Norman Blake first made his reputation as one of the finest guitar-pickers in bluegrass, a logical heir to the venerable Doc Watson. But Blake has restlessly explored other instruments and other genres. In his latest album, "Original Underground Music From the Mysterious South," he leads a string quintet, the Rising Fawn String Ensemble. Dominated by mandolins, the 12 original instrumentals are an unusual blend of old-timey mountain music and classical chamber music.

The album begins with "New Brick Road," with Peter Ostrusko picking out the high, sprightly melody on one mandolin. Carl Jones then picks it up on the mandola (tenor mandolin). Larry Sledge and Norman Blake add a lower harmony on the mando- cello. Blake double-tracks the fuller guitar part. His wife, Nancy Blake, doubles the lead mandolin line and overdubs a sonorous bowed cello.

In this and other pieces, a careful architecture supplants the carefree exuberance and improvisatory flair of many folk and bluegrass performances. Everything seems premeditated with the high, fluttering mandolins neatly balanced by a cleanly picked banjo or guitar and anchored by Nancy Blake's darkly colored cello drone. This is subdued, symmetrical, meditative music. ON RECORD, ON STAGE THE ALBUM NORMAN BLAKE -- Original Music From the Mysterious South (Rounder 0166). THE SHOW NORMAN BLAKE, at the Birchmere Friday and Saturday.