A simple DIY guitar tremolo pedal

Have you ever thought of making your guitar sound differently or just livening things up a bit? In addition to reverb and chorus, a subtle tremolo can help. Today, I will build a very simple tremolo pedal using three transistors.

Of all the time-based effects, electronic tremolo is the easiest to get.

The important thing is that one does not need to alter amplifier bandwidth for tremolo, like in wah pedals and phasers, by electronically reconfiguring frequency-dependent circuits containing active and reactive resistances—resistors, coils, and capacitors.

There is no need to implement, much less modulate, the signal time delay. This is done in delays, reverbs, flangers, and choruses using mechanical, analog (bucket brigade devices, BBD), and digital delay lines.

Moreover, there is no need to process the frequency of the input signal. Raising the frequency by an octave is relatively simple; it takes a full-wave rectifier, similar to those used in power supply units.

A full-wave rectifier folds the wave. It turns the lower half-wave up, and now there is a whole period of the output signal for each half-cycle of the original signal.

We have doubled the fundamental frequency and added some harmonics. After all, the output signal is no longer sine. The upper half-wave is smooth, and the lower half-wave is pointy.

The first octave did not use a bridge rectifier with four diodes but a half-bridge rectifier with two diodes and a transformer with the winding having a midpoint.

Such a transformer was easy to find. For example, a phase-inverter matching transformer from a pocket transistor radio could do. We've put together a receiver of this kind in a post about the Regency TR-1.

It is unnecessary to use a transformer if you build a full-wave rectifier using operational amplifiers.

And finally, when making a phase inverter for a full-wave rectifier, you can do it without operational amplifiers. One transistor is enough. After all, the output signal at its emitter has the same polarity as the input signal at the base, and at the collector, it has inverted polarity.

Suppose the signal frequency needs to be decreased by half, not increased. In that case, that is, by one octave, then synchronous D-flip-flops, which we've discussed in previous articles, will be helpful.

A circuit divides the oscillator frequency on a 555 timer by 2, 4, 8, and 16. That is, by two to the first, second, third, and fourth powers. And if we speak musical terminology, we lower the tone by one, two, three, and four octaves.

However, electric guitar pickups do not produce digital pulses or even a sine wave but an analog signal of a complex shape with many harmonics. So, to divide the frequency with flip-flops, the guitar signal must be preprocessed by converting it into rectangular pulses.

One possible circuitry answer to this problem is in the diagram of this old Japanese pedal. Here, the signal on its way to the latches passes through a compressor and a Schmitt trigger.

The compressor is needed to process both loud and quiet notes effectively. Interestingly, the CMOS NAND gate circled in green on the diagram, is used as an analog signal amplifier.

This is doable. The ratio of the resistor value between the output and the inverting input of the CMOS inverter to the impedance in series with the input determines the gain, just like in an operational amplifier.

An excellent example of a pedal that uses CMOS inverters rather than transistors, op-amps, or tubes to amplify the guitar signal is the Tube Sound Fuzz circuit, designed by Craig Anderton and published in his 1975 book "Electronic Projects for Musicians."

Frequency dividers on flip-flops work well with a single note. And for a chord or pitch shifting at an interval of a non-integer octave number, a digital signal processor is almost always required.

Perhaps the only exception is the perfect fifth. The original frequency must be divided by three and multiplied by two to obtain it. Frequency division into three is possible using a single CD4013 chip containing two synchronous D-flip-flops with asynchronous setup and reset.

At each of the points of the circuit, marked with numbers 2, 3, and 4, there is a frequency from point 1, divided by 3, in the form of a PWM signal with a duty cycle of 2/3 or 1/3. For music, this sounds even better than a duty cycle of 1/2.

The phases of the signals at these points are shifted between each other, which can also be used for interesting sound effects. But there is one problem.

Modern music uses not natural but evenly tempered tuning. Therefore, our electronic fifth, played by flip-flops, will be slightly out of tune relative to the fifth on the guitar frets.

The frequency of this equal-tempered fifth is not equal to two-thirds the frequency of the original note but to the frequency of the original note divided by the 12th root of 128. This system allows one to freely transpose melodies and chord progressions from one key to another. That is why it became universally recognized.

And to get a tremolo, you don’t need all these tricks. Two functional blocks are enough: LFO and VCA, a low-frequency oscillator, and a voltage controller amplifier.

Each of them can be implemented literally on one transistor. This is precisely what the developers of the Univox/Unicord U65RN combo amplifier, produced since 1971, did.

It had 15 watts of power, a 12-inch loudspeaker, and a spring reverb. It was assembled according to a relatively simple circuit by today's standards. However, at that time, it was considered quite complex.

Our attention should be drawn to the lower left corner, which shows the tremolo diagram. The leftmost transistor is used in the phase-shifting RC LFO. This generator produces harmonic sine waves, the frequency of which is controlled by the SPEED potentiometer.

Oscillations from the LFO output go to the modulation intensity control INT and to the second transistor's base. This transistor simply bypasses all input signals from the microphone, guitar, and organ. This is how the VCA is made.

And the tremolo pedal jack simply shorts the LFO signal to the ground, and the VCA transistor is completely turned off. This allows one to turn the tremolo on and off using a pedal with just a latching SPST button inside.

The diagram published by Anthony Leo in the November 1968 issue of Electronic Australia Magazine, 3 years before the Univox/Unicord U65RN, differs slightly from the tremolo found in this combo amp.

Since then, the circuit has been called the Electronic Australia tremolo or EA tremolo for short. It's a beloved one among pedal enthusiasts.

Here, the VCA comprises a common-emitter bipolar transistor Q1 and a JFET Q2 that bypasses most of Q1's AC bias resistance, thereby modulating the stage's gain.

I've assembled this DIY pedal kit from Landtone using just this scheme. In the video below, you can hear the resulting sound.

Due to such an exciting circuit design, the effect turned out to be very beautiful. This is the best tremolo I have ever encountered.

As we can see and hear with our own eyes and ears, excellent results can be achieved using simple means if you approach the project thoughtfully and with love.