Pierre Bensusan remembers the moment that changed his life.

As a young guitarist living in a small flat in Paris with parents and two sisters who listened to or sang music all the time, he was used to hearing all kinds of sounds.

"My father had played the trumpet when he was young and when he wasn't working he'd be out at concerts or listening to opera, swing or French accordion music. My mother had played violin and she always sang around the house, and my sisters, they played their records so loud that whatever they were listening to, we were all listening to," says the Algerian-born Bensusan.

"So I'd heard a lot of stuff by the time I was 12 or so, but when I heard Pentangle for the first time, I thought, 'Oh my God'. These two guitarists, John Renbourn and Bert Jansch, playing together were incredible and the way they played made me look at the guitar differently. From that moment, I became dedicated to the instrument full-time."

Forty years on from Bensusan's emergence with a precocious first album as a 17-year-old, he's a musician of international standing, revered in guitar circles for his mastery of the alternative tuning DADGAD and busy with concerts in all sorts of places and situations. As we speak he's in Washington DC, looking forward to returning to Scotland this weekend but preparing to perform that evening at a mathematicians' convention. He hopes, he jokes, that this doesn't say something cruel about his music.

The guitar wasn't his first instrument. He took piano lessons from the age of seven but became disenchanted after four years and gave up. A year later his father bought him a guitar and he taught himself to play.

"I learned this tune by ear," he says, singing the opening lines of Spanish guitar classic Romance Anónimo. "I didn't know it was a famous tune, but from learning it by ear I was able to work out how to tune the guitar, and from there I moved on to Bob Dylan, Donovan, Joan Baez, Cat Stevens, and Crosby, Stills, Nash & Young songs until Pentangle showed me that the guitar could be much more than an accompanying instrument."

British folk guitar heroes - Nic Jones, Martin Carthy and Davey Graham, as well as Jansch and Renbourn - became his inspiration. He's mildly envious of today's internet surfers having instant access to virtually every style and piece of music in existence but looks back at his own searches through blues and ragtime as part of the buzz of discovery. Hearing Planxty at a folk festival in Brittany that he'd hitch-hiked to opened another door and, although he can't explain why, he somehow felt very close to Irish music.

"At a certain point, in my mid-teens, I think, I'd been experimenting with different guitar tunings and listening to all this music and I thought, 'this is all great but who am I and where do I fit in?'" he says. "I felt I had to find a place where I could develop my own voice. So I decided I had to be more conservative. Coming from the piano, where you have all the notes in front of you, I realised that I had to have something similar with the guitar. I said to myself, okay, let's have one guitar, one tuning - I settled on DADGAD - and let's just concentrate on that."

DADGAD became, he says, his language and he became completely fluent in it by playing jazz standards, Bach, Debussy, anything, especially if it wasn't conventional guitar music and he could adapt it to suit this tuning.

"I played for hours and hours, although I didn't play enough in my opinion," he says. "And that's the way I would like to be still. People have this romantic view of a musician's life being about playing an instrument all the time but when you're travelling, you're not playing, and when I do get the time, I like nothing better than to sit and play scales."

He may not be able to play on journeys but he can compose because much of his music is constructed entirely in his head, often begun by trying to imagine something he'd like to hear.

"I think about voicings, colours, counterpoint, harmony, bass lines, melodies and only when I have all that together do I transfer it to the guitar," he says. "That way, I have the music in my mind but I can still be spontaneous and it's the same when I play live. Everything you do offstage - practising, composing, re-composing - is preparation for being onstage, where things happen in the moment and you have to be ready to grab the music and take the people with you."

Pierre Bensusan plays Milngavie Folk Club on Saturday, June 6 and Traverse Theatre, Edinburgh on Monday, June 8.