Build Your Own: Clean Boost Guitar Pedal

I’ve always wanted to build a guitar pedal from scratch. Why not start with the easiest of all: a clean signal booster

 

In this series, I will detail how I have built my own clean boost guitar pedal. I will detail the schematic, what parts are chosen, the circuit stages, and how they operate, as well as following through to the finished product.

The Beginning

A clean boost guitar pedal should accomplish a few things:

1. Take an AC signal around 300mV pk-pk and boost it by some multiple
2. The boosted signal should not be muddled or distorted/clipped in any way
3. The output signal should model the input signal, with the only difference being the amplitude
4. It should run on +9V (either a battery or a standard guitar pedal wall-wart)
5. It should have an adjustable output gain
6. It should have a bypass capability
7. It should not load the input source too much
8. It should have enough power to drive the output easily

So what’s a clean boost pedal for anyway? Think about driving your amp a little hotter going into a lead part, or maybe you just need a little extra volume. With an adjustable clean boost, you can get that perfect amount of pre-gain for any sound you want.

The Design

The design of the clean boost started with the above design considerations in mind.  The circuit needed to have a few key components to satisfy my requirements. First, the circuit needed to have a pre-filtering stage that would prevent loading the input signal too much. There would also need to be a boost stage, and lastly an output stage to provide the output specs I wanted. I came up with the following circuit to accomplish this.

Schematic

The circuit is comprised of very few components, and is made up of four stages.

Input Stage

The input stage does a few things for the boost pedal. Notice the power supply section. The diode provides a little circuit protect from reverse biasing the circuit. Next notice the capacitor C1 feeding into the the pull-up-down resistor network. This section of the circuit provides the input AC signal with a DC offset of 1/2 the supply voltage. This allows us to use GND for the negative reference of the op-amp and the supply voltage as the positive reference.

Pre-Boost Stage

The pre-boost stage is comprised of the op-amp, and a capacitor. This section of the circuit serves to satisfy the no-load requirement from the input source, as the op-amp is in a voltage follower configuration. Since the input signal to the op-amp is between GND and the supply voltage, with AC perturbations from the input stage, the output will follow that exactly. The capacitor at the end of this stage serves to block the DC component of op-amp output, and only pass the AC component on to the power stage.

Power Stage

The power stage does what its name implies. It provides the power for the boosted signal. The input from the op-amp perturbs the NPN BJT, and because the resistor network surrounding it has the BJT in its active region, the output taken on the collector is boosted. One thing to note at this stage, the output is inverted from the input signal. The signal amplitude has been boosted by several times. This is the essence of our clean boost pedal. Also note that the output signal has a large DC offset which is not what we want for the output.

Output Stage

This final stage serves to take the DC component out of our signal (C3) and to attenuate the signal. I have chosen to represent a potentiometer with two series resistors. When we move the wiper of the potentiometer, R7 and R8 will adjust to give us a variably attenuated output signal.

Simulation of the Circuit

To see how the circuit might perform, I used LTSpice to simulate the circuit under various circumstances.

I simulated the input signal to be a sine wave at 880Hz  with an amplitude of 150mV. The power supply was set to 9V. Basing my simulation on a B50k and going in 5% steps, I obtained the following data.

W-GND (kΩ)	Pk Amplitude (V)	RMS (V)	dB
(input signal)	0.150	0.1061	-16.48
2.5	0.094	0.0665	-20.54
5.0	0.187	0.1322	-14.56
7.5	0.281	0.1987	-11.03
10.0	0.377	0.2666	-8.47
12.5	0.471	0.3330	-6.54
15.0	0.565	0.3995	-4.96
17.5	0.660	0.4667	-3.61
20.0	0.754	0.5332	-2.45
22.5	0.848	0.5996	-1.43
25.0	0.942	0.6661	-0.52
27.5	1.036	0.7326	0.31
30.0	1.130	0.7990	1.06
32.5	1.220	0.8627	1.73
35.0	1.318	0.9320	2.40
37.5	1.410	0.9970	2.98
40.0	1.500	1.0607	3.52
42.5	1.600	1.1314	4.08
45.0	1.690	1.1950	4.56
47.5	1.780	1.2587	5.01

 

The Implementation

I knew I’d want to have a PCB printed for this project, just to make things really clean. I drew up the schematic (with a few logical modifications) in EagleCAD and laid out the board.

Schematic Capture

Notice there are a couple of key differences. I have added 0.1″ pitch terminals for the the signal in/out, 9V in, and two 3×1 0.1″ pitch terminals for a boost select and bypass select. Also, there are two op-amp; one is in use and the other is in a strange configuration. The series of op-amp I have elected to use has two amplifiers in the package. To keep the unused one from injecting noise, I put it in a known configuration so it will not be noisy in the circuit.

Boost select is where the potentiometer will wire into the circuit. Bypass select will allow me to put a SPDT switch in and connect common to the pole number 2. In position 1, the pedal will be engaged, in position 2, it will be fully true-bypassed.

Layout

The layout is based on the size of a Hammond 1590G project box. The 3-terminal part for the footswitch is located at the bottom of the board, while the 3-terminal part for the boost select is located at the middle-top area. The signal in is on the top right of the board, and signal out is on the top left. Power (9V) in is in the center of the top.

PCB Spin

I use OSHPARK for all my prototype PCBs because they’re quick, good quality, and have free shipping. Here’s what OSHPARK says the final product should look like:

 

Next Steps

Next, I need to order the PCBs, order my parts, and wait! I will make another post when the parts and PCBs come in, and show how it turns out. I ‘ll also include and audio test.

Thanks for reading!!