The tone stack is the equalization section of the amp. The most utilized tone stack today is the FMV (Fender-Marshall-Vox) stack. It consists of a combination of high and low-pass filters. Each filter combines a resistor, a capacitor and a potentiometer (variable resistor) and depending on how these are connected in the circuit, they become either a high-pass or low-pass filter. A high-pass filter passes frequencies above a certain frequency. That frequency is determined by the values of the resistor and capacitor used in the circuit. A low-pass filter likewise passes frequencies below a certain frequency. The potentiometers determine how much of the signal that is passed by the filters is passed to the output section of the amplifier. 

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