"I'm sure glad there's only one of me," said Joan Baez at Wolf Trap last night, following a terrific comedy set by the Smothers Brothers.

Throughout their act, the Smotherses engaged in artfully convoluted banter, pausing occasionally to sing a song, only to interrupt it with yet another dizzy display of sibling rivalry. Typical was a bit of Gilbert and Sullivan, earnestly sung by Dick. Tommy managed to lose sight of the melody altogether, and instead recruited the pianist for a bout of dueling guitar and keyboards. He also put the lesson of the Iran-contra hearings in a nutshell: "It's better to tell the truth about a lie than to lie about the truth."

Baez isn't the pure soprano she once was. Her voice is darker now, a little throatier. But it's still a remarkable instrument, in some cases better suited than it used to be to the pop materials she sings. She emphasized several of her own songs, lyrics with a distinctly personal edge, but apart from "Diamonds and Rust," nothing rivaled her Bob Dylan trilogy or the South African freedom song she sang to taped accompaniment. The low point came when she read several excerpts from her recent autobiography.