When the high note is taken with comfort and confidence, therefore without strain, that's a feat known as "letting off the top." When the stage is taken with comfort and confidence, therefore without shoes, those are the feet of a singer known as Phyllis Hyman.

She has just arrived at -- actually exploded into -- Room 4 of the principals' dressing rooms at the Kennedy Center Opera House, where she is appearing in the Duke Ellington musical revue "Sophisticated Ladies." She sits down and kicks off her shoes.

"You see," she says, explaining the gesture her audiences have not only come to expect, but demand, "by design, that is, by craftsmanship, I wear a size 10. By truth, I wear a 10 3/4 -- make that a 10 1/4 -- and they don't make attractive ladies' shoes in a 10 1/4, so my feet always hurt. One night in a club where I was singing I asked if I could take my shoes off, and they said yes, and I think it made everybody feel more at ease because it showed I wasn't self-conscious."

She still isn't. At 6 curvaceous feet, with wind-blown chestnut hair and full, just-glossed lips, Hyman is a smoldering siren, and she knows it. Yet she looks, in her black and tan separates, authoritative enough to preside over a corporate board meeting. Instead, she presides over the barbecued spare ribs and candied sweet potatoes she's just purchased from the cafeteria.

"One thing I learned at Morris," she says, referring to Robert Morris business college in her home town of Pittsburgh, "was that Phyllis, you're a lucky lady. You've got yourself, you've got a wonderful family, you've got a tremendous business sense and you know how to deal with people."

Hyman has conspicuously omitted the most valuable "got" she's got: the throaty thunder that won her the first music scholarship Morris had ever awarded, and, later, steady dates with clubs; commercial jingles for Sasson jeans, McDonald's, Burger King, Tuborg and Diet Pepsi; recording contracts with Buddah and later Arista, where she cut the albums "Phyllis Hyman," "Somewhere in My Lifetime" and "You Know How to Love Me"; the title of Best New Female Rhythm & Blues Vocalist of 1977 by Record World; and now, the beckon of Broadway.

With a voice like that, what was she doing in business school? "I didn't know I could sing," she says. "Not like Nancy Wilson, or Dionne Warwick. kI mean in terms of technique and quality and control, and Nancy has the technique of life. I was perfectly satisfied to have a normal life, a family -- not necessarily a husband and kids -- but you know, steady work, security, and I've never really struggled, so I was happy with that. That was the way I was brought up."

But sing she did, "at first in little clubs around Pittsburgh. Then I left college in '71 and went on the road for the first time with [Washington pianist] Dick Morgan. From then on I was hooked."

She credits Morgan with teaching her "everything" about show business -- "how to wear my hair, do my makeup. I believe when you work for someone you do what they tell you to do." She assumes a parody of her somewhat awkward stance. "I used to say, 'Dick, teach me how to stand.' And he's say, 'Forget it, Phyllis, that's how we found you, stand like that.'"

And she credits herself for summoning the courage to conquer her stage fright. "At first, when I started to do little clubs, I was okay as long as you weren't looking at me. I used to think people were out of their minds for paying good money to come and hear me sing." Then one night, she says, she got the nerve to really look at their faces. "And I realized how lucky I was. I thought 'You have something to offer that people enjoy and when they enjoy it you give it to them.' So I've been giving ever since."

Although she has had no "real vocal training," Hyman studied voice in Miami for six months in 1973 "to see what I was doing wrong and learn how to breathe. I didn't know that when you breathe correctly you're supposed to feel it in your back. It's supposed to expand. And of course talking hurts your voice, so naturally mine is always sore."

In 1975 producer Norman Connors convinced Hyman to join him in recording the Stylistics' "Betcha By Golly Wow" after hearing her at Rust Brown's club in New York. Her involvement with "Sophisticated Ladies" is the result of the show's music coordinator and assistant conductor, Lloyd Mayers, having heard her sing at Mickel's in New York last year. "It was really funny because Leslie Uggams and Marilyn McCoo and a host of other vocalists had auditioned for the show. Then Lloyd heard me and he said, 'I found my girl.'"

Her contribution to the show will be "a little dancing" and the Hymanizing of Ellington's "In a Sentimental Mood," "Prelude to a Kiss," "I'm Checking Out Goombye," "Tell Me It's the Truth" and "I Got It Bad and That Ain't Good."

Though she won't disclose her age ("there's an artistic handicap to consider: once you give it out, you get older"), Hyman is the oldest of seven children of a Pittsburgh barber and housewife. She is separated from her husband and former manager Larry Alexander. "He lives in Florida and I live in New York, but a lot of years have gone into our relationship, we're very close and I don't want to just forget him or that part of my life."

Although she claims not to be particularly anxious about her theatrical debut in "Sophisticated Ladies," she is excited by the professional departure it has marked for her. "To me, opening night was the first dress rehearsal we ever did of this show, which is a perception of someone who was not trained in the theater. Theater audiences are more reserved. I'm off tour for a year. I'll be doing records but I'll miss the immediate response you get at clubs and concerts. I'm enjoying this challenge. I've had to get used to the strict discipline of the theater. When the show starts at 8 and you have a half-hour break at 7, if you show up at 7:35 you do not go on that night."

Even with "Sophisticated Ladies" and her new album, "Sunshine in My Life" -- "guaranteed gold," according to Arista's Steve Ruben -- coming out this spring, Hyman is ever planning for the future. She's not at all vague about her ultimate destination: "It had better lead to Vegas and TV." She laughs. "What I love most about this business is that it allows you the versatility of having many different jobs. I didn't know how to put my own makeup on when I first started out. Now, I'm a professional makeup artist. I've done Jayne Kennedy, Wilma Rudolph and other women for magazine covers." She has translated her business sense into her own production company, Command Performances Inc., and a management firm.

What if Phyllis Hyman does reach the pot of green at the end of the rainbow? "I used to be afraid of success," she says, "because of the pain I saw it bring to other people. But I've studied being human, so when the big stuff hits, I'm gonna be ready for it."

Even the mob scenes and invasion of privacy? "Look, I like people. I can and want to deal with people. Besides, I can always disguise myself." Even . . . at 6 feet tall? "Sure," she says, "I can ride in a wheelchair."