difference between an active and a passive preamp?

I have a nad c272 amp and am looking for a good preamp to go with it, but I am on a very tight budget. I see lots of preamps that are acive and some passive - I have no idea of the difference? I have quad 22L speakers and listen to cd only. Any help understanding these differences would be great. I just want simple 2 channel preamp, with as tube like sound as possible. Please help, and many thanks,
And, as a result of having no active gain, passives are generally more transparent, and provide more bang for the buck if gain is not an issue. If you have only one source, check out Luminous audio axiom
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King Rex preamp under $400. As transparent as a passive with tube like mids and deep bass of the solid state pre it is. I believe it has 15 db of gain. A real fine sounding unit the sounds well above its price.
There are a lot of caveats when using a passive preamp. As Elizabeth said, you have to make sure your source component has a very robust analogue output section with low output impedance. In addition, your interconnect lengths from source to preamp to amp (short as possible) and capacitance of the interconnects (lower the better) and the input impedance of the amplifier (higher the better) are critical as well. There are passive preamps made of pots, stepped resistors, LDRs, transformers, and autoformers. I prefer the latter three. If all of these are taken into consideration and executed correctly, a passive preamp is better sounding than any active preamp I have ever heard. But I have not heard the mega expensive $15k+ preamps like darTZeel, Atma-Sphere MP-1 with upgrades, etc.
Very nice clear responses...the way they build upon one another...nice to see someone get a direct answer(s) to their questions. Educational too....One for the archives!