Wah Pedals
Wah Pedals are named for the distinctive "wah wah" sound they produce. Essentially a wah pedal is simply a band-pass filter with its center frequency manipulated by rocking the pedal. This focuses the guitar tone on a certain range of the harmonics of the direct guitar signal. When used in front of distortion, it not only produces the characteristic "wah" sound, but can also be used to change the nature of the distortion, similar to a boost pedal. Some artists prefer to use a "cocked" (fixed position) wah pedal as a boost.
Features
The basic wah pedal usually uses a toe switch to toggle it on/off. Otherwise, you only have the pedal motion to manipulate the tone. Some wahs go further, incorporating features such as:
- Automatically engaging/disengaging when moving the pedal out of/into heel position or stepping on/off the pedal
- A spring to automatically return the pedal to the heel position.
- Additional knobs to control the wah's width and min/max center frequency.
- Switch between operating as a wah pedal vs. volume pedal (or even a whammy pedal).
Buying Advice
While the wah pedal is conceptually simple, every wah pedal tends to sound a bit different. Besides the quality of components used, there are numerous circuits that can be used to create a band-pass filter, with various differences in how the filter operates. It is likely that the shape of the bandpass will differ depending on the pedal position - not as simple as sweeping a parametric EQ's frequency with constant Q and Gain parameters.
It's best to aquaint yourself with tone in a first-hand manner. How the wah sounds also depends on your guitar/pickups, your amp/distortion, and additional items in your signal chain that may affect the frequency response. Some amps respond differently to certain frequencies vs. others, which means that simply listening to demo clips or studio recordings may not accurately represent how the wah will sound through your gear.
Other things to keep in mind are the durability of the pedal - it's meant to be stepped on, so it may not last long if poorly constructed. Also, the shape of the pedal may make it easier to step on vs. others. Additionally, the range of motion may be uncomfortable or too small on some pedals - make sure rocking all the way from heel to toe and back is comfortable in a gigging situation. Also, check the feel - how stiff is the pedal's motion? Sometimes this can be adjusted by loosening/tightening a screw.
Signal Chain Placement
99% of the time, the wah pedal is placed 1st or close to first in the chain, before any kind of amp or pedal distortion, or compression. This gets the characteristic "wah wah" tone. For a clean guitar tone, the wah can be placed further back in the chain, even as far as the amp's effects loop without a dramatic change in tone. It may simply sound a little unnatural or odd being after some compression, but overall has the same effect. Placing a wah after distortion is a different story, creating a noticeably odd and filtered sound - think Rage Against the Machine - Bulls on Parade.