why does preamp improve sound quality?

I'm sorry if I'm not as clear on how impedance and other factors affect your situation. I've always thought that introducing the gain of the preamp between the two means that the amplifier has more power in reserve to accurately transmit the signal, with particularly better dynamics.

There would have to be more distortion introduced with the extra stage, but it sounds like there is more distortion when your power amp takes the whole load itself.

Elizabeth, you addressed the issue of impedance mismatch between the source and poweramp. Can you explain what kind of impedance mismatch? In terms of electronics, what does a preamp do with the signal before it enters the poweramp?

It probably has to do with matching the output impedance of the source with the input impedance of the amplifier. If you have all of the manuals, find the specifications for:

Mark Levinson no. 390s "output impedance"

Quad preamp "input impedance"
Quad preamp "output impedance"

Quad II-forty five amp "input impedance".

After looking at these, see if the numbers suggest that "CD to AMP" would be better matched with the preamp inline.

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.