All work

Guitar Boost Pedal

A single-transistor analog boost based on the Electro-Harmonix LPB-1. It started as a breadboard prototype and ended up as a fabricated two-layer board.

  • Analog Design
  • Breadboard
  • KiCad
  • PCB Layout
Detail of the boost pedal's KiCad schematic showing the 2N3904 transistor and its bias network

Overview

The circuit is a single-transistor common-emitter booster running off a 9V supply, with a volume potentiometer on the output. It is small, and it is based on a well-known design, the Electro-Harmonix LPB-1. Most of the work went into everything downstream of the schematic: getting from a breadboard that worked to a set of files a manufacturer would actually accept.

The circuit

Amplifier stage
A 2N3904 NPN transistor in a common-emitter configuration provides the clean gain.
Bias network
A 430k/43k voltage divider sets the transistor's operating point, with a 10k collector and 390Ω emitter resistor.
Coupling capacitors
100nF input and output capacitors block DC while passing the guitar signal.
Volume control
An A100k audio-taper potentiometer on the output sets the boost level.
Off-board wiring
Input and output jacks, the potentiometer, and the 9V battery connect to the board through pin headers.
KiCad schematic of the boost pedal, from the input jack through the 2N3904 amplifier stage to the volume potentiometer and output jack
The full circuit in KiCad. Signal comes in at the jack, through the coupling capacitor to the 2N3904 stage and its bias network, then out through the volume pot.

Build process

Breadboard prototype
Selected component values and verified the amplifier on a breadboard with real jacks, a potentiometer, and a powered supply rail before committing to a layout.
KiCad schematic
Captured the circuit, assigned through-hole footprints, added pin headers for the off-board components, and resolved every Electrical Rules Check warning.
Printed PCB
Routed a 2-layer, 29.5×23mm board with a stitched ground pour, passed Design Rules Check and the manufacturer's free DFM review, then exported Gerber and drill files and sent the board to fabrication in FR-4 with a HASL finish.
The boost pedal circuit on a breadboard, with the volume potentiometer and both jacks wired off-board and a bench supply feeding the rail
The prototype before layout. The 2N3904 stage sits on the board; the volume pot, input and output jacks, and power are all wired off-board, which is how they ended up on pin headers in the final schematic.

Outcome

The board is 2-layer FR-4, 29.5×23mm. It passed Design Rules Check and the fab house's DFM review on the first submission. Gerber and drill files went out and the board was manufactured with a HASL finish.

The fabricated boost pedal board populated and wired up, with the 9V battery, volume potentiometer, and input and output jacks soldered to its pin headers
Finished PCB.

What I'd do differently

I'd socket the transistor, because swapping it out for more or less gain currently means desoldering. I'd also put test points on the bias network. Probing a board this size with no dedicated pads meant clipping onto component legs, which is awkward and a good way to short something by accident. If I did another revision I'd add a true-bypass footswitch as well, since the jacks currently run straight to pin headers and the pedal sits in the signal path whether you're using it or not.