 Both upright basses and acoustic bass guitars usually use piezoelectric pickups rather than magnetic pickups, and in some cases, the instruments may use condenser mics to pick up the higher range sounds.To amplify instruments with piezo transducers and condenser mics, specialized impedance-matching preamplifiers are often required.Also, since both piezoelectric transducers and microphones are more prone to unwanted feedback than magnetic pickups, the bass tech may have to set up a notch filter with a parametric equalizer to reduce the frequency that is feeding back.