The purpose of the expander is to attempt to undo some of the compression used on many recordings and add in some dynamics. dbx was the main proponent of these gizmos and made a number of flavors. The bottom of the line was a single band version that applied expansion across the whole audio spectrum. Problem was that a loud low frequency sound could cause audible pumping in higher frequency sounds. To address this dbx introduced expanders that independently expanded different parts of the spectrum by splitting the signal according to frequency range, expanding each range, then recombining them. They produced several 3 band models and one 5 band model (5BX).
On top of this they added a separate transient expander called "impact restoration", which was supposed to increase the definition of transient leading edges. In the 3 band series it operated globally, in the 5BX it operated independently on each band.
In the late 80's I owned first a 3BX and then a 5BX. They could definite add punch to flat recordings if used in moderation, but as my ear improved I heard the artifacts (pumping and breathing), and found that the devices colored the sound. The cure was worse than the disease, for serious listening.
Also the proprietary dbx chips used in these are no longer available, so service is an issue.
On top of this they added a separate transient expander called "impact restoration", which was supposed to increase the definition of transient leading edges. In the 3 band series it operated globally, in the 5BX it operated independently on each band.
In the late 80's I owned first a 3BX and then a 5BX. They could definite add punch to flat recordings if used in moderation, but as my ear improved I heard the artifacts (pumping and breathing), and found that the devices colored the sound. The cure was worse than the disease, for serious listening.
Also the proprietary dbx chips used in these are no longer available, so service is an issue.