"people talk about us being the biggest band in the world in terms of a 'Time' cover and 'World in Action', and the papers are going crazy about whatever you've said recently. It all seems a bit removed in a way from what we do, because on stage it's just the four of us."

That's right. Just the four of them. Just four of the world's best-known rock megastars, playing collectively under the name U2. There's Bono up there chanting the audience into hysteria. There's The Edge strumming his guitar in his unique - but now much-imitated - style. And then there's the other two, wotsisname the drummer and thingy the bass player. It's "thingy" who we're talking to now: Adam Clayton, bassist with U2 ever since the band started, and now reflecting on the contrast between the status the group now enjoys as global rock heroes, and their humble origins as Irish pub rockers' who had a more meaningful message than "you're a woman I'm a man, I'm gonna love you all I can". These days, U2 not only sell millions of records, play to packed sports arenas and feature on the cover of 'Time'. They also have a habit of causing the odd blip on the world's Richter scale meters.

"There have been rumours of earthquakes that seem to occur on the nights U2 are playing in certain towns", Clayton says in disbelief. "I find it amazing that this can happen, but it's worth remembering how far those bass frequencies can travel!"

Adam puts his Earth-shaking bass sound down to what he calls his "secret weapon" - a Moog Taurus synthesizer pedal from the late 1970s. "I first heard about them from Sting, and I thought: it's a really glorious bass sound, it's so pure, and you get these waves of energy... It's quite a frightening effect - the energy that comes off those, and probably the bass drum as well. Bass frequencies travel a hell of a long way."

Adam Clayton started his playing career with an old acoustic guitar. "It sounded awful, but it didn't really matter, you could just plonk out the chords and learn how to tune.

"I really didn't like playing the bass, because you had to have an amp. It didn't sound exciting - you could really thrash an acoustic guitar and get some excitement out of it. Gradually though, I've been drawn closer and closer to the bass. I like the energy that you can get from bass - in the way that, say, JJ Burnel or Peter Hook played it; they'd play something that gave it a little more personality... More recently, I've been listening to James Brown records, which is another approach to bass that has a lot of personality. And some of the bass on the Marvin Gave records is absolutely fantastic."

But is Adam really happy with his lot, now that he has switched from six strings to four?

"It's a different mental thing to being a guitar player or singer", he explains. "I'm beginning to understand it now, to understand the role, and once you know the structure of the song you can get immense pleasure out of maybe just playing one note."

Clayton may have settled into his role as a bass player, admiring musicians who bring character and feeling to the instrument, but modern techniques like slapping don't appeal to him as a player.

"It's really worn a bit thin. I mean, Mark King or whoever... it's great to be a technician like that, but I don't really feel that U2, or me as a player, really have any grasp of that sort of thing. We really just do what feels comfortable, to reflect the arrangement of the song, and it's not really about that kind of technique.

"I BUY OLD GUITARS. I DON'T MEAN VINTAGE GUITARS; I MEAN A SECONDHAND GUITAR BECAUSE IT'S GOT A BIT OF LIFE TO IT - IT'S BEEN SOMEWHERE AND DONE SOMETHING."

"Music, for us, has to be kept simple because it is a simple form of expression. Very often you can disguise the simplicity of something merely to make it complicated and over-technical, and then you lose the essence of what you had in the first place. I do love players like that, but it's not something I would aspire to."

As big as U2 are, it seems they aren't the kind of band that get regularly asked to extend their musicianship onto the work of other artists. Does Adam Clayton get asked to play with other people?

"Sadly, never at all", he laughs. "The only thing like that I've ever done was when Robbie Robertson came to Dublin to do some backing tracks for his latest album, and the four of us were involved in that. That was my first session gig, I suppose, but I don't know if I could do it for anyone else, to be honest. I'd have to really like them and get on with them before I'd do it... My playing is about personality, and I have to work with people I like working with. They couldn't just show me a chord chart and say: you've got ten minutes to stick the bass down."

With all the big bucks that U2 must obviously be earning, Adam Clayton's attitude to session work isn't perhaps surprising. His attitude to buying equipment, however, is less easy to explain.

"I buy old guitars", he says, "and by that I don't mean vintage guitars; I mean I prefer a secondhand guitar to a new guitar, simply because the price is better, and it's got a bit of life to it - it's been somewhere and done something and it's not all shiny and new. As they say: if it ain't broken, don't fix it!"

It's also difficult to believe that, despite conquering the world, U2 still have a long way to go. Adam Clayton is still young, ambitious, and unquenchably optimistic.

"We're at the peak of where a band can be, and yet it's not a peak for us - we're not 30 yet! A lot of bands are just signing their deals at this age, and then it might take them ten years to get to the position we're in now. For us the next ten years are going to be amazing, because we've got so much time on our hands, and so much energy to do things with it. I think we're in a position to make a lot of good music in whatever idiom we choose - and that's freedom!"

Reflective about the past, calm about the present, confident about the future: being "the biggest band in the world" obviously has its benefits.