In the last week of August 1970, somewhere between six and seven hundred thousand people gathered on a hillside on the Isle of Wight. By the Guinness estimate, it was one of the largest gatherings in the history of the British Isles, a city the size of Glasgow assembled on farmland before a single wooden stage. Every watt of sound coming off that stage belonged to a balding, 47-year-old man from Balham who did not much care for rock and roll. His name was Charlie Watkins.

He had spent the war in the merchant navy and the years since selling accordions and guitars from a cramped South London shop. On that night he crouched behind a mixing desk, needles buried in the red, pushing close to 5,000 watts into the largest open-air audience Britain had ever produced. It was the biggest sound system in the world, and he had built it himself in a small factory off the Kennington Road with a staff you could count on your fingers and toes. On the third night, Jimi Hendrix walked out to play his last performance in Britain.

Within three weeks he would be dead in a flat in Notting Hill. Nobody on that hill knew it yet. What they knew was that the sound was not right. The wind had got up off the Solent and was throwing Charlie’s carefully stacked columns of noise backwards over the heads of the crowd toward the sea.

The people at the front heard music. The people at the back heard weather. Somewhere in the recordings from that night you can catch Hendrix’s voice between numbers, not singing, just asking: turn it up, Charlie. He could not turn it up.

The machine was already screaming. This is the strange thing about Charlie Watkins, and it is the thing the entire story turns on. He is arguably the man who invented the sound of the modern music festival: the wall of speakers, the mixing desk out front, the whole architecture of how half a million people can hear one guitar. The Rolling Stones used his gear.

The Who used it. Pink Floyd used more of it than anyone. And yet the events he made audible are legendary, while the man who made them audible is a footnote. He never wanted it any other way.

He was born in London in 1923, a working-class childhood between the wars. He left school early and went to sea, joining the merchant navy in 1938 at fifteen. A year later the war began, and the merchant navy became one of the most dangerous jobs a young man could hold: unarmed ships running the gauntlet of the North Atlantic convoys while German U-boats hunted them in packs. Roughly one in four merchant navy crews did not come home.

It was on a ship in the mess room that Charlie fell in love, not with the sea but with a steward who played the accordion to pass the long hours between watches. He begged to be taught. Decades later, when the empire was gone and the festivals were over, he would come back to that instrument. He survived the war and through the late 1940s made his living as a professional accordionist.

In 1949, with his brother Reg, he opened a record shop in Tooting Market. By 1951 they had taken bigger premises in Balham and began selling instruments as well as records: accordions naturally, and guitars. In that early-1950s Britain, postwar currency restrictions made it almost impossible to import American equipment, so a shopkeeper in Balham simply could not get the good amplifiers. Charlie decided he would build them himself.

He had no formal training in electronics, just a tinkerer’s confidence, a backroom that smelled of hot valves and solder, and by his own cheerful admission a Black & Decker drill that constituted the entire manufacturing capacity of the operation. In 1957 skiffle arrived, and suddenly every teenager in Britain wanted a guitar, and every guitar wanted an amplifier. The demand broke over the exact spot where Charlie happened to be standing. The machines that made his name were not amplifiers, though.

It was a box that made echo. Charlie had heard records by an Italian bandleader featuring a lush repeating slapback echo, and he found out how it was done: two tape machines wired together, one recording and one playing back a fraction of a second later. His genius was to ask a simple question: what if you could put that in a box cheap enough for any working band to carry to a gig? He handed the problem to an engineer named Bill Perkins, who cracked it in three months.

The unit they built in 1958 held a single loop of tape running past three replay heads, with a feedback loop that let the echo repeat and decay, all in a turquoise and cream case. They called it the Copicat. The first hundred sold out in a single Saturday, with the queue running down Balham High Road. Within a couple of years WEM was selling in the region of a thousand a month.

Everybody wanted to copy the Copicat. Rivals across Europe built their own tape echoes, and they all found to their endless frustration that they couldn’t quite catch the sound. Charlie knew exactly why and took a certain wicked pleasure in never telling anyone. The magic, he confessed years later, came from a bit of bad engineering, an accidental flaw that gave the machine its particular character.

The competitors kept doing the sensible thing: improving it, cleaning it up, refining it. And in refining away the flaw, they refined away the very thing that made it worth having. Charlie did not patent the Copicat. That fact looms enormous in the legend.

But it sold hundreds of thousands of units and stayed in production for over fifty years. By the start of the 1960s the little shop had become a proper firm. At its peak the factory employed around 40 people. With those 40 pairs of hands, WEM became one of the big three of the British musical instrument trade, alongside Vox and Selma, with a drum shop owner in West London named Jim Marshall rising behind them all.

And here the legend of Charlie Watkins as the trusting engineer robbed blind by copycats quietly turns inside out. Jim Marshall’s celebrated 18-watt amplifier is, when you put the two circuits side by side, very nearly a copy of Charlie’s Watkins Dominator from the late 1950s. In the amplifier world, the copying didn’t run away from Charlie Watkins. It ran toward him.

