Abbey Road was the only Beatles album mainly recorded on an 8-track tape machine, rather than the 4-track machines that were used for prior Beatle albums. This is noticeable in the better sound separation and mixing of the drum kit. EMI's conservative management had not yet approved the use of their then-new 8-track Studer deck, and that accounts for why this was one of the rare Beatles albums to be recorded at three different studios (Trident, Olympic, and Abbey Road). The album was also the first to be recorded and mixed entirely on a solid state sound board, giving the album's sound a noticeably different "feel" from its predecessors; Harrison later remarked that the new sound was too "harsh" for his liking. Also, the Moog synthesizer is featured on the majority of tracks, not merely as a background effect, but sometimes playing a central role, as in "Because" where it's used for the middle 8. It is also prominent on "Maxwell's Silver Hammer" and "Here Comes the Sun".