Gustav Leonhardt was an unassuming, private man, and his instrument, the harpsichord, may seem an improbably modest vehicle to bring about a sea-change in the way the music of an entire era is performed and listened to. But at the outset of Leonhardt's career, in the early 1950s, neither musicians nor instrument-makers gave much thought to the conditions in which composers like Bach and Handel might have worked: Baroque music was performed with little stylistic insight, often on instruments solid enough to withstand enemy fire. Leonhardt's superb musicianship – at the organ as well as the harpsichord, and as conductor, scholar and teacher – combined with methodical, self-effacing rigour to strip away two centuries of accreted performance practice and so allow modern audiences to hear the Baroque as it might have sounded in Bach's own day.
In a world already transformed by Leonhardt's insights, it is difficult to imagine just how thorough was his influence in the decades leading up to what is today called HIP: historically informed performance. But a younger colleague, the harpsichordist Mahan Esfahani, put it plainly: "His presence is so overwhelming and of such influence that we cannot deny how it totally changed our view of the harpsichord."
Leonhardt's early life foreshadows his career and personality. The Dutch Protestantism into which he was born remained important to him. His parents were music-loving amateurs (his father was a banker) and involved their children in domestic music-making. They bought a harpsichord for the family performances of Bach and Telemann and at 10 or so young Gustav made his first acquaintance with the instrument.
German occupation inevitably brought hardship, and although life in the Dutch countryside was easier than for city-dwellers, there was still no electricity and water and, for the last nine months of the war, Leonhardt spent stretches hidden between the floorboards so as to avoid being sent away for slave labour. The family harpsichord became a companion in his isolation, and as its fascination grew, he decided that it would be the focus of his life.
Although his parents insisted that he complete a normal education, they also supported his musical aims and so in 1947, aged 18, he moved to Basel to attend the Schola Cantorum, then virtually the only music school specialising in early music. For his debut he played The Art of Fugue, stoutly insisting that Bach had intended it for the harpsichord. But he still had anxious parents to satisfy: "They thought that I should take a conducting course and perhaps become a famous conductor, earning lots of money. So I was sent to Vienna and enrolled on a conducting course. It was not very successful ... Instead I spent a whole year from seven in the morning until seven in the evening in the National Library going through sources and music."
Those countless hours were to stand Leonhardt in good stead. He copied out thousands of manuscripts, many of music unperformed for centuries. They gave him access to a huge and unfamiliar repertoire, and so he was able over the ensuing years to open up the music of the past to an audience that had no idea of just how rich it was.
In Vienna he met Nikolaus Harnoncourt, now best known as a conductor, but then a cellist with similar concerns to Leonhardt, and before long they were performing together. In 1952 the director of the Hochschule für Musik appointed him to a professorship in harpsichord, in which capacity he remained in Vienna for three years, the last of them overlapping with a like position at the Amsterdam Conservatoire.
There he taught until 1988, acting also as organist of the Waalse Kerk, which has an outstanding instrument from 1733. His presence made Amsterdam one of the capitals of early music: students came to him from all around the world, to be received in the splendour of his 1605 town-house.
Leonhardt's time in Vienna coincided with a sudden upsurge of recording activity, as US labels took advantage of the strength of the dollar to make inexpensive recordings in Europe. One of them was the Bach Guild, using the label Vanguard, and Leonhardt made a number of LPs for them. The countertenor Alfred Deller was one of the musicians with whom he worked – learning much about phrasing and nuance. In 1955 he formed the Leonhardt Baroque Ensemble (later Leonhardt Consort), gathering around him and his wife, the Swiss violinist Marie Amsler, a group of musicians whose names are early-music touchstones: the recorder-player Frans Brüggen, cellist Anner Bylsma and the three Kuijken brothers, the violinist Sigiswald, cellist and gambist Wieland and flautist Barthold.
Leonhardt insisted on recording on instruments contemporary with the music he was playing (though he was also happy with faithful copies). Although he and Harnoncourt came to disagree on the principle of Baroque performance practice, with Harnoncourt prepared to compromise and Leonhardt remaining a purist, they collaborated on one of the monuments of the day, a complete cycle of Bach's 200-plus cantatas, recorded between 1972 and 1990 for Teldec. Leonhardt had no time with claims of "authenticity"; his outlook was squarely realist: "Our mission is to know as closely as possible what the composer might have wanted, or what was possible in the period during which he was composing".
His concert manner, stern and unsmiling, reinforced the image of the purist. But he would occasionally reveal a sly sense of humour, slipping in a bit of ragtime to end a recital, and he enjoyed fast cars and wine. In 1967 he appeared as Bach in Jean-Marie Straub's film The Diary of Anna Magdalena Bach – he was impressed by the film's musicological accuracy. But he was no absolutist, stating: "I can't stand pedants. A musician should make sure his basic principles are right, then express himself according to his temperament".
Gustav Maria Leonhardt, harpsichordist, organist, conductor and teacher: born 's-Graveland, Hilversum, Netherlands 30 May 1928; married Marie Amsler (three daughters); died Amsterdam 16 January 2012.