The purpose of a preamp is to match the impedance of the pickup and get a good signal level to the amp. You can use a piezo without a preamp (as I have tried with my K&K on a guitar), but it generally doesn’t sound very good. My guitar sounds bloated, tubby and very bassy without a preamp. I could set the EQ on the amp to compensate, but it doesn’t sound near as well as it does with the preamp. The preamp I got was an Orchid muting DI box. It is bare bones simple (no tone controls) but it does a great job. It was fairly inexpensive as well (a bit under $100).