volume knob, and a bass and treble control knob. Mid-priced models may add additional tone controls (e.g., one, two or three "midrange" controls and a "presence" knob for very high frequencies) and/or add a second type of volume knob called a "gain", "preamplifier" (or "preamp"/"pre"), or "drive" (short for "overdrive") control. A good selection of equalizer knobs and gain stages is standard on expensive amplifiers. If an amp has one or more preamp or gain knobs, the second volume knob may be called "master", "volume" or "post". Amplifiers for electric guitars are more likely than bass amps to have multiple "channels", but some bass amps also have channels. By providing two or more "channels", each with its own gain, equalization and volume knobs, a bassist can preset various settings (e.g., an accompaniment setting for playing a backing part and a solo bass setting for playing a bass solo). In a heavy metal band, a bassist may use a multi-channel amp to have one setting with an aggressive overdrive, while another channel has a "clean" sound for ballads. On some amps, setting the "gain" or "drive" control above a certain setting causes an overdrive effect, either due to the natural effect of overloading the preamplifier (or the preamp tube on a tube amplifier) and/or due to a distortion effect being turned on. Tube amplifiers typically also have a "standby" switch in addition to an "on/off" switch. Controls are typically mounted on the front of the amplifier near the top of the cabinet; often the knobs are recessed so that they do not project beyond the wooden cabinet, to protect the knobs during transportation. On amplifier "heads", protective metal U-shaped protrusions may be used to protect the knobs during transportation. On some amps, notably Roland models, the knobs and switches may be on top of the amplifier, at the rear of the top surface. Again, the knobs are usually recessed below the top of the wooden cabinet to protect them. Mid-to high-priced amps may have other switches (which on some amps are switched on by pulling an existing rotary knob out) that boost or cut some part of the frequency range, such as "bright boost", "deep boost" or "mid scoop" switches. Amps with an onboard audio compressor or limiter, which is used to protect the speakers from sudden peaks in volume and from damage due to power amplifier clipping, may have only