Fairlight CMI was kept on one song. Clayton said the synthesiser "was only really used as a means to an end, to see if an idea worked". Eno contributed synthesiser parts on a Yamaha DX7, while he and the Edge also played a Yamaha CP-70 electric grand piano. Bono recorded his vocals on Neumann U47 and U67 and AKG C-12 microphones. Lanois said that they captured a warm sound and a "bottom end and depth" to Bono's vocals. For "Promenade", one of the first songs for which vocals were recorded, Lanois encouraged Bono to sing quieter by increasing the volume of his vocals in his headphones. By doing so, the producer was able to force Bono to "draw on the fine points of the voice". The band finished recording at Slane Castle on 5 June and began a second phase of the recording sessions at Windmill Lane Studios the following day. Their original intent was to record the backing tracks at Slane before overdubbing and mixing at Windmill Lane. Clayton said this was because the mobile recording equipment did not offer them as many options or treatment possibilities as a traditional studio mixing console would. However, the group ended up crafting their songs at Windmill Lane more than they originally anticipated. The Edge reflected that when recording at a new location such as Slane, it took them a few weeks to "get in to the momentum of the new creative surrounding". Lanois said that in hindsight the recording sessions were "split in a slightly off-balanced fashion", with too much time being dedicated to experimenting and not enough to fundamental recording. Within the band, there were concerns that their material recorded at Slane was too skeletal to consider completing yet, prompting them to re-evaluate and in some cases re-record it. Eno, on the other hand, was more interested in recording as few takes as possible and preserving the rough, spontaneous nature of the music. Clayton said that Eno would get easily bored if he had nothing to do, while Mullen said he was impatient with U2's creative process. Tension grew between them and the production team, largely because the band "weren't at a point where [they] could finish anything". Throughout the recording sessions, Bono had been ad-libbing his vocals, without completing lyrics. Mullen explained that the group's reliance on Bono to help complete the musical composition of their songs took time