Solo for a melody instrument with feedback is a work for a soloist with live electronics (four technician assistants) composed in 1965–66 by Karlheinz Stockhausen. It is Nr. 19 in his catalogue of works. Performance duration can vary from 10½ to 19 minutes.
Solo was first sketched in 1964 and is closely related to Plus-Minus, Momente, and Mikrophonie I (Stockhausen 2002, 24 and 51). It was composed in March and April 1966 on a commission from the Japanese broadcasting network Nippon Hoso Kyokai, and was premiered in two different versions on 25 April 1966 in a public concert at the NHK in Tokyo which also featured the world premiere of Stockhausen's Telemusik. The soloists were Yasusuke Hirata, trombone, and Ryū Noguchi, flute; the assisting technicians were Akira Honma, M. Nagano, Shigeru Satō, and the director of the NHK studio, Wataru Uenami. The score is dedicated to Alfred Schlee, the director of the Vienna branch of Universal Edition (Stockhausen 1971, 85 and captions to plates 18 and 19, between pages 80 and 81).
For the Tokyo premiere a special table with adjustable guide rollers was constructed, to enable different delay times for the tape playback. In subsequent performances a single tape was threaded through seven tape recorders, and eventually an apparatus was constructed especially for Solo in the Institute for Sonology at the University of Utrecht. However, this piece of equipment remained private property and was not made available for hire (Stockhausen 1971, 87). By 1970 computers were already considered as a replacement for the tape delay, but was regarded prohibitively expensive (Stockhausen 2009, 192). Twenty years later, after initial difficulties, one such version was made at IRCAM in 1992, using Max/MSP on a NeXT computer, and successfully performed in public in February and March 1993. In March 1998 this program was adapted to run on commercially available computers. Pre-programming on the computer can fulfill the functions originally assigned to the assistant technicians, thus requiring only a single assistant to control the sound (Sluchin 2000, 42). In notes for a 2002 performance also using a computerised system, Stockhausen acknowledged the IRCAM version, emphasising that it was done live, but nevertheless concluded that "It will still be a long time until young musicians … will be able to learn the interpretation of SOLO with suitable mobile apparatuses" (Stockhausen 2002, 24 and 51).
Feedback (Rückkopplung) in this case refers to tape delay, through which music played by the soloist is made to return after periods of time specified in six different form plans, one of which is to be chosen for any performance (Stockhausen 1971, 86). The performer is given six pages of conventionally notated material constituting the "content" of the work, and selects material according to certain criteria, playing it into a stereo pair of microphones that feed into the tape-loop system. Three assistants choose one or both recording channels, the degree of feedback, and the level of sound to be emitted from the speakers. This results in a regular though transformed periodic recurrence of the initial material, while the soloist adds new matrial over it. The system of the feedback plan therefore is the piece, since any musical relationships present on the sheets of music are destroyed by the atomisation and reorganisation created by this system (Harvey 1975, 97–99).
Performance practice 
Stockhausen discovered early on that the originally imagined spontaneous performance of Solo was far more difficult than expected. Consequently, versions prepared in advance were used from the outset, following Stockhausen’s suggestions (Stockhausen 2002, 24 and 51). In the first commercial recording, with Vinko Globokar on trombone, Stockhausen supplemented the live performance with excerpts from his electronic composition Hymnen, following the method he had already used in Mikrophonie II, where he inserted tape recordings of his own previous compositions. In the case of the trombone recording of Solo, this involved a lengthy section from the Second Region of Hymnen, including its prefatory "bridge". The trombone phrases are initially answered by electro-acoustically distorted military-band sounds of the German national anthem, similar to the way in which sounds from Gesang der Jünglinge, Carré, and Momente played back on tape answer the technologically alienated live voices of the mixed choir in Mikrophonie II (Sadie 1968; Frisius 2008, 177–78). By 1969, Stockhausen had also taken to merging Solo with other works in a single composite performance (Mann 1969).
- Karlheinz Stockhausen: Solo, für Melodie-Instrument mit Rückkopplung; Vinko Globokar: Discours II pour cinq trombones; Luciano Berio: Sequenza V; Carlos Roqué Alsina: Consecuenza. Vinko Globokar (trombone). Avant Garde. LP recording. Deutsche Grammophon Gesellschaft 137 005. [Hamburg]: Deutsche Grammophon Gesellschaft 1969.
