DBX Expander

I have recently acquired a Teac reel to reel and although it's a great machine, soundwise, it just doesn't measure up to CD or LP's in my system. I have a chance to purchase a DBX sound expander that I'm told greatly enhances the sound of the tape. I have no experience with this box at all. Does anyone have any knowledge of what this does?
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.
I agree as well. The DBX does to much pumping of the signal.
Although the ONE BAND DBX seemed to not do as much pumping as the others. Teac use to make an outboard noice reduction device for their machines as well which may have lowered the noise another 5 or 6 db without the pumping affects. If your a tweaker, you can try replacing the resistors and or the caps in the recording board, which is what I did to my Teac and had noticed a definate improvement in lower noise.But this is very time consuming indeed.
Well, I totally disagree with the 2 comments above. I have demo-ed properly adjusted DBX DS Series expanders for at least 15 "vinyl people" - and only ONE did not hear a dramatic improvement in A/B auditioning. Yes, they will "pump" if you overdo the levels - so don't overdo it.

If you bi-amp vertically, you can also just use them on the mids and highs, eliminating any bass pumping - but I have not found this to be an issue. Factors to consider here are the Damping Factor of you amps, and also the speakers themselves. Mine are sealed box bass cabinets, and pumping has not been a problem. Transmission line and ported cabinets may be more of an issue.

I consider the DBX an absolutely essential part of my system. Many of the recordings of the '70's and '80's were run through STACKS of DBX, or similar, equipment in the studio - EQ, compression, and peak limiting AT A MINIMUM! Proper use of one more quality device that attempts to reverse the "processing" on the output side does "color" the sound (that's what it's supposed to do) but not detrimentally IMO - just the opposite. They DO include op-amps in the circuit, but again, nobody who has heard mine can point out any negatives. One guy who is a hard core minimalist simply decided he liked the sound without the 3BX better, but admittedly had no rational explanation. Then on a 2nd visit, he was pretty much neutral on the 5BX. Quite a few people have bought 3BX's immediately after hearing mine.

That being said, a DBX will not elevate a cheaper, worn out, or misalignes RtR to the level you're looking for.

Unless you have much money to burn, try a 3BX-DS first. The DS version is the ONLY one to consider - the older ones are not as good. It should only cost around $200 to $225 plus shipping, on Ebay. There are lots around and they're affordable. If you don't like it, you can very easily resell it for what you paid.
A mint 5BX may cost you close to $1500 or more, so it's a bit riskier to start out with - especially if buying from an Ebay dealer and/or shipping via UPS or Fedex is part of the equation! The 5BX's Impact Restoration circuit is better, but the 3BX-DS's Dynamic Range Expansion is better IMO. So it's kind of a toss up anyway. I know someone who uses one 3BX on each channel and the results are superb.