There are living composers older than Alan Hovhaness, who turned 75 on March 4, but there can be very few who have been composing music longer or who have produced more. His first work was a short organ piece written when he was 4 years old, and since then his opus numbers have climbed up to 300 and beyond.

Last night at the Departmental Auditorium, the Armenian General Benevolent Union celebrated its own 80th anniversary and the composer's 75th birthday with a concert that featured music of Hovhaness in the first half and two brilliant young Armenian performers in standard classical repertoire after the intermission. The atmosphere was very much like that of a large family gathering; the music-making was uneven but often compelling. The program could present only a small sample of Hovhaness' vast and varied work, but it gave a fair idea of the gentle, exotic, mystical flavor that belongs uniquely to this composer.

The voice of Hovhaness' wife, soprano Hinako Fujihara, has been the inspiration for some of his most exquisite music, and it was heard last night in three of his finest vocal works, "How Long Wilt Thou Forget Me," "A Presentiment" and "Alleluia," with Hovhaness at the piano. His piano technique seems as fine as ever, even in his 76th year, but his wife's voice (a more fragile instrument) has suffered the effects of time. It was good, though, to hear it on this special occasion, and the remains of what must have been an outstanding voice could be heard in occasional phrases. Bass Ara Berberian, one of the Metropolitan Opera's most stalwart performers, was outstanding in two works with Hovhaness at the piano, and violinist Peter Oundjian and pianist Aroutiun Papazian presented superbly the folk-flavored melodies and rhythms of his rhapsodic "Varak," Op. 47.

After intermission, Papazian soloed with exquisitely disciplined style and flawless technique in Mozart's Variations in D, K. 573. His interpretation of Chopin's Scherzo in B flat minor, Op. 31, was brilliant and individualistic almost to the point of eccentricity. In Franck's Sonata in A, with violinist Oundjian, his interpretation was more uneven; he is a superb pianist but sometimes seems to forget that self-effacing partnership is the essence of chamber music.