamber Orchestra at the Music Center at Strathmore on Tuesday. Her instrument, though only lightly miked, sounded tubby and unfocused during solo passages, and tended to become submerged when the full ensemble kicked in. She didn't help matters by playing, much of the time, near the fingerboard (where plucked strings sound their mellowest), or by keeping her dynamics on the soft side.

But with her immaculate technique, non-showy naturalness of phrasing and intelligent ornamentation, Isbin made the transcriptions of Vivaldi's Lute Concerto in D and Bach's Violin Concerto in A Minor sound apt and lovely. The orchestra wrapped her in a halo of graceful string filigree.

In fact, under Howard Griffiths's baton, the 23-member string ensemble produced a singularly lithe and transparent sound all evening. Mozart's Divertimento, K. 136, was an airborne delight, all silken violin tone and elegantly tapered phrasing. Frank Martin's "Pavane Couleur du Temps" had an enigmatic, coolly seductive beauty. And Bartok's Divertimento for Strings -- no less gorgeously turned than the rest of the program -- closed the concert with athletic vigor and wonderfully tight ensemble playing.

-- Joe Banno