By Joshua Kosman

Playing the music of the 18th-century masters on instruments they themselves might have recognized, and perhaps even at tempos they might have heard, is a far more common phenomenon today than it was a quarter of a century ago. The entire movement surrounding the use of period instruments—call it “authentic” performance, or historically informed performance, or what you will—has morphed from a fringe activity into a specialized but integral part of the musical landscape. Performances of music from the Baroque and Classical periods are now as likely as not to be played on replicas of older instruments, or at least to be informed by historical ideas about phrasing, texture, and tempo.

Yet even today, few performing groups bring to this repertoire the sort of gusto, suavity, and spirit of adventure that mark nearly every endeavor by the Philhar- monia Baroque Orchestra. Under its longtime music director Nicholas McGegan, the 23-year-old organization has carved out a niche as the nation’s liveliest and most dexterous purveyor of period performances. Playing as many as 50 concerts annually in the San Francisco Bay Area and throughout California, Philharmonia has infused the historical-performance enterprise with a vitality and zest that are worlds removed from any sense of dull conformity to the dictates of scholarship.

Much of the group’s success has to do with the uncommon brilliance of the instrumental playing. The string players in particular muster a clean, forceful ensemble sound that boasts all the tonal richness of gut strings without the tuning problems that all too often go with them. Woodwind and brass sections are populated by musicians who have figured out how to exploit the coloristic possibilities of their instruments while avoiding the attendant technical pitfalls. In recent years, the Philharmonia Chorale, under director Bruce Lamott, has developed into a cogent and immensely responsive chorus.

And presiding over it all is McGegan, the lithe, diminutive, and tirelessly energetic Englishman who has done more than anyone to give this orchestra its distinctive artistic profile. Whether tenderly shaping the vocal lines of a Handel oratorio—and Handel’s music remains the group’s greatest strength by far—or zipping through the airy measures of a Vivaldi concerto, McGegan’s conducting is witty, rhythmically buoyant, and full of vibrancy and nuance.

Perhaps it is no surprise that Berkeley, always a hotbed of both early music and fearless experimentation, should have given rise to such an ensemble. Founded in 1981 by harpsichordist Laurette Goldberg, the group took a few years to find its legs before McGegan came on in 1986. Since then, the orchestra’s activities have continued to expand, both in its home base by the San Francisco Bay and throughout the wider world. A series of appearances in New York City’s Lincoln Center culminated in August with a visit to the Mostly Mozart Festival for a fully staged production of Il rè pastore (one performance, unfortunately, was canceled because of the 2003 blackout). Scheduled for 2005 are debut appearances in Carnegie Hall’s Zankel Hall and Walt Disney Concert Hall in Los Angeles, as well as an appearance at the Proms in London.

Over the decades, Phil-harmonia has attracted some of the finest singers of the early-music world. Most unforgettable, surely, is the great mezzo-soprano Lorraine Hunt Lieberson, who as an unmarried so- prano named Lorraine Hunt provided Philhar-monia’s audiences with some of the most exquisite Handelian singing any of us had heard. Those performances can still be savored on the Harmonia Mundi recordings of Susanna, Theodora, Messiah, and Clori, Tirsi e Fileno, not to mention the breathtaking compilation disc Arias for Durastanti. Later, when Hunt began to reposition herself as a mezzo, she marked the shift with a 1995 performance of Berlioz’s Nuits d’été marked by unearthly beauty and attention to detail. Other memorable artists who have graced the Philharmonia schedule include the late baritone Will Parker, sopranos Lisa Saffer, Christine Brandes, and Dominique Labelle, and countertenors David Daniels and Andreas Scholl.

Probably the orchestra’s most fruitful partnership has been with the Mark Morris Dance Group, a collaboration that dates back to 1988. In settings of Handel’s L’Allegro, il Penseroso ed il Moderato, Purcell’s Dido and Aeneas, Rameau’s Platée, and Vivaldi’s Gloria, among others, McGegan and the orchestra have provided an essential musical counterpoint to Morris’s visionary choreography.

Although the work of these composers and their contemporaries has always figured prominently in Philharmonia’s activities, the “Baroque” part of the orchestra’s name has long been a misnomer. Music of Mozart, Haydn, and Beethoven forms just as notable a cornerstone of any given season, and excursions into the music of the early Romantic era have grown more frequent and more assured: Schubert, Mendelssohn, and Berlioz have all been illuminated by the combination of period instruments and McGegan’s incisive, probing leadership. Only the Symphonie fantastique remains notably unattempted (“We could certainly do it,” says McGegan with a laugh, “but we’d have to take out a mortgage to get hold of all those instruments”).

Alongside the familiar masterpieces of the 18th and 19th centuries, Philharmonia has also shown an exciting readiness to explore the lesser-known musical figures of the period, from Charles Avison to Jan Dismas Zelenka, as well as newly discovered works by established masters. The current season, for instance, concludes with the North American premiere of Alessandro Scarlatti’s Vespers of St. Cecilia. “One thing that a period instrument orchestra can do,” McGegan says, “is to serve as the performance wing of the musicology world. People keep digging up these wonderful new pieces for us to play, and for us it’s the equivalent of getting a new piece out of John Adams.”

Future plans call for an even further expansion of the repertoire: For its upcoming 25th anniversary, the orchestra is commissioning a new work from San Francisco composer Jake Heggie, creator of the acclaimed opera Dead Man Walking. “It will probably scandalize the recorder players,” says McGegan, “but that’s fine. I think if one only plays the safest repertoire, that doesn’t challenge anybody. It’s nice to do some weird stuff now and again. I think one 21st-century piece in 25 years is OK.”

Joshua Kosman is the classical music critic for the San Francisco Chronicle.