signal: Settings on the guitar itself (passive tone controls, active equalizer circuits in built-in preamps, pickup selector switch position, etc.) Devices between the guitar and the preamp stage, such as a wah-wah pedal or other effects units, such as chorus or reverb. Between the preamp and power stages (an effects loop or some dedicated amplifier tone circuits) Between multiple stacked preamp stages (also called “gain stages”) In feedback loops from a post-preamp signal to an earlier pre-preamp signal (as in the case of presence modifier circuits) Tone stages may also provide electronic effects—such as equalization, compression, distortion, chorus, or reverb. Amplifiers may use vacuum tubes (called valves in Britain), solid-state (transistor) devices, or both. The two common guitar amplifier configurations are a combination ("combo") amplifier that includes an amplifier and one or more speakers in a single cabinet, and a standalone amplifier (often called a "head" or "amp head"), which passes the amplified signal via a speaker cable to one or more external speaker cabinets. A wide range of speaker configurations are available in guitar cabinets—from cabinets with a single speaker (e.g., 1×10" or 1×12") or multiple speakers (e.g., 2×10", 4×10" or 8x10"). Guitar amplifiers vary widely in price and quality. Many music equipment companies import small, low-powered practice amplifiers for students and beginners that sell for less than $50. Other companies produce expensive custom-made amplifiers for professional musicians, which can cost hundreds or even thousands of dollars (USD). Most combo amplifiers have a carrying handle, and many combo amplifiers and cabinets have metal or plastic-reinforced corners to protect the amp during transportation. Control knobs and buttons are typically on the front of the cabinet or chassis, though in some cases, the knobs are on a recessed panel at the back of the top of the amplifier. The most basic amps only have a few knobs, which typically control volume, bass, and treble. More expensive amps may have several knobs that control pre-amp volume (or "gain"), distortion or overdrive, volume, bass, mid and treble, and reverb. Some older amps (and their re-issued versions) have a knob that controls a vibrato or tremolo effect. The 1/4" input jack is typically mounted on the front of the amplifier. In the simplest, least expensive amplifiers, this 1/4" jack is the only jack on the amplifier. More expensive amplifiers may have a patch bay for multiple inputs and outputs, such as a pre-amp out