How does an explorer discover effects? What's it like swapping unforgettable theories with Brian Eno? And how does the Edge get that fantastic sound? You too can hear the answers as Adam Sweeting (words) and Paul Rider (pics) take pride in their work.

You probably have your own favourite moment of The Edge's guitar in action — the opening clarion-call of "I Will Follow", perhaps, or the palpitating flurry of echo in "The Electric Co", or possibly the noble chime of "Pride". One thing is certain. Once heard, Edge's playing is not forgotten. Edge — real name David Evans — puts the tingle in U2.

U2 have weathered a torrent of hostile criticism in their time: they are "too rockist", they "preach", singer and lyricist Bono is "pompous" or "egotistical". Their album "War", released at the beginning of 1983, saw U2 reaching the high tide of this polemical phase in their career, with blunt-edged songs like "New Year's Day", "Seconds", and especially "Sunday Bloody Sunday". Stirring as some of this material undoubtedly was, it began to look as though U2 had completely lost the magic and mystery which had made their 1980 debut album "Boy" such hypnotic listening. Edge's playing, too, was harsher and less impressionistic.

But then in October 1984 came "The Unforgettable Fire", an LP on which U2 had for the first time dispensed with the services of producer Steve Lillywhite. This time, they'd locked themselves away in the majestic pile called Slane Castle, near Dublin, with Brian Eno and Daniel Lanois. "Pride", the single which preceded the album, was a return to the classic U2 virtues of fluid melody backed up by Edge's striding guitar figures, with the benefit of a little additional atmosphere. The album was a different kettle of fish entirely.

Edge himself is well aware of the transformation in U2's approach. "That record 'War' in a way has enabled us to make this record," he mused, over morning tea in a plush hotel in Paris. "If we hadn't made 'War' and created the sort of economic freedom we now have, there is no way we could have made 'Unforgettable Fire', so I'm very glad we did.

"But I'm also aware that it was almost a one-faceted side of the group that we showed on that record, and we are definitely multifaceted as a band. We will probably never make a record like 'War' again — it's done us harm in some quarters but it's also done us an awful lot of good."

It was essentially "War" which broke U2 through to a mass American audience. "It's kind of ironic that that record was a reaction to what was happening in England and Europe," says Edge, "which was a lot of mush and pap, and everything that we became a band to fight against in a strange way."

"The Unforgettable Fire" finds U2 exploding preconceptions and going widescreen and open-ended. If you'd never heard U2 before, the LP would give you a misleading impression of their frame of reference. "4th Of July" or "Elvis Presley And America" build insidious atmospheres while you're not looking, "Wire" finds Edge assembling layers of jittery rhythm guitars which sting like a swarm of starving bees, while "MLK" — like "Pride", a song about Martin Luther King — is a ballad of softly haunting lushness.

Much of the album came about via Edge and Bono closeting themselves away with instruments and tape machines to construct demos, which were then passed on to bassman Adam Clayton and drummer Larry Mullen for their contributions. One senses that Clayton was not entirely happy about this assumption of creative leadership by the group's pivotal duo, while it doesn't seem unreasonable to infer a good deal of studio interaction between Brian Eno and Edge (the latter is, after all, developing a forehead of impressively Enoid proportions). Tactfully, Edge denies this.

"Well, certainly I spent a lot of time with Brian during the course of the record, but it certainly wasn't a case of us running away with the project. Nobody was interested in taking a back seat at all. When we first started working with Brian and Danny Lanois we had about 10 or 15 pieces of music, some with melodies, some with ideas for melodies, and none of them was really finished. Songs like 'Pride' and 'Unforgettable Fire' had been demoed in rough form, so we knew more or less what we were dealing with there.

"But immediately when we started working with Brian, instead of finishing the 15 songs we had started, we actually started writing another 15, so within about a month of starting work we found ourselves with 30 songs, none of which was finished. That was extremely good for us, I think, extremely creative. I think we got the best out of Brian in that first month, 'cos he was throwing a few cats among the pigeons regarding our methods of writing, some of the axioms on which we base a lot of our work." Lanois too, who owns his own recording studio in Hamilton, Canada, was able to chuck his knowledge of production, engineering and musical theory into the pot.

According to Adam Clayton, U2's previous songwriting procedure had often consisted of little more than the group jamming in a room, then Bono singing over the top. For "Unforgettable Fire", assorted unexpected procedures came into play. Consider "A Sort Of Homecoming", for instance. Edge explains.

