I recently bought a kamaka tenor, and what a beauty. However, I feel incomplete with out a pickup on it. Today I have asked my local music shop to install a pickup onto it, of course they agreed and will do it for me for a fee. However, I asked for them to install the Mi-Si pickup, and I want to know, what a preamp is. I know the Mi-Si pickup is an active pickup that you just plug into your amplifier once it's installed. No batteries whatsoever, but what is a preamp? what does that do