What Are Tremolo Guitar Effects Pedals & How Do They Work?


Tremolo is an awesome effect that has the potential to really give a guitar or bass that extra character it deserves in a mix. This modulation/dynamic sort of hybrid effect can be easily accessed via a pedal.

What are tremolo guitar effects pedals and how do they work? Tremolo pedals are stompbox units designed for guitar and/or bass (though they work with other electric instruments) that produce the amplitude-varying tremolo effect. Tremolo is a modulation effect that varies a signal’s amplitude over time and is defined by the rate and depth of its modulation.

In this article, we’ll further our understanding of tremolo and tremolo pedals along with the other effects types that vary signal amplitude in one manner or another. I’ll share a few pedal examples along the way and tips on how to get more out of your tremolo pedals.


Table Of Contents


What Is Tremolo?

Tremolo, in music, is broadly defined as a trembling effect. This effect is largely achieved through variations in the amplitude of a sound source.

This wide definition can be divided into two main types of tremolo, though both act to vary the amplitude rapidly.

The first type of tremolo is that of “rapid reiteration”. Think of tremolo picking (alternating upward and downward picking at a fast rate), for example.

The rapid reiteration style of tremolo achieves the effect by the nature of transients. In the majority of cases, producing a note on an acoustic instrument, synthesizer or with our voice will produce transient information.

Transients are the high amplitude, short-duration peaks in the sound wave (or audio wave) at the beginning of a waveform. They hold much of the character of the sound source.

Rapidly playing a musical note will produce transients at a fast rate. Just as the transient of the wave falls, the next transient is produced. This yields a sort of tremolo (rapid amplitude varying) effect.

This isn’t how tremolo pedals go about things. However, this type of effect is achievable by playing technique (tremolo picking, for example).

The second type of tremolo, which is the way tremolo pedals are designed, is that of amplitude variation. Think of the vocal tremolo technique while a single, long note is being held.

This type of amplitude modulation tremolo can be relatively easy to produce as an audio effect.

Guitarists, I should go beyond the explanation above to state that tremolo is not vibrato and vibrato is not tremolo. Don’t let Fender fool you!

Vibrato is a rather fast modulation of a signal’s “pitch”. Tremolo, as we’ve discussed, is a rather fast modulation of a signal’s amplitude.

The “vibrato” effect built into some amplifiers (you, the one that varies the amplitude of the signal) is really tremolo.

The “tremolo arm” (sometimes referred to as the whammy bar) that affects the pitch of the guitar is really vibrato.

Labeling of tremolo and vibrato can become tricky in the world of guitar. It’s important, then, to not only know the true definitions of the two effects but also that you’ll likely run into improperly-labeled units during your lifetime.

To learn more about vibrato pedals, check out my article What Are Vibrato Guitar Effects Pedals & How Do They Work?


What Are Tremolo Pedals & How Do They Work?

Tremolo pedals are stompbox-style units designed to receive guitar, bass or other instrument signals and produce a tremolo effect by rapidly altering the amplitude of the audio signal.

These pedals work by effectively reducing the amplitude of the input signal and bringing it back up to unity at a very fast rate.

We can think of tremolo, in terms of audio electronics, as turning the volume knob up and down very fast. The rate of a tremolo unit reflects the frequency at which we turn the volume up and down. The depth of a tremolo unit reflects the amount we turn the volume down before bringing it back up.

Tremolo pedals (and tremolo circuits within guitar amplifiers) are designed in one of two ways:

Each of these methods utilizes an LFO (low-frequency oscillator) at the heart of the design.

Low-frequency oscillators, as their name suggests, oscillate at low frequencies. Typically this means below 20 Hz (the lower limit of the audible spectrum).

The speed/rate control of a tremolo pedal affects the frequency of the LFO. The amplitude of the LFO will control the amount/depth of the tremolo’s attenuation of the guitar/bass/instrument audio signal.

A typical tremolo LFO frequency range is between 3 – 10 Hertz (cycles per second). The typical waveform of a tremolo LFO is a sine wave. Of course, these generalities are simply generalities and are not always the case.

Now that we know what an LFO is, let’s move on to the two types of tremolo units.

Signal Modulation Tremolo

The signal modulation tremolo circuit uses its LFO to control an opto-isolator (aka optical assembly).

