the dilemma under-saddle preamp consists of a pressure-sensitive coax cable which sits in the bridge slot under the saddle. It is slightly more compressable than piezo, so there is less risk of difference in signal-strenght from the individual strings. A FET preamp is directly attached to the pickup; this preamp reduces the impedance which results in perfectly balanced basses and it reduces humming. The preamp is powered by a 9V battery and is switched on when the plug is inserted in the jack.

piezo pickup with preamp

FET preamp