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An Equalizer adjusts the frequency response in a number of different frequency bands. A graphic equalizer (or “graphic EQ”) provides slider controls for a number of frequency region. Each of these bands has a fixed width (Q) and a fixed center-frequency, and as such, the slider changes only the level of the frequency band. The tone controls on guitars, guitar amps, and most pedals are similarly fixed-Q and fixed-frequency, but unlike a graphic EQ, rotary controls are used rather than sliders.Most parametric EQ pedals (such as the [1] Boss PQ-4) provide semi-parametric EQ. That is, in addition to level control, each band provides either a center frequency or Q width control. Parametric EQs have rotating controls rather than sliders.Placement of EQ in a distortion signal processing chain affects the basic guitar amp tone. Using a guitar’s rotary tone control potentiometer is a form of pre-distortion EQ. Placing an EQ pedal before a distortion pedal or before a guitar amp’s built-in preamp distortion provides preliminary control of the preamp distortion voicing.For more complete control of preamp distortion voicing, an additional EQ pedal can be placed after a distortion pedal; or, equivalently, the guitar amp’s tone controls, after the built-in preamp distortion, can be used. An EQ pedal in the amp’s effects loop, or the amp’s tone controls placed after preamp distortion, constitutes post-distortion EQ, which finishes shaping the preamp distortion and sets up the power-tube distortion voicing.As an example of pre-distortion EQ, Eddie Van Halen places a 6-band MXR EQ pedal before the Marshall amplifier head (pre-distortion EQ). Slash places a Boss GE-7, a 7-band EQ pedal, before his Marshall amp. This technique is similar to placing a Wah pedal before the amp’s preamp distortion and leaving the Wah pedal positioned part-way down, sometimes mentioned as “fixed wah,” (pre-distortion EQ), along with adjusting the amp’s tone controls (post-distortion EQ).If a dummy load guitar-amp configuration is used, an additional EQ position becomes available, between the dummy load and the final amplifier that drives the guitar speaker. Van Halen used an additional EQ in this position. This configuration is commonly used with rackmount systems.Finally, an EQ pedal such as a 10-band graphic EQ pedal can be placed in the Insert jack of a mixer to replace the mixer channel’s EQ controls, providing graphical control over the miked guitar speaker signal.Equalization-related effects pedals include Wah, Auto-Wah, and Phase Shifter. Most EQ pedals also have an overall Level control distinct from the frequency-specific controls, thus enabling an EQ pedal to act as a configurable level-boost pedal.

Some EQ pedals include:

  • MXR 10-band EQ
  • Boss GE-7, PQ-4, EQ-20