It was Marshall, not Watkins, who went on to conquer the world with it. All of this was prologue. In the mid-1960s Charlie started doing something no one in Britain had properly done before: making it possible for a crowd of half a million people to hear a band at all. It began with a humiliation in 1965, when a WEM public address system failed to carry the American band The Byrds over the screams of their fans.

Charlie came away determined to solve the problem of live sound. Working with an engineer named Norman Sergeant, he developed the WEM SL100, a power amplifier that could be daisy-chained, one master unit feeding a whole row of slaves, each driving its own column of speakers. A wall. Then came Windsor in August 1967, where Charlie assembled what is generally reckoned to be the first 1,000-watt PA in the world.

He wanted, he later said, the loudest sound heard since Hitler addressed the rallies at Nuremberg. The sound rolled off the field and into the town, and the authorities came for him. He was escorted from the mixing position by police mid-festival, and the case went to court. Charlie was defended by Quintin Hogg, one of the most eminent QCs in the land, and he won.

The loudest sound in Britain was declared lawful. That same festival gave him another invention, entirely by accident, when a singer with the band Family grabbed a side speaker and hauled it round to face himself so he could hear over the wall of noise. Charlie watched and understood instantly what he was looking at: the stage monitor, the wedge-shaped speaker angled up at the performer’s feet. He simply started building it into the system.

The golden run came in 1969 and 1970. In April 1969 Janis Joplin played the Royal Albert Hall with WEM sound; she was so moved by hearing herself carried properly to an audience that she reportedly wept with gratitude afterwards and had the rig flown back across the Atlantic. That July the Rolling Stones played their free concert in Hyde Park before a quarter of a million people, the sound carrying clean out of the park and reportedly heard seven miles away. The historian Gary Cooper observed that without Charlie, that concert could not have happened at all.

Pink Floyd became his biggest customers, arranging their WEM columns across the back cover of their 1969 album Ummagumma. David Bowie was a familiar face around the operation. Mark Bolan wrote him a book of poems. And back at the factory, the low frequencies the system could produce were so powerful that the young women working in the upstairs office were sometimes reduced to tears by the sheer bodily force of the bass being tested below them.

Then came the Isle of Wight and the summit and the wind. On that windswept hill, Charlie’s largest rig ever built, close to 5,000 watts, the definitive proof of the wall of sound, was a partial defeat. The wind scattered his masterpiece across the water, and Hendrix, three weeks from death, kept asking for more. The single largest thing Charlie Watkins ever built is also the place where his sound failed most publicly, defeated not by any rival or patent but by the weather.

His absolute peak and the beginning of his disillusion were one event. And then the ground began to shift. The hire firms arrived, renting rigs to bands instead of selling them, and were often better. People were starting to ask whether these enormous volumes were dangerous; in 1973 sound levels were measured at a constant 128 decibels.

Charlie had a conscience about it. He went privately to a Harley Street specialist to ask whether the thing he had built might be causing permanent harm. The technology was turning too. The bucket brigade chip could produce echo with no tape at all, and by 1983 digital delay made the Copicat obsolete, not through a Japanese conspiracy but through the quiet, unstoppable arrival of a better way to make the same sound.

And Charlie’s own health was failing, a thyroid condition draining the energy on which the whole enterprise ran. He had never built a successor. There was no single fatal mistake. The unpatented Copicat, which outlived everything built to replace it, was arguably the shrewdest call of his career, because a patent would have required him to publish the exact secret of the flaw that gave the machine its magic.

The one product he did aggressively patent, a guitar organ, flopped. In 1964 Norman Sergeant talked him out of building a big valve amplifier, and he watched Jim Marshall ride that wave to worldwide fame, but Charlie never really chased fortune. He lent his gear to bands for nothing, mixed festivals for free, built a firm designed to produce fame instead of capital. And in 1974, at the very height of his fame, he looked at the hire firms and the noise laws and the flying bottles and the failing thyroid, and he decided to stop.

Not because he was beaten, but because he had had enough. He paid off his staff, sold the factory, and went home to the accordion. There was no bankruptcy, no liquidators van, no foreign corporation carrying the assets away. A firm that had made the Rolling Stones audible to a quarter of a million people was closed by the deliberate, sober decision of the man who owned it.

And the Copicat did not die. It went solid state, went digital, and Charlie himself near the turn of the millennium made a fresh run of the original analog machines; among the first to ring up was David Gilmour of Pink Floyd. In 1985 Charlie remarried, and he spent his retirement happily developing MIDI accordion systems. He was given a lifetime achievement award by Audio Pro International in 2011, recorded an oral history for the NAMM archive, and died at home in Balham on the 28th of October 2014, aged 91.

The original Isle of Wight PA was tracked down and acquired by a private collector, preserved under exhibition lights. The live sound industry, now worth many billions, stands on foundations Charlie Watkins laid: the slave amplifier, the portable mixing desk, the stage monitor, the wall of speakers. The industry scarcely knows his name. Nobody stole the empire.

There were no billions to lose. He gave much of its value away on purpose, because he cared more for the engineering than the money, and when the world he had made turned loud and violent and troubling to his conscience, he set it down and walked home.