These tapes were secretly recorded by John D. Ehrlichman, President Nixon's chief domestic policy adviser, in his office on the fourth floor of the White House. The collection consists of 88 Dictabelt recordings and one double-sided audio cassette recording.
Most of the recordings were made on a Dictabelt system, but at least two recordings were made on a recorder that used an audio cassette. The system reportedly consisted of two microphones in the ceiling with the recorders hidden in a cabinet behind his desk. Ehrlichman may have also used a hand-held microphone hidden under papers for some of the recordings. The earliest recording in the collection is from 12 February 1970 and the last is from 30 April 1973.
Ehrlichman's recording system was set up independently of President Nixon's and was apparently operated and maintained by Ehrlichman himself. It was first revealed publicly by columnists Rowland Evans and Robert Novak who reported in July 1973 that Melvin Laird had ordered the system removed upon discovering it when he moved into Ehrlichman's old office.
NARA's finding aid for this collection is available here. So far, the Nixon Library has processed and released only the two conversations recorded on the audio cassette. A number of the recordings in this collection were transcribed either by Ehrlichman's staff or as part of the various court cases related to Watergate. One of these transcripts has thus far been released by NARA (link below). Special thanks to Steve Greene of the Nixon Library for his assistance in obtaining these materials.