Success is a funny thing in the classical music world. Consider the case of Philip Glass.

Acclaimed as one of the guiding lights of minimalism, his success with dance pieces and film scores keeps earning him commissions, while his operas and theater pieces are slowly working their way into the repertory. Yet of his three operas, only one, "Einstein on the Beach," is available on record (although a recording of "Satyagraha" is due in June). A recent theater piece, the Robert Wilson collaboration "CIVIL warS," has been mounted only twice in this country. Other work, such as his score for the Paul Schrader film "Mishima," simply hasn't been released yet. Thus, unless you live in New York, Amsterdam or one of the few other cities that have seen performances of his major pieces -- or are both devoted and peripatetic -- odds are you haven't heard much of what Glass has been writing lately.

Which is why he maintains the Philip Glass Ensemble, the group he will bring to the Warner Theatre this Friday.

Formed in the early 1970s, the small, self-contained ensemble provides Glass with a private forum for his minimalist ideas and an economy that makes his works no more difficult to present than the average rock concert. Ironically, while success has meant that Glass need no longer restrict his musical imagination to five instrumentalists and two vocalists -- now he can indulge in large choruses, orchestras and the like -- this new freedom has restricted the Philip Glass Ensemble, which isn't suited to all of the composer's recent work.

"Just today, we were rehearsing a selection from 'Akhnaten' that I was able to adapt to the ensemble," Glass said by phone, "but that can be a problem. With the opera 'Satyagraha,' I just have never been able to make an adaptation."

Synthesizers, he said, have been "crucial" in translations of larger works, "because as I become more involved with symphonic pieces, [with synthesizers] at least you can aspire to a symphonic sound. See, it's not simply synthesizers -- it's synthesizers plus acoustic instruments plus live voice. By mixing acoustic instruments with synthesizers, we're able to carry off a feeling of an authentic, symphonic sound."

Glass doesn't believe he compromises his work by adjusting the scoring. For one thing, the ensemble has long relied on amplification to maintain instrumental balance, so the electronics will hardly overshadow the acoustic sounds. For another, Glass is perfectly comfortable with writing for synthesizers. "Sometimes I write for woodwinds or flutes, sometimes I write for synthesizer," he said. "In a recording, I would rarely substitute synthesizers for the real instruments. I might back them, to get a little more depth electronically, but I wouldn't try to get away without the real instruments."

Unfortunately, all that hardware has led some critics to suggest that Glass owes more to pop music than to the more lofty traditions of western composition. True, he has made a record for a rock record company (1974's "North Star"), and he has commissioned David Byrne, Laurie Anderson and Paul Simon to write lyrics for a new work. But this no more makes him a rock 'n' roller than his work with Doris Lessing on an operatic version of "The Making of the Representative from Planet 8" makes him a science fiction author.

Still, his compositional style -- with its slowly unfolding rhythmic structures manipulating relatively simple harmonic material -- galls certain segments of the classical music establishment. His work has been called simple-minded, redundant, annoying and childish, everything that the establishment's old guard, the serialists, weren't.

Part of the problem, according to Glass, is that these critics don't understand what they're hearing. "They haven't figured out how it works," he said. "They don't understand the rhythmic structure of it at all, they don't understand any of the structural principles of the work, and to them, it simply doesn't sound like what they do."

Indeed, much of his inspiration comes from outside the western musical tradition. After a job transcribing Indian music with Ravi Shankar as a student in Paris, he became interested in the principles behind eastern music, "the musical language and the ideas of rhythmic structure, which I could easily begin to use with western technique," he said. "When you listen to 'Einstein on the Beach,' it never occurs to you that it has anything to do with India, when in fact the rhythmic structure of a lot of the piece I learned directly from when I was in India."

Mostly Glass sees his critics as simply a generation behind, reviling his work the way the classicists rejected Beethoven or the Brahmsians despised Wagner. "You can see how that would happen," he said. "We're talking about people who are 10 to 15 years removed, in terms of generations, and who were formed in a different world. They just don't hear it. I mean, I hear it, you hear it, a lot of people hear it, but they don't hear it. And then they go in print and tell us they don't hear it.

"It's funny, isn't it? But that's the way it's always been."