The Edge began using the Korg A3 multi-effects processor during the making of Achtung Baby and it has been in his rack since (as of end of Vertigo 2006 tour)

 

It is a multi-effects processor and many of it's sounds, in conjunction with the SPC-01 card were used for the Achtung Baby and Zooropa albums; still used today to replicate those sounds live.

 

This unit is responsible for the swooshing auto wah sound of "Mysterious Ways", the intro to "Lady With The Spinning Head" and the crunchyt flanged sound on Zooropa's "Numb".