A few years ago, famed pianist Santiago Rodriguez was driving on the Capital Beltway in his Datsun 240Z, listening to the last movement of Rachmaninoff's Third Piano Concerto.

"The sound was first rate," he said recently. "It was just a sensational performance. I figured if it was from Washington, it was probably the National Symphony Orchestra."

At the conclusion of the live piece, after enthusiastic applause, the announcer introduced the orchestra: the Fairfax Symphony Orchestra. Rodriguez, a New Yorker, was stunned. He recalls, "I said to myself, 'The Fairfax what?' "

That is a reaction common among those who associate anything below the level of major symphony orchestras with screeching violins and high school auditoriums, but the Fairfax Symphony Orchestra is regarded, by community supporters and nationally known musicians, as much more than that.

It has sellout concerts, bookings at the Kennedy Center, overwhelming community support and a conductor who "just keeps moving the bar on the high jump," said Barbara Serage, managing director of the orchestra.

Janos Starker, regarded as one of the world's two greatest cellists, said that the orchestra's development has been "incredible," and that it is "among the top three community orchestras in the country." He would not name the others.

Although Starker referred to the orchestra as a "community group," according to the American Symphony Orchestra League it is in the larger "metropolitan" orchestra category. The league has five classifications for orchestras, based on budget size, from "major orchestra" to "community orchestra."

In the United States, there are about 100 metropolitan-size groups, those said by the league to have budgets between $265,000 and $950,000. The Fairfax Symphony, with an annual budget of $457,000 and 108 musicians, is the only metropolitan orchestra in this area. The closest otherwise is in Harrisburg, Pa.

If the Fairfax Symphony Orchestra enjoys a reputation as one of the best of its kind, the prime cultural organization in vast Northern Virginia -- it was not always. Fifteen years ago it was considered one of the worst.

Those were the days when there sometimes were more musicians on stage than fans in the audience. Even the American Symphony Orchestra League, Fairfax's parent organization, recommended that it disband or, at least, stop playing for the public, according to Conductor William Hudson.

"They were, at that time, what we used to call a 'Mickey Mouse' orchestra," Starker remembered.

But the Fairfax group had some advantages over similar groups in the country. Its location gave it access to a well-educated population from which to draw both musicians and listeners, and the county government and business community offered financial support.

A large part of the transformation from the "Mickey Mouse" orchestra that was formed in 1957 into a reputable music group, observers say, is attributable to Hudson, the orchestra's full-time conductor since the 1971-72 season.

"He is the reason Fairfax is what it is," said Karen Sutterer Thornton, a 35-year-old French horn player. "In an area where it is easy for a community orchestra to be just that -- 'Let's have fun' -- he is a perfectionist."

To improve the musicians, Hudson, a University of Maryland music professor with a graduate degree in music from Yale University, began requiring yearly auditions. "It's coldblooded," he said, "but you have to do it if you want to keep growing."

Next, he sought to change the orchestra's image by using endowment money to bring in big-name soloists such as Rodriguez and Starker, pianist Jorge Bolet and violinist Aaron Rosand.

Hudson also gave his musicians the opportunity to have their work criticized by the visiting artists, and he encouraged the most talented among them to try solos.

The orchestra derives a third of its budget from ticket sales, advertising and benefits, and 25 percent from grants given by Fairfax City, Fairfax County and the area's growing business and arts community.

The rest comes from private contributions, which were up 40 percent last year from the previous year after a direct-mail campaign.

The size of its audience has increased from 1,000 its first year to an estimated 80,000 last year, and the number of performances and the size of the repertoire is greatly expanded.

Despite how far Fairfax has come, a recent rehearsal made it clear there are still obstacles to its continued success.

Without a concert hall of its own, the orchestra rehearses in a cinder-block school band room, with linoleum floors and a poster near the blackboard that says "Get Ready for a Totally Awesome School Year." When it performs in Fairfax, its concert hall is Fairfax High School.

More than than 20 of its musicians struggle to get to the 7:45 p.m. rehearsals after long days at other jobs.

"Often, it's very difficult, because I'm tired at the end of the day," said Newton Pacht, 60, who teaches law at Howard University and is the orchestra's principal double bass.

"Sometimes it's hard to shift gears," agreed Sharon Like, 28, a banking regulatory lawyer with the FDIC and the principal second violin.

Another problem is that an increasing number of players belong to the musicians' union, which means that they are available for high-paying free-lance engagements, such as playing for ballets and operas at the Kennedy Center.

This lucrative work sometimes means skipping Fairfax rehearsals, and it is one reason Hudson hopes someday to be able to pay union-scale wages. Currently, Fairfax musicians make a minimum of $75 a concert, compared with the union's approximately $260, he said.

Rehearsals for a part-time orchestra such as the Fairfax one are sparse enough as it is. On a Monday, five days away from a concert, a discouraged Hudson lowered his baton and declared that the Khachaturian Flute Concerto sounded "awful, absolutely awful." The cellos, especially, needed work, he said.

Yet another obstacle is that while most of Fairfax's musicians possess skills that make them "able to make Beethoven sound like Beethoven," Hudson still frets about those who cannot keep pace and those who for personal or professional reasons leave the metropolitan area -- about 15 each year.

Another problem is Fairfax's proximity to the top-quality National Symphony Orchestra. Most Fairfax players admit that the National is of a distinctly higher quality, yet they say that not all music lovers want to drive into the District.

"The real catalyst will be when we get our own concert hall," Hudson said. "When that happens, the audience will double. People will take us more seriously."

Whatever is in store for the Fairfax Symphony, its musicians hope the organization will not lose the sense of enthusiasm that makes people like John Relman, 27, a law clerk, give up his evenings to play the cello in a cramped school band room. Said Relman, "There's a real enjoyment of the music for its own sake."