"The core of that particular number is the melody I think, it's really strong. I think there will eventually be a very different version of that song that we do live, that will be really exciting. That song in the studio was such a fluke... in fact, when we recorded the backing track, Danny was just messing around with treatments and sounds of instruments, and without the vocal he just did a mix of the instruments on to quarter-inch tape, just guitar, bass and keyboards, and I had that on a cassette for a while. It was always my favourite version.

"When we started doing the vocal, the rough mixes were never as good, so when we finally finished the vocals of that song Danny said, 'That's a great backing track. Why don't we just bounce Bono's voice on to it and we'll see what that sounds like?' So that's what we did, and that's the version we put on record."

"Unforgettable Fire" itself Edge regards as more or less an instrumental track over which Bono adds improvised melodies. "The melody came as a layer on top of the music," he says, "so it's just as legitimate to replace that final layer as you wish at a later date, which is what we're gonna be doing. I think Bono's writing his best melodies for that song onstage, and he would admit that."

And then there was the case of "4th Of July", which is just Clayton and Edge doodling in the studio. Eno, fiddling about with treatments in the control room at the time, simply rolled the quarter-inch tape. Everybody forgot about the piece until the end of the recording sessions, when a final rummage unearthed it again and Bono at once insisted on its inclusion on the album. It was another piece that never made it on to multitrack.

Would it, I queried, be reasonable to construct some sort of link between Edge's input to "Unforgettable Fire" and the work he contributed to "Snake Charmer", a bizarre little item he recorded in 1983 in collaboration with Jah Wobble, Big Apple production whizz Francois Kevorkian, Holger Czukay and Jaki Leibezeit? Edge pondered, then opined that whereas "Snake Charmer" had been unguided chaos in the studio, "Unforgettable Fire" had been propelled purposefully by both Bono and Eno, men who know what they're after.

"Snake Charmer" was an education nonetheless, even though Edge was only able to commit three days to the project which he estimates was approximately half the time he'd have liked. "When I arrived the bare skeleton was already there, so in a way my billing on it was a little embarrassing. But what I did enjoy was the collisions within the project, because it was great to work with Holger and Jaki who are such incredibly sort of gypsies. They're great guys, real music theorists."

More specifically though, might there not be a parallel between the "Snake Charmer" track "Hold On To Your Dreams" and U2's nervy "Wire"?

Edge: "I think we as a band were going through a bit of a renaissance concerning rhythm — I mean, we were the band who were described as taking an 'anti-dance stance'. To be fair to us, we've never been disco-orientated, we were never interested in black music originally. Our roots in terms of music were very white. This has changed, but around that time we were getting more and more aware of the rhythm — the lack of rhythm — in what we do.

"I'm not saying 'Unforgettable Fire' is an incredibly rhythmic record, it certainly isn't, but that led us to experiment more in that area and consequently tracks like 'Wire' I think are more rhythmic than stuff on our other LPs. I think Francois Kervokian had a very good sense of rhythm and a lot of his input to 'Snake Charmer' was a rhythmic one. I don't think U2 will ever be a dance band. I think it's down to aptitude really, and ability. I don't think U2 has it in it to be incredibly rhythmic, and maybe we shouldn't try."

As you can see, Edge is not a pretentious man. When I probe him about the origins of his striking guitar style, he is similarly keen to stress that it was born out of the limitations of circumstance.

"My guitar style has always been the product really of the rest of the group. Adam is a very ostentatious sort of person, y'know, very extravagant, so when he started playing bass he wasn't interested in taking the bottom end of the sound spectrum at all. He wanted to be right up there in the mid-ranges, so his bass sounds were always extremely full with a lot of top end — very different to say Simple Minds or any of the other bands around that era, the Bunnymen or anybody like that.

"In order to give the group any sort of clarity, therefore, I had to stay away from the bottom end of the guitar as much as I could. So I tended to work around those high chords, that ringing sound, and we were never really rhythmic so there seemed to be no opening for that kind of rhythm guitar that Mick Jones has done very well, or Keith Richards. I was never really interested in that.

"So when we started writing our own songs it just developed, this style of using high chords with that sort of ringing quality, not particularly rhythmic but more just a harmonic wash over little hook lines picked out within the songs, essentially very much as it is now. Then we started making a bit of money on gigs so I bought an echo unit and various other boxes, and most of the other boxes I discarded very quickly. I hate effects, the kind of things that jump out at you."

You mean obvious flanging and chorus? "Yeah. We call all those 'squidge'. We have a very low squidge threshold. With echo, it has a certain purity to it because it is a natural thing — or it's synthesising a natural phenomenon anyway. I found that echo was the most rewarding and most interesting, so I just started developing along those lines."

