On board bass preamp. The OBP-1’s boost-only design gives you the tone shaping benefits of active electronics without losing the inherent sound of the instrument. The OBP-1’s discrete, all-FET circuitry features up to 18dB of boost at 40Hz and 4kHz, creating a sonic environment high in dynamic headroom and low in noise. Two bands of boost. Maximum amount of boost: 18db, occuring at 40 Hz for the bass, and 4kHz for the treble. Output impedance: 600 ohm. Dimensions: 0,98“W x 0,98“L x 0,59“D. OBP-1TK version: Includes separate Treble and Bass pots.