Guitar Pedalboard

Tech Tip: Effects Basics—Part 2

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Learn how tremolo, phase shifters, flangers and chorus effects work and how to use them to give your guitar or bass a signature sound.

In Part 1 we looked at the history of effects and delved into time-based effects such as reverb, delay and echo. In this installment, we’ll shed light on so-called modulation effects that can range from subtle chorus and tremolo sounds to the mind-bending phaser and flanger effects beloved by psychedelic rock fans.

Modulation Effects

Modulation effects take the original waveform coming out of the guitar and process it using filters and other circuits. On the output side of the effects processor the original “dry” signal is blended together with the affected signal to create a variety of sounds based on the control settings you dial in. Most such effects have time and depth controls that determine the overall level of the effect and the speed at which it operates. Read on for a rundown of the most popular effects that use this kind of technology.


If you’ve heard Tommy James and the Shondells “Crimson and Clover,” you’ve heard a fairly extreme example of tremolo in action. The wobbly, shimmering effect that many people think of as an “underwater” sound is achieved by rapidly altering the modulation of the signal to create a pulsing sound. Swamp-rock sounds such as those created by Creedence Clearwater Revival often incorporate tremolo. Listen to John Fogerty’s guitar on CCR’s “Born on the Bayou” to hear a more subtle, highly effective use of tremolo alongside some modest levels of tube-amp distortion.

An early example of a tremolo circuit was the one Leo Fender built into his 1950s Vibrolux and Tremolux guitar amps. The pulsing sound was created by rapidly changing the biasing of tubes, either in the preamp or output stage to create what was called a “bias wiggle.” Confusingly, Fender labeled the effect with the word “Vibrato.” In reality, vibrato is achieved by rapidly changing pitch—not volume or modulation. To really confuse matters, Fender called the whammy bar used on the Stratocaster guitar a tremolo arm. Because of the popularity of Fender amps and instruments, this terminology confusion continues to this day.

Basic tremolo mimics the sound you can generate by playing a note or chord while rapidly turning your volume up and down. On most tremolo stompboxes adjusting the speed knob changes how rapidly your volume fluctuates; the depth knob controls the range of volume from loud to soft as the tremolo oscillates between those preset limits.

Modern tremolo effects often aim to emulate the sound of those vintage tube circuits Leo Fender built into his amps. Many units use resistor-based circuits and some models offer more aggressive flavors of tremolo that generate square wave and sine wave sounds. Some, such as the BOSS TR-2 Tremolo feature an LFO control that allows you to alter the waveforms from square to triangle shapes with sweepable points between those settings.


You can dial in a wide palette of tremolo sounds using the BOSS TR-2 Tremolo stompbox that go from old-school swamp rock sounds to outer space surf music.


Also known as phase shifters, this effect takes the audio signal and doubles it then shifts one signal out of phase with the other. On the output side both waveforms are recombined while being passed through an oscillator circuit that moves the two signals rapidly in and out of phase with each other. The result are soaring and “whooshing” sounds that were an important component in ‘60s psychedelic sounds. The Small Faces song “Itchycoo Park” is often cited as classic psychedelic-era example of a phaser in action. In reality, as we discuss below, that sound was actually created through a tape-flanging technique.

In the 1970s, stompbox phasers began to appear with Brian May of Queen and Eddie Van Halen becoming avid users. Van Halen’s “Eruption” guitar parts used a vintage version of the MXR Phase 90 to help place the band at the top of the hard-rock heap.


With graphics borrowed from Eddie Van Halen’s axe, the limited-edition MXR EVH Phase 90 generates the head spinning, swirling tones that were a key element of Eddie’s tone.

More recently, Radiohead’s Jonny Greenwood employed an Electro-Harmonix Small Stone EH4800 Phase Shifter on the band’s landmark album, OK Computer.


The Small Stone Phase Shifter, as heard on Radiohead’s “Paranoid Android,” adds a broad range of spacey sounds. Engaging the Color switch lets you scoop out the frequency spectrum for more sonic options.


Flangers mimic the studio trick that was used on “Itchycoo Park” and other psychedelic rock chestnuts in the late ‘60s. The engineer would place a hand on the tape recorder’s reel for a moment or two during playback and then release the reel to let it catch up with a dry (unaffected) signal of the same part being played back simultaneously. Physically braking the reel caused the two signals to be out of phase with each other producing jet-plane like whooshing sounds.

Modern flanger effects boxes typically have a depth setting that adjusts the intensity of the effect while a rate knob controls the speed of the cycles. They operate by delaying one of two identical signals, usually by 20 milliseconds or less. Varying the delay time causes the output to sweep up and down the frequency spectrum. Digital flangers use digital signal processing to emulate the sound of analog gear. Many flangers feed the output signal back into the input, which increases the effect’s intensity.

Classic examples of electronic flanging include Jimmy Page’s guitar on the Led Zeppelin tracks “Nobody’s Fault But Mine” and “Kashmir.”

Modern flangers usually offer a diverse range of sounds. A good example is the MXR M-117R Flanger, which has a four-knob control panel that includes a Manual knob with which you can dial in the frequency center when you disengage the auto sweep function. A Regen knob heightens the effect.


Tweaking the controls of the MXR M-117R Flanger gives you a suite of flange sounds that can be tinged with delay and chorus effects.


Long before the age of electronics, chorus effects were created by choirs singing in roughly the same timbre and very nearly the same pitch. As the voices coming from various points mingled, they created a shimmering effect. Similarly, a 12-string guitar with its six string pairs tuned an octave apart creates a complex form of chorus effect. Pianos likewise employ multiple strings for each note that are slightly out of tune with each other. Accordions and mandolins also take advantage of this psycho-acoustic phenomenon.

Electronic chorus effect do essentially the same thing by altering a duplicated waveform in relatively subtle ways to add space and shimmer to the unaffected sound. The affected waveform will sound similar to the original, but be different enough to sound like multiple voices playing the same notes or chords.

Chorus can be a great way to fatten the sound of a bass or solo guitar adding harmonic complexity to the music. Often acoustic guitar amps have a built-in chorus effect that imparts warmth and complexity to the amplified sound. Many guitarists overcome the “quacky, ”thin tone that piezo pickups can impart to an acoustic-electric guitar by using a chorus effect.

The Electro-Harmonix Small Clone Chorus has been around since 1980, and thanks to warm-sounding analog circuitry and a very simple control layout, it has earned a place on the pedalboard of rockers and acoustic musicians alike.


With its ability to create the shimmer of a 12-string acoustic or the warble of a Leslie rotating speaker, the Electro-Harmonix Small Clone Chorus was a go-to stomp in Kurt Cobain’s arsenal of effects.

Stay tuned for Effects Basics—Part 3 in which we’ll enter the crunchy domain of distortion effects and their cousins including overdrives, boosters and fuzz boxes.


Learn more about getting your pedalboard in order with guitar magician Steve Vai.

Want to dive deeper into effects? Our Guitar and Bass Effects Buying Guide looks at the entire spectrum of stompboxes and effects multiprocessors.

Tags: Electric Guitars

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