Def Leppard guitarist Phil Collen has credited producer Robert John "Mutt" Lange with being responsible for "probably about 99%" of the band's success during the 1980s, in a new interview with Detroit radio station WRIF. Lange, who first made his name producing AC/DC's "Highway To Hell" and "Back In Black", worked closely with Def Leppard from 1981's "High 'N' Dry" onwards, shaping their sound and songwriting during their commercial peak. Collen's comments underline the outsized influence a producer can have on a band's identity, describing Lange as effectively a fifth member who taught them to sing and play.

Collen said Lange contributed vocals and guitar ideas across the band's records and helped rewrite weak choruses into hits, citing the ten days spent perfecting "Pour Some Sugar On Me" for 1987's "Hysteria" as an example of his hands-on approach. He also noted that "Pyromania" (1983) marked the point where the band found its own distinctive sound rather than echoing AC/DC, and referenced earlier comments from a 2024 podcast in which he described Lange as recognising the band members were "malleable" and willing to learn from him over years of close collaboration.