On Bourbon Street in New Orleans, there are signs in the windows of clubs that say, "Jazz Preserved Here." Most of the signs are lying. You can't preserve the old New Orleans ensemble jazz unless you have the old musicians who developed the style--no, not even in New Orleans, where jazz was born.

Because jazz has become steadily more cosmopolitan during the last 40 years, migrating north to Chicago and later New York and finally exploding world-wide on records. The jazz anyone plays today depends so much on what happened in those years--on the rise of overpowering soloists like Louis Armstrong, the big-city, big-band style of Duke Ellington, the bebop innovations of Charlie Parker, even the European heritage brought in more and more by the Modern Jazz Quartet--that, while young musicians can strive toward a self-consciously primitive jazz style, they cannot duplicate the attitude and style of the working-class men who, in the first quarter of this century, played "jass" on weekends and drove trucks or laid bricks during the week.

Only two bands made up of New Qrleans old-timers still remain. They're both based at Preservation Hall, on St. Peter Street just off Bourbon, and so many of their members have died in recent years that the two bands have to share their trombonist, clarinetist, and drummer. These ancient jazzmen play with the vigor they must have had in the barrelhouse saloons and honky-tonks where they played in the twenties. But nowadays they go on tour and play at Lincoln Center and at Symphony Hall, where they were March 2.

The two Preservation Hall Jazz Bands play an antique jazz preserved, yes, but able to infect a couple of thousand people in Symphony Hall with an enthusiasm that leads to dancing in the aisles. The members of the bands are not innovators, at least not these days. They improvise from the same spirituals and rags and blues they've played for a halfcentury and throw in a "Hello Dolly" so that everyone hears a song he knows. Their playing sounds like the earliest jazz records, though you can hear more than the blended brass screech and the knock of the woodblock that were often all that acoustical recording could capture.

DeDe Pierce's trumpet leads the touring group, which takes its impulsive rhythm from Billie Pierce's constant piano figures and from the steady drumming provided by Cie Frazier on snare, cymbals, bass and woodblock. The music emphasizes collective improvisation, with Willie Humphrey harmonizing the upper voice on the clarinet and Big Jim Robinson filling in on trombone below, with the lowest harmony coming from Allan Jaffe--who runs Preservation Hall and manages the group--on bass horn. The earliest groups has other instruments, like banjo or string bass, and these can be heard on the groups' two most recent records, sold at the concert and by mail ($5 each) by Preservation Hall.

There aren't any opportunities for major virtuoso showpieces, though each player shows what he can do during the short solos that are all part of the group effort. Billie Pierce and Frazier keep each song moving forward, whether in a fun piece like "Ice Cream, You Scream" or in gospel blues like "Closer Walk With Thee," and their ability comes off through their restraint and consistency.

Robinson gets more solo opportunities than is usual for the trombonist, perhaps because, though over 80 years old, he is the spryest of the group. He was alive when Buddy Bolden formed the first real jazz band, back around 1895, with 16-year-old Bunk Johnson as second cornet. Johnson took over the band after Bolden's health failed, and some years later Robinson played with him.

Billie and DeDe sing on all the blues and many of the other songs. Whenever they sing, they show how closely the early jazz instrumental sound modelled the voice of the jazz singer. The twists of melody and even the harmony are less important to the music than the setting of the one instrumental tone against another, while the intense rhythm carries the song forward to its abrupt end.

With the passage of time the music these groups play has become broad in its appeal. The up-tempo songs have become a general expression of joy and enthusiasm, the blues one of sadness in the abstract. When jazz began, its emotions seemed more specific, even functional, engendered by the saloons on the New Orleans waterfront and the brothels in the Storyville district and the people found in either. Whites and middle-class blacks hated the music, just as they despised the life it represented.

But by the sixties the music was so separate from its original low-life milieu that my parents could take me, as a very small boy, to Preservation Hall. Sweet Emma the Bell Gal and Her Dixieland Boys were playing that night. I was nine years old, so it was already late at night when we sat down on folding chairs in the front row. I noticed a small sign on a pegboard wall that said, "Traditional requests $1. Others $2. The Saints $5." My father explained the sign to me, and while the band played a lot of bouncy songs I didn't know, I watched the banjo player and each soloist in turn and waited to hear "When the Saints Come Marching In." It would come soon, I hoped, but not too soon, because as soon as they played "The Saints" the evening would be over.

At Symphony Hall, "The Saints" was the first encore, and everyone knew it would be the last song of the concert. Those who could fit in the remaining space in the aisles left their seats and pressed forward. Robinson danced around the stage, waving his white handkerchief, directing short oom-pah bursts at people in the front. Putting down his trombone, he lifted some little children onto the stage and danced around with them.

Humphrey and Jaffe disappeared for a moment and then came down the center aisle, while the clapping audience followed. Robinson played his clearest, strongest solo of the evening, and Humphrey answered it from the rear balcony. The song lasted fifteen minutes, and when it was over, those in the front reached forward to press hands with Willie and Big Jim.

Jazz doesn't usually play its past; it leaves the past behind as the background for the development of new forms. No one today would try to recreate King Oliver or Jelly Roll Morton, and the great jazzmen still alive play even their old songs in new ways, always experimenting. The Preservation Hall Jazz Band is the remnant of the original jazz--so far removed from current jazz as to be an anachronism, a piece of history that is Storyville and New Orleans and Dixieland, which captivates even children too young to understand nostalgia.