An opto-isolator is an electrical component with 4 legs: two for an internal lamp (yes, one that produces light) and two for a light-dependent resistor (LDR).

LDRs are interesting. They’re typically made of cadmium sulfide or a similar photo-resistor placed inside an enclosure. A lens in the enclosure will allows light to reach the photo-resistive material.

When there’s no light shining on the photo-resistor, its resistance is very high. When there is light shining on the photo-resistor, its resistance drops to very low levels.

The LFO, in this case, sends a varying voltage to the lamps of the opto-isolator. As the voltage varies, the brightness of the lamps varies. As the brightness of the lamps vary, the resistance of the LDR varies.

The LDR, then, is a variable resistor, which allows more or less voltage to pass through the tremolo circuit. Resistance and voltage have a linear inverse relationship. As resistance increases in a circuit, the voltage decreases and vice versa.

Since the analog audio signal amplitude is defined by voltage, this oscillating resistance causes an oscillating amplitude.

A second variable resistor is put in series after the LDR. This potentiometer is not controlled by an LFO but it does work to increase or decrease the overall resistance of the circuit.

This potentiometer controls the intensity of the vibrato. By increasing the resistance of the pot, we reduce the overall effect of the LDR on the signal and, therefore, reduce the intensity of the tremolo.

So a simplified diagram of a signal modulation tremolo would look something like this:

The frequency of the LFO controls the speed/rate of the vibrato. The amount of resistance of the potentiometer controls the intensity (with inverse proportionality).

The Electro-Harmonix Stereo Pulsar (link to check the price on Amazon) is a classic tremolo pedal that utilizes signal modulation to produce its effect. It also has a stereo output that, when utilized, outputs two different tremolo signal: one with the LFO in-phase and the other with the LFO out-of-phase.

Electro-Harmonix Stereo Pulsar

Electro-Harmonix is featured in My New Microphone’s Top 11 Best Guitar/Bass Effects Pedal Brands To Know & Use.

Bias Modulation Tremolo

Bias modulation tremolo is far less common in both tremolo pedals and the tremolo circuits of guitar amplifiers.

In this design, the LFO varies the bias voltage going to a triode vacuum tube or output preamp. These tubes have been used and are still used as audio signal amplifiers. “Output preamps”, as I’ll refer to them here, are typically n-channel JFETs (junction-gate field transistors) that mimic the workings of a triode but are based on transistor technology.

Of course, we’re not looking to amplify the input signal to drive a speaker. However, we can still use a tube/preamp to alter the amplitude of the signal and cause vibrato.

Here’s a simple illustration of a triode vacuum tube:

  • A: anode
  • G: grid
  • K: cathode
  • H: heater

Here’s a simple illustration of a JFET:

  • S: source
  • G: gate
  • D: drain

Though these are two very different devices, they work similarly. The tube’s anode, cathode and grid are somewhat analogous to the JFET’s source, drain and gate, respectively. This is not full true but is a good way to envision how they both serve the same purpose in a tremolo pedal.

The bias voltage is a DC voltage applied to the grid of a tube in order to set a baseline current through the tube (between the anode and cathode). The AC voltage of the input audio signal is then superimposed on the DC bias to control the “output” of the tube (between the anode and cathode).

By bringing the bias voltage down, we effectively drop the output voltage. Bringing the bias voltage back up will bring the output voltage us as well.

A tube, when heated and by itself, will typically have a fixed bias and a steady output current. Applying an [often lower amplitude] AC audio signal at the gate will control this [often higher amplitude] output signal. That’s essentially how a tube works as an amplifier.

However, if we’re sending an audio signal to the gate and oscillating the bias voltage, we can “amplify” the audio signal while also producing the tremolo effect.

In order to cause the oscillation necessary for tremolo, there must be an RC network (basically a resistor and capacitor in series) between the grid (input) and anode (output) of the tube.

The LFO will control the resistance of the variable resistor, which will change the capacitor’s charge time, which in turn changes the rate of the tremolo. More resistance means slower charging, which results in slower tremolo.

Another variable resistor (this time a potentiometer) is put between the RC network and the grid (in series). Increasing the resistance of this resistor reduces the intensity of the tremolo. Reducing the resistance increased intensity.

The basics of this circuit is very similar to the signal modulation circuit in that they both have a variable resistor for speed control followed by a potentiometer for intensity control.

