Asked how to make a guitar, the celebrated luthier Wayne Henderson offered a straight-up answer: "Just get a pile of really nice wood and a whittling knife. Then you just carve away everything that isn't a guitar." (footnote 1)
The making of a preamplifier seems more or less the opposite. You start with a simple volume control and a couple of jacks, then add whatever you think constitutes a preamplifier. Choices might include electronic source switching, line-level gain, phono-level gain and equalization, tone controls, tone-defeat switches, a balance control, a headphone jack, an iPod input, and maybe even a digital-to-analog converter with a USB receiver. The sky is pretty much the limit.