By the time of "War", Edge's soaring echo-treated playing had become so influential that loads of people were doing it. Edge, mindful of sounding like a parody of himself, decided to give it a break.

"With 'War' we decided we'd leave that aside for a while, we could afford to do that for an LP, and just stick to more straight hard-hitting guitar sounds. With this new record, as I say, we wanted to show off all the different sides of the group, so I decided to develop the echo thing. So with this album I used a lot of different guitars, whereas before it was mainly the Strat and the Explorer. I used an awful lot of a Washburn acoustic with an internal pickup, and in the studio for some reason it sounded really good. It's a sign of a good instrument, actually, if it's one that inspires you when you pick it up, and the Washburn certainly did that.

"Also, there was Eno's ability to affect the sound of what you were doing with subtle treatments. A perfect example of that is '4th Of July'. I read a review in some American paper where it's talking about the keyboards on that track, but in fact there's just two instruments on it — bass guitar and guitar, there's no keyboards at all. Everything that sounds like a keyboard is Brian's treatments, which are complex chains of various ancillary gear. It's not easy to explain what they are, but they work very well.

"To use effects in a subtle but effective way, that's the difficult thing. As Holger Czukay said, 'People who use effects should be shot! I never use an effect!' What he was trying to say, in his broken English, was that you cannot just slap a whole load of something on. That's what they do with video effects now, 'Right, let's slap a bit of this on that shot.' That's how effects were formerly used in music, but people's sensibilities now are more highly tuned. It's definitely the effects you're not really aware of that are the really successful ones."

What gadgets does the fastidious Edge allow in his armoury nowadays, then? "Well, I use digital delays, I also use a harmoniser, a device which changes the pitch of what you're playing — I tend to use it in octaves above or below. But you can also take out, say, a fifth harmony. I know Stuart Adamson uses one as well quite a lot for his lead passages, so you get that bagpipe sound he uses. I also use a digital reverb unit which reproduces the sound of the auditorium, which in a live context isn't quite so necessary, but it adds to a sound. The trick is really the chain of effects, using them in what order. Like I've got stereo amps, so I maybe have two slightly different echoes coming from the two amps, so you get this interesting sweep to the sound.

"I want to get into quadrophonic effects, that's the next thing. I think someone has been using quad PA systems, which I think is a very interesting idea. I think it was Pink Floyd, unfortunately."

Edge was brought up on a diet of just about everything, music-wise — he mentions Zeppelin, Yes and the Mahavishnu Orchestra as formative listening. As he developed as a musician, he found himself listening to a narrower spectrum of music with a more critical ear. "Which is a shame, because I think now probably my enjoyment of music is limited because it's obviously affected by my own work. I listen to something and I don't just appreciate it as it is, I tend to analyse it."

He was able to find music which stood up to his penetrating gaze, fortunately, and a lot of it was coming out of New York. "When the band formed it was around the time of things like Patti Smith's 'Horses'. 'Marquee Moon' by Television was one of my favourite LPs for a long time. In fact 'Before And After Science', the Eno LP, was out then, and that was another of my favourite records. There was a sort of trend there. Even though Patti Smith didn't use that ambient thing, she's very hard-hitting, there was a certain romanticism in her work, certainly lyrically, that was a point of reference. She was on the same merry-go-round, I suppose."

Indeed, when U2 crossed the water for their first visit to a London supposedly electrified by punk, they were sadly disillusioned — there was nothing going down at all. "We discovered that people were just as closed and self-conscious — self-conscious as opposed to other-people conscious — and negative as well, as they'd ever been. It seemed to us and it still does that the provincial bands were the ones who broke that. The London groups all seemed to be really trivial."

But that was an age ago. These days, Edge doesn't really think of himself merely as "a guitar player" any more. "I think of myself as kind of broader than that," he explained carefully. "I'm not an incredibly versatile guitar player, but I've made best use of what limited talents I have. I think my talent is possibly applying my abilities in a new way, so that could be production, it could be songwriting, it could be guitar playing, it could be anything. I enjoyed Eno because I could see he did that as well.

"He's not a great keyboard player, he doesn't write great songs really, he doesn't have the craft that say Bowie has to write a song, or Paul McCartney. His engineering and technical abilities are limited as well. In fact he knows very little about an awful lot, but it's how he applies that knowledge. I suppose it's down to confidence, too."

Eno worked with U2 because, although no great rock fan, he was struck by the spirit within the group. With "The Unforgettable Fire", U2 have managed to evolve artistically while keeping the core of the group intact. Meanwhile, it seems that Edge may now be developing capabilities which will lead him outside U2's collective format. Whatever course he takes should prove beneficial to all parties.