Note that if the bias voltage is too negative, the tubes will cut off. Conversely, if the voltage is too positive, the tube may overheat and burn up in a relatively short amount of time.

The Origin Effects Revival TREM (link to check the price on Amazon) is a bias modulation tremolo pedal that utilizes discrete transistor-based (JFET) circuitry to replicate the tube sound. Its tube-amp style signal path is designed using all-analog components (only minus the tubes).

Origin Effects Revival TREM

A Note On Digital Tremolo Pedals

Tremolo pedals can also be digital and have chips that emulate either of the methods discussed above.

Digital pedals that offer tremolo often offer it as an additional effect to other modulation-type effects. These pedals will typically have extra functionality over analog tremolo pedals such as clock syncing, tap tempo, and ratio/rhythm controls.

The Seymour Duncan Shape Shifter (link to check the price on Amazon) is an awesome example of a digital tremolo pedal.

Seymour Duncan Shape Shifter

Tremolo Pedal Parameter Controls

Tremolo, as we’ve discussed, is largely defined by the speed and depth of the amplitude oscillation. The speed is controlled by the frequency of the LFO while the depth is controlled by an additional variable-resistance potentiometer. Let’s look at these parameters along with the other controls we’ll commonly find on a tremolo pedal:

Depth/Intensity

Depth/intensity refers to the amount of amplitude variation a tremolo will cause.

A deeper tremolo means there will be a larger difference between the high and low signal amplitudes outputted by pedal. The effect will more noticeable.

A shallower tremolo will cause less variation in the output amplitude and the effect will be more subtle.

Rate/Speed

The rate or speed control affects the rate at which the tremolo circuit will drop the amplitude and bring it back up again. It’s defined by the frequency of the LFO and can be thought of as how many times the tremolo completes one cycle of amplitude variation per second.

Shape

Shape controls are available on some tremolo pedals. They will effectively alter the waveform of the LFO.

Typically a tremolo will work with a sine wave LFO. However, we can change the waveform to achieve different styles of tremolo. Other common basic LFO waveforms include the square, triangle and even the sawtooth wave.

Sometimes this control can be discrete and other times it can be continuous. It may also be labeled as “slope”.

Tap Tempo

Tap tempo allows users to tap a foot switch in order to establish a set tempo for the tremolo effect.

Ratio

Ratio is very similar to speed/rate.

Using the set tempo, the ratio control will alter the speed of the LFO to match some subdivision of the tempo (quarter note, eighth note, dotted eight note, triplet quarter note, etc.).

Level

The level parameter of a pedal controls the output level of the signal. Tremolo has a tendency to lower the natural transients of the guitar signal so a bit of added level can be great to remove any perceive drop in loudness as the tremolo pedal is turned on.


Tips On Using A Tremolo Pedal

Here are a few points to consider to help you get the most out of your tremolo pedal.

Subtlety Is Often Best

We may want to really show off our new tremolo pedal and the great effect it can have on our sound and tone. However, subtlety often yields the best results with tremolo. Try using slower rates and less intensity to find a sound that adds character without being overbearing.

Max Out The Intensity And Speed For An “Underwater” Effect

This is the opposite of the last tip. Cranking the controls on a tremolo pedal can yield an awesome “underwater” effect. Try it out for yourself!

Sync Up The Speed

This can be done rather easily with the digital tremolo pedals that offer tap tempo.

If you’re using an analog pedal (or a digital pedal without a tempo control), take some time to learn the sound of the various speed/rate positions. Set these manually to match the tempo of the song.

Play With The Rhythm Of Tremolo In Relation To A Delay Pedal

Tremolo and delay together can yield very dynamic results. We can set it up so that some delay repeats are dropped in level compared to their successors rather than the typical gradual drop off of repeats.

Try putting the tremolo before the delay in the pedal chain and vice versa. We can find lots of cool rhythmic patterns by combining delay and trem.

To learn more about delay pedals, check out my article What Are Delay Pedals (Guitar Effects) & How Do They Work?

Try A Tremolo Pedal After A Reverb Pedal

Some guitar amplifiers have tremolo circuits built into their design (sometimes mislabelled as “vibrato”, as we’ve touched on earlier in the article). These circuits are often put after the reverb circuit (if the amp has both).

We can emulate that effect by putting a vibrato pedal after a reverb pedal in our pedal chain.

