Here’s a proposition, possibly reductive, probably true: the German duo Cluster (Dieter Moebius and Hans Joachim Roedelius) were the only Krautrock act to make the usual artistic trajectory, from incipient experimentalism to more approachable, populist moves, without trading in one iota of their humour or their peculiar genius. Can and Tangerine Dream lost it; Faust and Ash Ra Tempel watered down most of their ideas (Faust’s “Krautrock” and Manuel Gottsching’s E2-E4 notwithstanding); Amon Duul II aren’t worth mentioning; perhaps only the other great Krautrock duo NEU!, of Michael Rother and Klaus Dinger, made it to the late 1970s with their aesthetics intact. Tracing Cluster’s history is a grand thing to do. Starting as Kluster, with third member Conrad Schnitzler, they released several albums of hard-nailed, formless analogue electronics, Klopfzeichen (Schwann, 1970), Zwei-Osterei (Schwann, 1971) and Eruption (1971), which were as vast and cold as the Arctic tundra, forbidding and steely in their gravity. After losing Schnitzler, the first two duo Cluster albums, 71 (Phillips, 1971) and Cluster II (Brain, 1972), explored similar terrain — an improvised meta-music that satellites out from the big bang of late ’60s counter-cultural disruption, where academic electronic music and the freedoms of rock at its most structurally footloose met on even terrain.