Passive Preamp?

A search of the archives will yield all the info and opinions you need. Passives can be had cheap enough that an audition in your system will tell you more than any written words can convey. Many feel passives lack dynamics but again, the sonics are very system-dependent. Set up properly, they can offer great sound that may be incomparable to an active piece. As always, YMWV.

Do a search here and at AudioAsylum,all the info you will ever need on passive preamp is here and there

A passive preamp has no amplification stage. It is actually something of a misnomer. Typically they do not plug into an electrical source. I own two of them and they are both just switch boxes with a volume control. If you need more than this fundemental info, then you should follow Herman's advice above.

Check out the thread at: http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1159194844&read&keyw&zzno+regrets

It sounds better than most active units, it goes between your CD player or DAC and your amplifier.

It is mainly a volume control with no gain since the amplifier can handle the DAC perfectly well by itself without adding another gain stage and the problems that can come with it. A passsive volume control then will sound more transparent...you will have to do some reading in order to insert one in your system because they are system dependent:

What is your system?