To learn more about reverb pedals, check out my article What Are Reverb Pedals (Guitar Effects) & How Do They Work?


Where Should Tremolo Pedals Go In The Signal Chain?

Tremolo is an amplitude modulation effect and will likely perform best among other modulation-type effects. That is, after dynamic, pitch-shifting, synth and gain-based effects.

Alternatively, try adding tremolo after time-based effects (reverb and delay). Some guitar amps that offer tremolo have it after their reverb, which can sound really nice.

Of course, these are just suggestions, take some time to experiment and listen for what sounds best to you when setting up the order of the pedals in your rig!

To learn more about ordering pedals in the signal chain, check out my article How To Order Guitar/Bass Pedals (Ultimate Signal Flow Guide).


Other Amplitude Modulation Pedals

Before we wrap up the article, I’d like to discuss a few other effects pedals that work with by affecting the amplitude of the signal.

More specifically, let’s go over the following pedals that alter signal amplitude:

Volume Pedals

Volume pedals are generally designed as expression pedals (with treadle-type foot controller). They control to volume or amplitude of the signal passing through their circuit.

This volume control is achieved by means of attenuation rather than by applying gain.

Volume pedals are easy to understand. The expression pedal will allow maximum signal in either toe-down or heel-down position and no signal at the opposite position, depending on how the pedal is set up.

Volume pedals do not only allow for muting, which is great between songs, while tuning, etc. They also allow for volume swells and other changes in dynamics due, in large part, to their continuously variable nature.

The Ernie Ball VP JR. (link to check the price on Amazon) is a popular volume pedal with a relatively small footprint (hence the name “Jr.”). This expression pedal will effectively control the volume of the guitar or bass signal.

Ernie Ball VP JR.

Ring Modulation Pedals

Ring modulation is a bit more tricky to understand. Let me break down the effect in relation to tremolo.

I like to think of it this way: ring modulation is to tremolo what frequency modulation synthesis is to vibrato. It’s not a perfect analogy but it works for me.

So with tremolo, we have an LFO that modulates the amplitude of the signal. Ring modulation is a similar kind of amplitude modulation effect, only with faster oscillation.

What happens when we speed up the oscillation of a signal’s “tremolo”?

Eventually, as we increase the oscillator into the audible range, there will be less of a noticeable tremolo effect and more of an effect on the waveform of the signal itself.

At this point, the modulator signal (what would be the LFO) begins to shape the carrier signal (the audio) into a new waveform with different harmonic content.

Ring modulation effectively summed the carrier and modulator signals together to create two brand new frequencies which are the sum and difference of the two signals. The modulator wave is typically a simple wave selected by the effects unit while the carrier signal is the guitar/bass/audio signal at the input.

So with a ring modulation pedal, we have our input signal (the complex waveform of from the guitar) and we have a modular signal (often a sine or square wave produced within the pedal).

The modulator signal acts to modulate the amplitude of the input signal. This results in a modulated signal made up of the sidebands from the sum and difference frequencies (similar to how amplitude modulation works).

The Fairfield Circuitry Randy’s Revenge (link to check the price on Amazon) is a great example of a ring modulator pedal. We can control the frequency of the modulator signal and its waveform (between a square and a sine wave).

This pedal also has a “hi” and “lo” switch, which makes it fitting for an article focused on tremolo. The high mode takes the modulator frequency into the audible range and causes a ring mod effect. The low mode puts the modulator frequency into LFO territory and the pedal effectively becomes a tremolo unit!

Fairfield Circuitry Randy’s Revenge

What is the difference between tremolo and vibrato? Tremolo and vibrato are both time-varying modulation effects. The difference is that the tremolo effect modulates the amplitude (volume) of the signal while the vibrato effect modulates the frequency (pitch) of the signal. A “tremolo bar” on a guitar is confusing because it actually produces a vibrato effect.

What does a phaser pedal do? A phaser pedal is a stompbox unit that affects the input signal with a phaser circuit (analog or digital). The phaser effect utilizes an LFO to modulate multiple all-pass filters in series to sweep the phase of frequencies within the signal (or DSP that mimics this) to produce the phase-shifting effect known as phaser.

For more information on phaser pedals, check out my article What Are Phaser Pedals (Guitar/Bass FX) & How Do They Work?

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