Mrs Peate, 34, of Runcorn, Cheshire, a euphonium player, had denied killing Mr Phillips, 62, a bass trombonist with the Frodsham Silver Band, on the night of 15 November 1994.
Outside Chester Crown Court yesterday Mrs Peate, 34 and a mother of two, refused to make any comment. Her husband, Graham, 36, said: "We are very relieved it is all over and that Polly has been vindicated."
During the four-day trial, the court heard that the couple regularly had sexual intercourse in his car after band rehearsals. The affair was "an open secret" among their fellow musicians.
Robert Fletcher, 27, a bass player with the group, had told police: "It became common knowledge throughout the band that a relationship had formed. They were inseparable."
However, on the night Mr Phillips died, the couple had gone to a lane as usual in his car and Mr Phillips had told her that it was to be the last time they would have sexual intercourse together.
The prosecution alleged that Mrs Peate struck him about the head at least four times, causing the heart attack that killed him. But the jury accepted Mrs Peate's claim that she had not attacked him and that he had died while they were making love.
She told the court: "He died having sex. I'm sorry, but that's how it happened. I have nothing to admit."
The affair had been hidden from their spouses. Mr Phillips's wife, Sybil, met Polly when the band was performing, but was totally unaware of the relationship with her husband, whom she married almost 40 years ago when she was 19.
She kept away from the court during the trial, but had told police: "I was totally unaware . . . that Polly and my husband were having an affair . . . It has come as a great shock to me. I have never suspected him . . . of having any affairs."
Mrs Peate told the court that she loved her husband and had been happy with him, although he had been a gambler.
When police had questioned Mr Peate he said: "I would describe my marriage as perfect. My wife and I get on together and I trust her implicitly."
Among the witnesses in the case were the band vice-chairman, Terence Hoppley, and his wife, Carolyn. They met through the band and left their partners to live together in Saltney, near Chester. Mrs Hoppley had told police that she was aware of a "chemistry" between Polly and Brian, and had heard her husband and Brian joking about "going down to the marshes".
After Mr Phillips died in the car, Mrs Peate walked back into Frodsham and called a taxi home. She told police later that she had gone to bed with her husband and made love - but claimed that she was thinking about Brian.Reuse content