Adapted from the book Guitar Effects Pedals: The Practical Handbook (Backbeat Books)

The last installment of Gibson's Effects Explained series deals with the occasionally overlooked “effects” created by filtering and EQ circuits. A relatively simple manipulation or filtering of the frequency spectrum in which the guitar operates is capable of producing some of the most emotive effects heard in music. We know this category best in the form of the wah-wah pedal, taken to an art form by guitarists from Jimi Hendrix to Eric Clapton to Slash to Zakk Wylde. Filtering also gives us voltage-controlled envelope follower auto-wah sounds. Down at the simpler end of the scale, plenty of guitarists seek the aid of an active tone control in the form of a graphic EQ.


Wah-wah pedals are often explained to us as being a tone control with a rocking pedal attached, but there’s a lot more to it than that. In truth, the wah circuit is a sweepable peaking filter, that is, a bandpass filter that creates a peak in the frequency response that the player can manually (should it be “pedially”?) sweep up and down the frequency spectrum. When that peak is swept through the portion of the spectrum that contains the notes we are playing, it emphasizes those frequencies and produces the “wah” sound we hear. The differing amounts of resonance produced as this peak is swept also contributes to the characteristic sound of any given wah-wah pedal, and this is something that can vary considerably from model to model.

Mackie 1604-VLZ ProBecause the wah-wah is dealing with guitar, its frequency spectrum usually runs from around 400 Hz to about 2.2 kHz. This is well within the range of the average sweepable midrange control on a standard mixing desk, and a look at one of these can help you better understand the function of a wah-wah pedal. A popular, affordable home-recording and PA desk such as the Mackie 1604-VLZ Pro, for example, has an EQ section with a mid frequency sweepable from 100 Hz to 8 kHz. Set the Mid boost control to max (most wahs don’t allow manual control over the extent to which the peak is boosted, it is preset in the circuit design), find the range between 400 Hz and 2.2 kHz on the frequency sweep knob, and twist this rapidly up and down between those levels while a friend vamps some guitar riffs through the channel. You should hear a dramatic emphasis when your frequency range hits that of the notes being played—in short, something like the sound a wah-wah pedal makes.

Of course the bandwidth in this mixing desk EQ control is broader than that of the peak produced by most wah-wahs (1.5 octaves on the Mackie), so the pedal’s frequency emphasis should be more focused, and therefore more dramatic. Some wah-wahs offer control over this bandwidth, as do Dunlop’s Cry Baby 535Q and the Boss FW-3 Foot Wah.

Vox WahThe most revered wah-wahs of all time, the Vox Wah and Cry Baby of 1967 and 1968—and Thomas Organ’s Clyde McCoy Wah-Wah that preceded them by a year or so— achieve this with a circuit that consists of just a couple of transistors, a coil inductor, a few resistors, and a capacitor—and of course a potentiometer with a gear on its shaft so a rocker treadle can set it in motion. Seems like pretty simple stuff, and in principle it is, but there’s plenty of magic and mystery associated with good wah-wah circuits, and it can take a lot of effort to get the things sounding just right.

Many players rave about early Italian-made Jen Vox Wahs with Fasel inductors in the circuit, and claim that nothing made after them has sounded as sweet or expressive. There are plenty of myths afloat in gearsville, but some flaw in these early Fasel inductors—or basically just that they were cheap—has been credited with enabling them to achieve asymmetrical clipping with some even-order harmonic content, as opposed to the spikier-sounding odd-order harmonics of other inductors in clipping, and this could indeed be responsible for any magic. Dunlop has reissued a Cry Baby Classic wah-wah with a Fasel inductor, but this is apparently a new generation of Fasel and I have yet to hear whether they perform the same as the originals. Plenty of guitarists also flip for the silver metal “trash-can” inductors found in early Cry Baby wahs.

Roger Meyer, Mitch Mitchell, Jimi Hendrix and Noel ReddingTweaking and tuning wahs is considered something of a black art, and a number of modifications exist—some user-installable—to help players make the most of both vintage and new models. Roger Mayer’s name inevitably crops up in any in-depth discussion of wah-wah modification (yes, again, but what can you do: the guy was a pioneer, he was in there at the start of transistorized effects, he worked with Jimi Hendrix, Jimmy Page, Ernie Isley, Stevie Wonder… and suffice to say he knows his stuff). When they worked in the studio together, Mayer would “tune” Hendrix’s wah-wahs to emphasize the key that the song was in, or tweak the frequency spectrum affected by the travel of the treadle. Today he offers the Red Lion Wah kit that fits into a standard Cry Baby pedal to allow a continually variable sweep. Many vintage wah-wahs also have a less-than-ideal input impedance for use with guitar, and can be guilty of severe “tone sucking”, so some guitarists also modify them to eliminate this. Boutique maker Fulltone also offers wah-wah mods, and its own Clyde Wah is a clone of the original Thomas Organ Clyde McCoy wah-wah.

