What are the differences in a buffer, line-stage, pre-amplifier and front-end

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Reading posts and looking in the diyAudio store there are references to buffers, line stages, pre-amplifiers and front ends. While they seem to have things in common, I think the specific terminology indicate that there are differences, i.e., they aren't just different names for the exact same things.

So:
1) Could someone explain the differences between the devices listed above?
2) In what circumstances is one more applicable than another?
3) I know there are some headphone amps that are used as pre-amplifiers. What characteristics would make the use of a headphone amplifier acceptable as a preamplifier? (For example, maybe a certain gain of the headphone amp would make it unsuitable, or maybe some characteristic of an amplifier would make most headphone amps unsuitable).
 
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1) A buffer generally means an input stage having high input impedance (so it does not load the source) and having a low output impedance (so it can drive pretty much anything) and having a voltage gain of unity (1) meaning it does not increase the signal level.

Line stage is vague and can just mean circuitry dealing with line level signals.

Preamplifiers can mean a stage (like a buffer) but this time having some voltage gain to increase lower level signals or provide a higher than usual output signal for example to drive a power amp of low voltage gain. The popular ACA power amp is one such low gain amplifier and one that might need a preamplifier. A full preamplifier might include source selection for several inputs and also tone controls and balance and so on.

Front ends are another vague one and could refer to something like a phono stage (lots and lots of gain that go in front of a buffer or line stage) or the input and input selection part of a preamp.

2) The above pretty much covers when and why you would use one over another.

3/ Noise could be one important factor here. Also the output of most headphone amps is pretty much 'direct' off a relatively high current output stage whereas most dedicated preamps and line stages and buffers use devices with less current ability but often generally better performance in other regards such as noise and distortion.
 
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