What is the real function of a preamp, and why use it?

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Alright, this is going to be a really naive thread for a lot of you, but hang with me.

I don't quite understand the purpose of a preamp. Is it only for switching sources, settting volume, etc? I can understand with inputs like phonograph, where you have to correct for RIAA weighting, you need some sort of pre converter. Also, if you've got low-voltage signals, such as guitar or microphone pickup, I can see the use of an amplifier before the main power amp.

But say I've got 1 source (a multifunction device such as a PC with a wonderful sound card), and that source can vary it's own volume. Then what good is a preamp in this situation? Is there a function I'm not seeing here? Am I perhaps incorrect in assuming that a normal amplifier (any of the zillions of schema on the net) cannot deal with line level (1 volt or so)? Are all these amplifier designs assuming that you're preamping your signal first, and I'm just not picking up on it? Is this some sort if impedance issue? Is the preamp function built into most amps, as the input stage? Whats the story?

Thanks to anyone who can provide help here.
- Jonathan
Most amps average out to about 26-28dB gain. Some are down in the teens, some are over 30dB, but the majority cluster in the upper twenties. If that is sufficient gain for your input signal, and if you can arrange for a volume control & switching, then you probably do not need a preamp.
Yes, a preamp can function as a buffer/impedance converter, but I'm assuming that your sound card has a reasonably low Zout. If you're happy with the sound quality of your card, then there's no need to complicate things.

In principal there is no need for a preamp with modern day sources, however most power amps are not really designed with quite enough gain to cope with all situations. Typically a power amp is designed with an input sensitivity of 0.775Vrms to 1Vrms in for full output. The standard for a cd player with fixed output is Odb on the disc gives 2Vpk out. This would only drive the amp to full output if the disc was recorded to within 3dB of max level. This is very rare. Mostly levels reach only to within -10 to -12dB of full level to give a safety threshold and avoid clipping the a/D coverter. So now your 100W amp could only deliver as little as 10W at full volume!
If your source component has a variable level out, then this is normally arranged to have a max level at full volume setting in excess of the 2V standard, thus compensating for under recorded discs. Your soundcard probably fits into this category, as do some cd payers such as those from Meridian. Otherwise you will need a preamp with a gain of around 20dB. In principle this extra gain could be included in the power amp, along with a volume control and input switching. This is called an INTEGRATED AMP !!
Often the two functions are separated in order to allow a freer choice of preamp functionality to match your requirements, separate from the issue of how much power you need.
Geoff, you may well be correct. It's been a while since I read the spec, so my memory may be tarnished, but the conclusions are basically the same. The 100W amp would deliver no more than 25 watts for a -12dB signal.
The old Quad amplifiers (303 and 405) were always designed with a 300mV input sensitivity, to comply I think with the DIN standard, so were more useful in passive preamp situations
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