Californian composer Terry Riley’s In C, first issued on vinyl in 1968, is widely acknowledged as a Minimalist landmark that altered the course of twentieth-century music. His influential album A Rainbow In Curved Air, which appeared the following year, is a vivid fusion of rock and raga, jazz and psychedelia realised through overdubbing on an 8-track machine which had recently been installed at the studios of Columbia Records.

In recent decades Riley has composed a series of critically acclaimed pieces for string quartet. He has also written for full orchestra and explored a variety of instrumental combinations. But during the 1970s, he concentrated on solo keyboard performances, continuing to make music yet writing down almost nothing. Riley selected a mode, chose a few motifs or basic patterns and then, seated on the floor in front of his audience, improvised on electronic keyboard. That instrument became an essential part of his musical identity for a while, although it was chosen in part for practical reasons. The electric organ, superseded at later concerts by a synthesizer, was portable and consistent. This enabled him to avoid unreliable pianos in venues which were less formal and more variable than the standard concert hall circuit.

By the early 70s Riley had come to feel that scores were a distraction. Faithful interpretation of an already written piece was a deviation from the true purpose of making music, which was spiritual quest. Fortunately, some of those live performances, personal journeys towards a state of transcendence, were captured on tape. Persian Surgery Dervishes, issued initially on the French label Shandar, features two such concerts for electric organ and reel-to-reel delay, one recorded in Los Angeles on 18th April 1971, the other in Paris on 24th May 1972.