<P> The Wall of Sound acted as its own monitor system, and it was therefore assembled behind the band so the members could hear exactly what their audience was hearing . Because of this, Stanley and Alembic designed a special microphone system to prevent feedback . This placed matched pairs of condenser microphones spaced 60 mm apart and run out of phase . The vocalist sang into the top microphone, and the lower mic picked up whatever other sound was present in the stage environment . The signals were added together using a differential summing amp so that the sound common to both mics (the sound from the Wall) was canceled, and only the vocals were amplified . </P> <P> The Wall was very efficient for its day, but suffered from more drawbacks than its sheer size . Frequent guest keyboardist Ned Lagin (best known for performing experimental interludes with various permutations of Lesh, Jerry Garcia and drummer Bill Kreutzmann throughout 1974 set breaks) preferred to play through the powerful vocal subsystem (considered to be "the best part of the entire...PA"); however, the group's sound crew often neglected to switch between his quadrophonic input and the vocal input during long sequences, resulting in few of his contributions being recorded . Additionally, as many as two channels of his input would still be lost in the mix when the system was working properly . The Wall's quadraphonic format did not translate well to soundboard tapes made during the period, as the sound was compressed into an unnatural stereo format and suffered from a pronounced tinniness . </P> <P> The rising cost of fuel and personnel, as well as friction among many of the newer crew members and associated hangers - on, contributed to the band's October 1974 "retirement ." The Wall of Sound was disassembled, and when the Dead began touring again in 1976, it was replaced with a more logistically practical sound system . </P>

When did the grateful dead stop using the wall of sound