Edwin (Ted) Flath, 56, founder of the California Bach Society, a choral group he organized in 1971 for a benefit concert that helped purchase an organ for St. Mark's Episcopal Church in Berkeley. By the early 1980s the group had grown to 123 voices and was widely known in Northern California and elsewhere. A native of Toronto, Flath was a graduate of the Royal Conservatory there and studied at Syracuse University. Flath also studied with major conductors in the East and was a concert organist and conductor in the San Francisco Bay Area for 20 years. In San Francisco on Sept. 24 of AIDS.