Bo Michael Tretow (born 20 August 1944 in Norrköping) is a Swedish record producer and audio engineer, musician and composer, best known for his work with the Swedish pop group ABBA (1970–1982), and with the musical Chess. Tretow experimented with different recording techniques, and played an essential part in creating the "ABBA sound". He has also composed several themes and jingles for Swedish national radio and television.

Source: Wikipedia (https://en.wikipedia.org/wiki/Michael_B._Tretow)