If a player uses a separate preamplifier and power amplifier, she or he can buy a power amplifier intended for a sound reinforcement system or PA system or pick a power amplifier designed specifically for bass instruments. These preamps and power amps come in two formats: 19 inch rack-mountable units and units with their own wood or metal case. If a player uses a rackmountable preamp and power amp, these units and any effect units, such as an audio compressors, can be mounted in 19" rack mount road cases.