Siegmund Nissel, the second violinist of the Amadeus Quartet, one of the world’s most prestigious chamber-music ensembles, has died in Britain at age 86. The quartet was highly unusual in playing without a change of personnel over a period of 40 years, from 1947 until it disbanded in 1987. The quartet had an extensive recorded repertory and toured widely, giving performances for the Dallas Chamber Music Society among many other musical organizations.