His publisher, Faber Music, announced the death but did not give a cause.
Mr. Knussen was a literal and figurative giant of contemporary music. A towering and lumbering bear of a man, he was known for his meticulous compositions and his conducting of groups including the London Sinfonietta, BBC Symphony Orchestra and Chamber Music Society of Lincoln Center. He also became head of contemporary music at the Tanglewood Music Center in western Massachusetts, among other prestigious positions.
When granting Mr. Knussen an honorary doctorate last week, Britain’s Royal Academy of Music declared, “There is no composer more refined in technique and imagination, no conductor more precise and fastidious, no mentor more generous, and no one who understands the repertoires of new music better.” It went on to call Mr. Knussen, a professor at the school, one of the “nation’s greatest cultural treasures.”
In 1968, Mr. Knussen was touted as a boy wonder when he conducted his First Symphony with the London Symphony Orchestra, where his father was the principal bassist and the conductor had taken ill. He was 15.
“It sounds ridiculous, I know, but I’ve been writing music for more than 10 years,” he told United Press International that year, before conducting his symphony at Carnegie Hall. “Inevitably, some will think I’m a bit of a freak, but I don’t. I just started earlier than most composers, that’s all.”
Drawing from modernists such as Igor Stravinsky, Mr. Knussen developed a highly individual style that seemed to emphasize concision above all else. He was known to spend years on his compositions, blowing past deadlines but delivering pieces that — despite rarely running more than 20 minutes — made him one of the most celebrated British composers since Benjamin Britten, an early mentor.
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In addition to his three symphonies, all completed before he was 30, Mr. Knussen wrote chamber works including “Ophelia Dances” (1975), “Coursing” (1979) and “O Hototogisu!” (2017), adapted from Japanese haikus; “Flourish With Fireworks” (1993), a brief concert opener; and popular concertos for horn and violin.
Mr. Knussen also recorded more than 50 albums that aimed to spotlight composers he believed were woefully underappreciated, including Magnus Lindberg, Toru Takemitsu and Modest Mussorgsky. But he was probably best known for his collaborations with Sendak, whose works he discovered in his 20s at a bookshop near Tanglewood.
In a 2005 interview with the San Francisco Chronicle, Mr. Knussen recalled that the children’s writer and illustrator introduced himself in an unexpected phone call, saying, “This is Maurice Sendak. Can we just start by me asking you what you think is the best children’s opera ever written?”
“I said, ‘The second act of “Boris Godunov.” ’ He said, ‘Right answer,’ and from that point on we became very close.” (As a kind of musical joke, Mr. Knussen quoted from Mussorgsky’s “Godunov” in his own opera.)
With sets and a libretto by Sendak, “Where the Wild Things Are” premiered in Brussels in 1980. The production was a disaster, with a malfunctioning dry-ice machine, overheated costumes and an unfinished score that Mr. Knussen continued tweaking for another four years.
In its polished form, however, the opera proved a success, capturing the whimsy of Sendak’s tale of a boy who is sent to bed without dinner, sails to a faraway land and generates a “wild rumpus” with a group of horned beasts.
“Knussen’s music definitely intensifies the story,” wrote Washington Post music critic Joseph McLellan, reviewing a 1985 recording for Arabesque. “It supplies vivid musical substitutes (if not equivalents) for the pictures in the original, and the result is a ‘Wild Things’ that is wilder than ever — a study of childhood, its insecurities, frustrations and fantasies with (as the libretto says) ‘a contained violence that could at any moment get out of control.’ ”
Stuart Oliver Knussen, who went by Olly, was born in Glasgow on June 12, 1952. Raised in London, he was immersed in music from a young age, surrounded by performers who dined and sometimes boarded with the family. “It never occurred to me that not everybody thought it was the most important thing in life,” he told the Guardian in 2012.
Though an average piano player (by his standards, at least), he began composing as soon as he could read sheet music, and received private lessons from composer John Lambert after studying at what is now the Purcell School for Young Musicians. He also studied under Gunther Schuller at Tanglewood, the summer home of the Boston Symphony Orchestra.
Mr. Knussen later served on its staff and was an artistic director of the Aldeburgh Festival, held each summer near his home in rural Snape.
His work at Aldeburgh provided further grist for critics who viewed him as a successor to Britten, who co-founded the festival and gave Mr. Knussen his first commission. Both men displayed a knack for writing music that appealed to children: In addition to “Wild Things,” Mr. Knussen composed the chamber work “Hums and Songs of Winnie-the-Pooh” and an operatic adaptation of the Sendak book “Higglety Pigglety Pop!”
Mr. Knussen married Sue Freedman, an American who played the French horn and edited many of his scores, in 1973. The couple separated in the mid-1990s but remained close; after her death in 2003, Mr. Knussen said he was driven to write “Requiem — Songs for Sue” (2006), incorporating lines from poems by Antonio Machado, W.H. Auden, Rainer Maria Rilke and Emily Dickinson.
Survivors include his daughter, contralto Sonya Alexandra Knussen of Baltimore; a brother; and a sister.
He was appointed a commander in the Order of the British Empire in 1994, and in 2016 was awarded the Queen’s Medal for Music. By then, his output had slowed significantly — not for lack of inspiration, he said.
“If I feel comfortable writing a piece, I get extremely suspicious, because it means that I’ve probably done it before and probably better,” he told the Detroit Free Press. “There needs to be a bit of grit to produce the pearl.”
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