These units may be housed in a "stompbox" pedal format, in a small rackmount unit (often less than a full rack space), or in units designed to be clipped to a belt or attached to an instrument. These devices are often marketed as preamplifiers, even though they also contain DI box features. Preamplifiers for electric bass typically contain gain knobs, sometimes including an overdrive effect unit, equalizer knobs and, for some higher-end units, multiple channels (e.g., a "clean" channel and a "dirty" channel, with the latter containing an overdrive effect). 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. Direct boxes are typically used with electric instruments or other electronic musical devices that only contain an unbalanced 1/4" phone output which needs to be connected to an XLR input of a mixing board. Multiple direct box circuits can be mounted inside one housing. These are used for multiple unbalanced outputs, such as for a bank of electronic keyboards. DI boxes can be used on instruments with electronic circuitry and pickups that do not contain an XLR balanced output. An example of this application would be an electric keyboard that needs to be connected to a mixing board, either directly or through an audio snake. Another example would be an acoustic guitar with pickups, an electric guitar or bass guitar, or a double bass with piezoelectric pickups. These instruments could be plugged into a DI box, and the DI signal would be mixed through a mixing console into a main or monitor mix. Some instrument amplifiers, particularly bass amplifiers, contain built-in DI units, and can be connected to a mixing console directly without needing an external direct box. This would be a typical setup for a bassist who wanted to connect their instrument through a public address system (PA system) or sound reinforcement system at a live show, while keeping the unique sound of the amplifier's preamplifier and equalizer circuitry and the custom settings.