Bob Rock worked with Metallica on the 1991 album at One on One Recording Studios.
Metallica began recording at One on One in Los Angeles in October 1990. The facility was the principal studio for the project, although the band also spent about a week at Little Mountain Sound Studios in Vancouver and later used A&M Studios in Hollywood for additional overdubs and mixing.
Rock’s involvement shaped both the performances and the final sound. He asked the musicians to record more as a unit and encouraged harmonic vocals from James Hetfield. The process involved extensive retakes, editing, and arguments over musical details.
The resulting record sounded slower, heavier, and more refined than Metallica’s earlier thrash-oriented albums. The studio setting is therefore part of the album’s production story, but the producer most closely associated with that transformation remains Bob Rock.