Christophe Rousset doesn't quite fit the mould.

For decades the early music movement was synonymous with socks and sandals, Ryvita and too much facial hair, but when I first catch sight of the spry French harpsichordist he's wearing gold Nike high tops and red low-slung skinny jeans. He's on stage at the Versailles opera house rehearsing with his period instrument ensemble, Les Talens Lyriques, half perched at the keyboard dashing out flamboyant continuo lines, half on his feet conducting with lithe, springy gestures. At 52, he's an eternally youthful maverick of the early music world.

As a budding harpsichordist, Rousset migrated in his 20s from his family home in Provence to the baroque music mecca of Amsterdam, where he turned heads with playing that was overtly charismatic and sensuous. He drew international attention in 1992 with a dazzling recording of Rameau's Pièces de Clavecin, then again two years later when his soundtrack for Farinelli (Gerard Corbiau's film about the Italian castrato) sold more than a million copies worldwide. But it's his opulently colourful approach to French harpsichord music of the 17th and 18th centuries that has earned him a reputation as something of a baroque bohemian.

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And yet Rousset has always been a purist at heart. As a child he wanted to be an archaeologist, and says it's a similar passion for unearthing and fastidiously reviving ancient rarities that drives his work as an early music performer. For all his puppyish energy, his street style and his interpretative bravado, when it comes to the nitty-gritty Rousset is a stickler for authenticity.

He founded Les Talens Lyriques in 1991 in the model of baroque ensembles that were typically led from the keyboard, and is plainly dogmatic about using the correct instruments for the group's repertoire. "I'm not interested in mixing and matching," he says, rather bluntly, when I ask whether some of the musicians in Les Talens also play modern instruments in other groups. "If they want to play in this ensemble, they must be absolutely committed to authentic performance techniques."

Les Talens are in Versailles to record Amadis, the latest in their series of Jean-Baptiste Lully operas recorded at the palace's opera house. Lully was court composer here to Louis XIV: this is about as close to an authentic, in situ baroque performance as you're likely to find.

And the results? Earthy and organic, with beguiling vocal trios, tender instrumental chaconnes, lilting choruses and scintillating dance rhythms. Rousset's continuo playing in recitatives is magnificent - every bit as ornamental as the Versailles setting but never obtrusive or extraneous. His lead singers (tenor Cyrill Auvity and soprano Ingrid Perruche) are both visceral, full-throttle, enthralling performers. The recording is released on the French label Aparté next year, and if it captures even a fraction of what I heard at the rehearsal in Versailles it will sound resplendent.

Long before then, Rousset comes to this year's Edinburgh International Festival for a Queen's Hall concert with Les Talens Lyriques and two solo recitals in St Cecilia's Hall, and for all three he has been given rare access to Edinburgh University's historic keyboard collection. His eyes light up when I mention the instruments. "It was such a pleasure to come to Edinburgh to try out the harpsichords. They're wonderful, kept in very good conditions, beautifully restored. Such an asset to your country."

Before choosing his EIF programmes Rousset wanted to get to know the instruments. "I needed to feel something from each of them. I chose the ones that I had a strong affinity with." What does that mean, exactly? "It's about sensuality. It's about what you feel at the end of your fingers - more important than what painting is on the lid! Sometimes the instrument tells you how to play, sometimes you tell the instrument how to play. With an instrument it's always a push and pull."

Eventually he narrowed it down and selected his repertoire accordingly. He'll use the 1709 Thomas Barton harpsichord for Purcell - "a little rough and nutty" - and the 1769 Pascal Taskin "with its beautiful rounded French sound" for Rameau and Balbastre. He matches a late 16th-century Italian virginal with Frescobaldi and a 1620 Neapolitan spinet with Scarlatti sonatas. And he uses the famous 1764 Goermans/Taskin harpsichord, jewel of the university's collection, for an all-Couperin programme with Les Talens. "Couperin's chamber music is very delicate, very intimate, with harmonic language and rhetoric and gestures that are very refined. You have to get into his world to enjoy it. With this instrument I think you will be able to."

Old instruments tend to be eccentric and temperamental; isn't it tricky to adjust from one to the next during the same recital? "Yes, old instruments have strong personalities," he says with a smile, "so I have to find the voice of each one. It's difficult and exciting at the same time. But I've got a few days to get to know them again before the performances."

Anyway, this is exactly the kind of challenge that Rousset rather likes. Rumour has it that his house in Paris is a treasure trove of obscure historic instruments. "That's true, but I always tend to play the same one" - a harpsichord made for him by David Ley - "because it's important that I have a benchmark. My home instrument. It's the anchor that allows me freedom to experiment elsewhere."

Christophe Rousset and Les Talens Lyriques are at the Queen's Hall on August 21. Rousset's solo recitals at St Cecilia's Hall are on August 22 & 23