- Stockhausen, Karlheinz: Solo (Version für Flote); Solo (Version für Synthesizer); Spiral (Version für Oboe). Dietmar Wiesner (flute), Simon Stockhausen (synthesizer) Catherine Milliken (oboe). CD recording. Stockhausen Complete Edition CD 45. Kürten: Stockhausen-Verlag, 1995.
- Knut Sønstevold, bassoon. Knut Sønstevold; Miklós Maros; Carel Brons; Arne Mellnäs; Lars-Gunnar Bodin; Karlheinz Stockhausen; Sten Hanson. [Solo recorded at Danviken Hospital Church, 23–26 June 1977]. LP recording Fylkingen Records FYLP 1011. [Stockholm]: Fylkingen Records, 1977.
- Sönstevold Plays Stockhausen. Karlheinz Stockhausen: Solo, In Freundschaft, Spiral, Tierkreis. Knut Sønstevold (bassoon); Kina Sønstevold (piano). Nosag CD 042; [Solo recorded by Swedish Radio on 4 October 1985 during the EAM Festival, Berwaldhallen]. [Sweden]: Nosag Records, 2000.
- Frisius, Rudolf. 2008. Karlheinz Stockhausen II: Die Werke 1950–1977; Gespräch mit Karlheinz Stockhausen, "Es geht aufwärts". Mainz, London, Berlin, Madrid, New York, Paris, Prague, Tokyo, Toronto: Schott Musik International. ISBN 978-3-7957-0249-6.
- Harvey, Jonathan. 1975. The Music of Stockhausen: An Introduction. Berkeley and Los Angeles: University of California Press. ISBN 0520023110.
- Herbort, Heinz Josef. 1987. "Klänge jede Menge: Neue Namen, neue Initiativen, neue Hoffnungen—aber alte Strukturen". Die Zeit, no. 44 (23 October).
- Kurtz, Michael. 1992. Stockhausen: A Biography, translated by Richard Toop. London and Boston: Faber and Faber. ISBN 0571143237 (cloth) ISBN 0-571-17146-X (pbk).
- Maconie, Robin. 2005. Other Planets: The Music of Karlheinz Stockhausen. Lanham, Maryland, Toronto, Oxford: The Scarecrow Press, Inc. ISBN 0-8108-5356-6.
- Mann, William. 1969. "Palimp—What?" The Times (1 August): 9.
- Sadie, Stanley. 1968. "Musicianship in New Guises". (6 December): 17.
- Sluchin, Benny. 2000. "A Computer-Assisted Version of Stockhausen's Solo for a Melody Instrument with Feedback". Computer Music Journal 24, no. 2 (Summer): 39–46.
- Stockhausen, Karlheinz. 1969. Solo, für Melodie-Instrument mit Rückkopplung, Nr. 19 (score). Vienna: Universal-Edition.
- Stockhausen, Karlheinz. 1971. "Solo für Melodie-Instrument mit Rückkopplung (1966)". In his Texte zur Musik 3, edited by Dieter Schnebel, 85–91. DuMont Dokumente. Cologne: Verlag M. DuMont Schauberg. ISBN 3-7701-0493-5.
- Stockhausen, Karlheinz. 2002. "Solo (1965/66) Version für Trompete mit Rückkopplung (Dauer ca. 15½ Min.)" / "Solo (1965/66) Version for Trumpet with Feedback (Duration ca. 15½ Minutes)". Programm zu den Interpretations- und Kompositionskursen und Konzerten der Musik von Stockhausen 27. Juli bis 4. August 2002 in Kürten / Programme for the Interpretation and Composition Courses and Concerts of the Music of Stockhausen July 27th to August 4th 2002 in Kuerten, English translations by Suzanne Stephens, 23–24, 50–51. Kürten: Stockhausen-Verlag.
- Stockhausen, Karlheinz. 2009. Kompositorische Grundlagen Neuer Musik: Sechs Seminare für die Darmstädter Ferienkurse 1970, edited by Imke Misch. Kürten: Stockhausen-Stiftung für Musik. ISBN 978-3-00-027313-1.