Tone stack

A tone stack is a specialized type of audio filter incorporated into the circuit of an audio amplifier to alter its frequency response. The term is primarily used in reference to instrument amplifiers such as guitar amplifiers.

Guitars can produce sounds from 80Hz up to 10KHz, while guitar amps produce very little sound above around 5KHz. For discussion of tone controls, everything above 600Hz is considered to be in the treble band. Notes at 200Hz and below are in the bass range, while the sounds between 200Hz and 600Hz make up the middle band.

Tone controls work by using adjustable band-pass filters to raise or lower the perceived volume of sounds in a specific range of frequencies (or band). For example, the high-pass filter selectively lets frequencies in the treble band "pass" through the amp more easily than bass frequencies. A Bass control is a low-pass filter that produces the same effect over lower pitched sounds.