(for sending to another guitar amplifier), a second low gain input, to use with active basses, an in jack to create an effects loop (when used with the pre-amp out jack), an external speaker output (for powering an additional speaker cabinet), and stereo RCA jacks or a 1/8" jack, for connecting a CD player or MP3 player so that a player can practice along with recorded music. Some amps have a 1/4" jack for connecting a pedal to turn the amp's onboard overdrive and reverb on and off or to switch between channels. Some amps have an XLR jack for a microphone, either for the guitar amp to be used for singing (in effect as a mini-PA system), or, for acoustic guitar, to mix a mic signal with a pickup signal. The vast majority of guitar amps can only be powered by AC mains power (plugging into a wall outlet); however, a small number of practice amps are designed for buskers also have battery power so they can be used for street performances. Types A combo amp contains the amplifier and one or more speakers in a single cabinet. In a "head and speaker cabinet" configuration, the amplifier and speaker each have their own cabinet. The amplifier (head) may drive one or more speaker cabinets. In the 1920s, guitarists played through public address amplifiers, but by the 1940s this was uncommon. Besides instrument inputs and speaker outputs (typically via 1/4" jacks), an amp may have other inputs and outputs. These can include an auxiliary input jack (sometimes with its own level control, for a drum machine), "send" and "return" jacks to create an effects loop, a “line out” jack, and an extension speaker jack. Practice amps sometimes have a 1/4" headphone jack, or stereo RCA or mini jacks for connecting a CD player, portable media player or other sound sources. Some guitar amps have an XLR input so that a microphone can be plugged in for singing. Guitar amps that include a mic input are in effect small, portable PA systems. Some amps, typically bass amps, have an XLR connector to provide a balanced output from the preamp section to a PA system or recording input. Instrument amplifiers are available in a wide range of price, quality, and performance levels. Some are designed for beginners, such as small, low-wattage practice amps, which typically have a single 8" speaker and about