why does preamp improve sound quality?

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

Harleys book is what several members of gon recommended to me as I was getting back in the field - I found it very useful admittedly not so much for the nitty gritty electronic analysis - as more general overview of each components issues and what to look for in putting together a system

One thing that no-one seems to be mentioning is that the line section of a preamp also has to control the interconnect cable. The better they do this is often also part of the measure of quality a preamp can portray. Passive controls do not control the cable at all (hence the difference in sound from low to high volume) so if a preamp is effective at controlling the cable *and* is competent at the other tasks of a line section (low distortion, wide bandwidth, etc.) then it can *easily* sound better then a passive.

Some might look at this as an impedance issue, and to a large degree it is, but it is not to do with how the unit drives the amplifier so much as whether or not the construction issues of the interconnect are adequately bypassed enough so as to be minimized.

And mind you, not all line sections are up to this task. If not, a passive will be better...