why does preamp improve sound quality?

Bufus, I Mark Levinson/Quad experience took place in a store demo. I have a similar Mark Levinson (no.39)/Jadis (SE845) setup, and am considering a Jadis preamp (JPL2), so wanted to know if the Jadis preamp would make my system sound better. I won't have a chance to test it out, so wanted to know if adding a preamp in this kind of situation generally improves the sound. I certainly don't want to buy an expensive and unnecessary piece of equipment, and find out it degrades the quality of my system! Unfortunately, I won't have access to any manuals before my immediate purchasing decision, since I'm in a different city.

Thanks for the explanations about impedance matching and extra gain, it really begs the question about my understanding how a preamp works in the first place. Does anyone know any website was a 'primer' or 'basics' about hifi explaining this? I wish audiogon had a section explaining technical basics for beginners like me.

Does a preamp actually add any more gain than the volume control on my CD player does? Or is it 'better quality' gain? I heard somewhere that the volume control on my CD player can both attenuate and add gain to the signal. So for low volumes, why would having a preamp be better if the improvement were primarily due to gain? At low volumes, would the direct sound be better, if it weren't for impedance mismatching?

Did you know that the signal comes out of your components at 'full volumn' and that your volumn control knob does not increase but rather it reduces the strength (volumn) of the signal?

A really good pre will do this better than some. My own Cd player has a very sophistaced volumn control so it serves as a pre even for signals from other sources, such as my DVD and satellite. Tone and bass controls are not not needed.

Getting the book Robert Harley's "The Complete guide to High End Audio" would give you a good introduction to the basics of electronics and many other useful bits of information. For me pre's have always improved the sound. Without pre might be a bit cleaner but with is richer.

Yes, a preamp will add some distortion or cause the signal to "lose" something. But like any other active component the power supply, regulation and output devices will determine everything. A top-notch preamp will overcome any loss that occurs with adding another compnent to the chain. The quality of the output of a good preamp in terms of dymanics, frequency response and voltage swing will be much better in many cases than the output of cdp's all because good preamps have better power supplies.

With respect to impedance matching, the lower the output impedance of the preamp means that more voltage is transferred to the amp. This is because power is eaten by resistance - and the more output resistance a preamp has, the more power it consumes from the signal, leaving less voltage at the amp. The amp is now forced to swing a higher voltage difference in a fininte amount of time making things more complicated and affecting the sound quality. So a preamp with a lower output impedance than a cdp is an overall plus - even though there is a slight loss in signal transfer.

Bottom line - good active preamps will be a plus because if they weren't, then who would buy them? We would all have passives or cdp's with volume control.

I just read reviews of Robert Harley's "The Complete guide to High End Audio" on amazon.com, and there was some pretty serious criticism there about his lack of technical knowledge and techincal errors in the book... makes me wonder if it would be the best source to learn about impedance matching? Any other suggestions for a good primer? It would be great if audiogon had a FAQ. Another option would be to use wikipedia... here are a couple of links:

http://en.wikipedia.org/wiki/High_end_audio
http://en.wikipedia.org/wiki/Audio_power
http://en.wikipedia.org/wiki/Tube_sound