An instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal from musical instruments such as an electric guitar, an electric bass, or an electric organ into an electronic signal capable of driving a loudspeaker that can be heard by the performers and audience. Combination ("combo") amplifiers include a preamplifier, a power amplifier, tone controls, and one or more speakers in a cabinet, a housing usually made of hardwood, plywood, particleboard, or, less commonly, moulded plastic. Instrument amplifiers for some instruments are also available without an internal speaker; these amplifiers, called "heads", have to be plugged into an external speaker cabinet. Instrument amplifiers are available for specific instruments, including the electric guitar, electric bass, electric/electronic keyboards, and acoustic instruments such as the mandolin and banjo. Some amplifiers are designed for specific styles of music, such as the "traditional"-style "tweed" guitar amplifiers used by blues and country musicians, and the Marshall amplifiers used by hard rock and heavy metal bands.