While the Vox/Cry Baby wah remains the classic template, makers have offered a range of variations on both the circuit and the mechanical operation of the pedal. Morley had an occasionally popular wah in its imposing, chromed range of effects that used quiet photo-resistors instead of potentiometers for sweep control. Roger Mayer’s Vision Wah is also a potentiometerless pedal, with a low-profile housing and a treadle positioned ergonomically for a more comfortable action while standing. And for pure wild inventiveness, it’s hard to beat Z.Vex’s Wah Probe, which uses a theremin-type antenna in the form of a copper plate mounted to the sloping front face of the pedal for totally contact-free wah action.


Of course the king of the treadleless wah-wah is the envelope follower, voltage-controlled filter, or auto-wah. These effects contain a sweepable peaking filter much like that of the traditional wah-wah, but use the intensity of the incoming signal—in other words, the guitarist’s pick attack—to generate the control voltage that sends the peak up and down the frequency spectrum. With most such devices, pick lightly and the sound remains bassy and muted; hit the strings hard, and brighter wah-like frequencies leap out.

Musitronics’s Mu-Tron IIIMusitronics’s Mu-Tron III, introduced in 1972, was the first widely available envelope follower, and remains one of the best-loved. Electro-Harmonix followed with a range of models such as the Doctor Q, Zipper, Bass Balls, and Y Triggered Filter, and most major makers of the 1970s joined in.

While the treadle pedal is generally considered the rocker’s wah (despite the disco-era’s clichéd appropriation of the effect), the envelope follower auto-wah is the archetypal funk machine. Think Parliament-Funkadelic, just about anything from bassist Bootsy Collins, or Stevie Wonder’s “Higher Ground” (a famous early use of a Clavinet through a Mu-Tron III). George Clinton, a singer, makes an unlikely endorser of recent-era E-H’s Tube Zipper envelope follower/distortion pedal—until you consider the man oozes more funk than a warehouseful of Mu-Trons, “mere” singer or not. Overall, the effect is probably less expressive than a treadle wah—or, perhaps more accurately, offers less fine manual control over its expressiveness—but is more effective in certain circumstances. Something about the rhythmic way your natural pick attack can induce a pumping feeling in pulsating rhythm parts can often sounds more natural than the methodical rocking of the typical disco-wah rhythm guitar part. Outside the realm of the funkateer, however, the auto-wah is consigned mainly to the “novelty” shelf.

Graphic EQ

The mini guitar-pedal-sized equalizer is not an effect as such, but an inline tone circuit. Still, it would seem churlish not to acknowledge it here, and many guitarists have made great use of graphic EQs over the years. The technology behind these units doesn’t need a lot of description. We could delve into the intricacies of circuit and design topologies, but that would get boring fast. Suffice to say a graphic EQ pedal is a multi-band active tone control with sliders rather than rotary potentiometers for graph-style presentation of the equalization settings. The frequency bands assigned to the sliders are fixed, and tailored to be useful to the frequency spectrum in which the guitar operates, and the bands are logarithmically related to correspond to the way the human ear perceives frequencies. As such, they provide a simple, intuitive means of tweaking your tone settings.

Graphic EQs were surprisingly popular in the 1970s and ’80s (when too many current-manufacture guitars and amps were sounding worse and worse, some would argue, and needed severe tweaking to sound decent). Players used them to cure ill-sounding frequency responses, tailor a rig for consistency in differing room acoustics from night to night, or provide a boost in a specified frequency band for soloing. They were de rigueur on amps at around the same time, creeping onto Mesa/Boogies and other new makes, but when players chose one in pedal form they chose an MXR more often than anything else. The six-band model was the standard for guitar, but players with a chip on their shoulder sometimes insisted on the ten-band unit (these usually had Flock of SeagullsFlock-Of-Seagulls haircuts and secretly believed that the guitar was dead, and that the synth would soon rule the known universe). All of the usual suspects offered their own versions, so you also find vintage graphic EQ pedals from DOD, Boss, Ibanez, Electro-Harmonix and others.

If you’ve got an unfriendly hump in your frequency response that you just can’t eradicate, are loosing bass or treble at the hands of a chain of tone-sucking pedals, or feel your own slice of sonic heaven could lie in a ten-band graphic EQ pattern that emulates the waves of the double-helix of baboon DNA, one of these pedals might be for you. If your guitar, pedalboard, and amp are in sound shape and well connected, with desirable impedance matches and reasonable cable lengths, and you haven’t over-egged the signal chain pudding, you can probably live without a graphic EQ.

Thanks for taking this ride through the guts of the world’s most popular effects pedals. There are many, many great tools out there that can add some sonic spice to your stew, but ultimately you shouldn’t feel compelled to use the “flavor of the month”, or to purchase another pedal just because it seems all the cool dudes are playing through one all of a sudden. Use whatever makes you smile, and get out there and play some music with it.

Want more Effects Explained?

•    Effects Explained: Modulation—Phasing, Flanging, and Chorus
•    Effects Explained: Modulation II—Vibrato, Tremolo, Octave Divider, Ring Modulator 
•    Effects Explained: Overdrive
•    Effects Explained: Overdrive, Distortion, and Fuzz
•    Effects Explained: Booster and Compressor