Preamp-DI units for double bass and other acoustic instruments often omit the overdrive features, but add additional features that help to produce a good sound and tone for acoustic instruments, such as an audio compression effect, a phase inverter switch and a notch filter (the latter two features designed to help reduce unwanted audio feedback "howls"). Preamp-DIs for acoustic instruments often include two channels and a simple mixer, to enable the player to use both a pickup and a condensor microphone. Some preamp-DIs provide phantom power, in case this is required to power a condenser microphone. Preamp-DI units may be battery powered, have an AC mains plug, or both.