MUSIC/Previn is back in London -- making music with the Royal Philharmonic Orchestra

Andreas Ludwig Prewin (alias Andr'e Previn) is back, with a vengeance. He's been celebratiang his assumption of the Music Directorship of London's Royal Philharmonic Orchestra with a series of concerts called (what else?) the Andr'e Previn Music Festival.

Flamboyant self-advertisement? Well, perhaps. But there's no gainsaying that the London concert scene has sprung to life in a way not seen since those heady days in the early '60s when Previn and the London Symphony Orchestra embarked on what was to be an artistically, and financially, fruitful relationship. It's the old story of right people, right time, right place.

The Royal Philharmonic badly needed someone with Previn's crowd-pulling power to erase their reputation as a basically fine outfit in the doldrums, and in dire financial trouble. There was even talk of the RPO being either disbanded or banished to a provincial base such as Leeds or Sheffield.

Equally, it was an open secret that Previn yearned for another London post. He likes working with British musicians; he has a half-timbered 18th-century house in Surrey; his fourth wife, Heather, is English. So, after a diplomatic minuet, a deal was done. Previn gave up the Pittsburgh Symphony Orchestra -- and the outcome, in an astonishingly short time, is a rejuvenated RPO.

They not only look different, resplendent in dazzling white tuxedos (what would their founder, Sir Thomas Beecham, have said about that?) but they sound different, morale restored, and playing to a standard that must give the other London orchestras food for thought. Their performance of that Previn specialty, Walton's First Symphony, whetted the appetite for their forthcoming new recording. Another -- of Elgar's First -- should have silenced the doubters who've suggested that Previn's love of English music is just a token affair more or less confined to Walton and Vaughan Williams.

There's no doubt that the RPO post comes at a turning point in the one-time wunderkind's career (later this year he also succeeds Carlo Maria Giulini as music director of the Los Angeles Phil.). Previn's later years with the LSO were bedevilled by complaints that he neglected the staple, classical repertory in favor of Shostakovich, Prokofiev, Vaughan Williams, Walton, and other modern masters. Clearly, he's profited from his sojourn in Pittsburg by coming firmly to grips with Hayden, Beethoven, and Bra hms. No Bruckner yet -- but then, he once told an interviewer that he didn't see the point of tackling Bruckner when Eugen Jochum did it so much better than anyone else.

It's to be hoped that Previn's future London programs will include American music, and a wide variety of it.

There was, sadly, too little of that in the American Festival staged over three weeks simultaneously in London, Glasgow, and Cardiff. It was billed as ``the greatest celebration of American arts and entertainment ever held outside the United States'' -- returning the compliment for the ``Britain Salutes New York'' festival of 1983.

The celebration began at London's Royal Festival Hall with two speeches, two national anthems, and a message from President Reagan. Then, the St. Louis Symphony Orchestra sat down to play. The critics warmed to the ``European'' sound and to their conductor, Leonard Slatkin. He's become a more familiar figure thanks to a series of taped concerts broadcast by BBC Radio Three, and to his appearance in two TV films about the Tanglewood summer music school. The Dallas Symphony Orchestra came next -- conducte d by Eduardo Mata in the gracious presence of Larry (J.R.) Hagman. They, too, aroused much interest -- but where, apart from some bits of Barber and Bernstein, was American music to add something to the average concert-goer's knowledge?

For real revelations one had to venture to the Almeida Theatre, a beautifully restored former vaudeville house in the London district of Islington. Here, for starters, Yvar Mikhashoff gave an extraordinary seven-hour marathon of 70 piano pieces covering 70 years of American music, and including composers as diverse as Charles Ives and Zez Confrey, Copland, and Cage. But the Almeida's real coup was to lure to Britain two of American music's greatest individualists. Virgil Thomson introduced his own works

-- and showed that, at 88, he has lost none of the astringency that once made him such a feared and revered critic. Conlon Nancarrow arrived bringing his entire family, plus his cook. After fighting against Franco in the Spanish Civil War, Nancarrow lived in exile for 40 years. In the early '70s, he was ``discovered'' along with his exuberant works for player-piano, which can make one instrument sound as though it were being played by many hands.