The Kakuhomaru Bridge Pickup for Electric Violins

The Kakuhomaru (角宝丸) bridge is a piezoelectric pickup for electric violins, designed with a ‘one-sensor-per-string’ approach to capture each note with clarity and richness.

🔊 Listen to the open string sample here:

open.string.sample.mp4

Bill of materials

Each Kakuhomaru bridge is composed of 4 main parts:

1. Piezoelectric rods and shielded cable (repurposed from guitar undersaddle pickups).
2. 3D printable case (available in both low profile and high profile).
3. Custom printed circuit board (PCB) with castellated holes.
4. Semi-rigid electronically conductive foam (I'm using this one which has a 30 ohm-meter (Ω·m) volume resistance).

While some parts may require a few days of lead time, the assembly is straightforward and DIY-friendly once all components are in hand.

Ordering the PCB

To get started with your PCB order, upload the production file to your preferred PCB manufacturer, such as JLCPCB or PCBWay. Ensure you select Castellated Holes—essential for the functionality of our bridge pickup.

Here are the specifications I used for our prototype:

PCB Thickness: 1.6 mm 
PCB Color: Black
Remove Order Number: Yes
Castellated Holes: Yes
Edges: 2

For reference, ordering 10pcs from JLCPCB cost approximately $40. This should help you budget for your build.

Assembly Instructions

• Step 1: Attach the Piezoelectric Rods

  Carefully slice open the heat shrink tubing of the under-saddle pickup with a utility knife to retrieve the piezoelectric ceramics. Position the ceramics with the positive end towards the E string (treble side) and the marked negative end towards the G string (bass side) . Fix them in place on the PCB using a tiny drop of CA glue at each end.

  

• Step 2: Solder the Shielded Cable & Insert the Conductive Foam

  Strip back a small portion of the shielded cable to expose the central positive wire. Solder this wire to the positive through-hole on the PCB. Attach the braided mesh, which acts as the negative wire, to the GND (ground) through-hole. Then, insert conductive foam into the eight castellated holes, using approximately 5mm cubic pieces.

  

• Step 3: Close the Protective Case

  Place the PCB assmebly inside the 3D printed case and secure it with two M2x10 screws.

  

🔊 Test the bridge assembly with an amplifier once you complete the assembly to ensure proper functionality.

preamp.testing.mp4

Setting up

• Shielding: There's no need for copper shielding tape due to the PCB's integrated ground plane, which reduces interference.
• Grounding: Establish a grounding connection by connecting the shielded cable to the tuning pegs (if applicable) or directly to the string ends, depending on your violin setup.
• Bridge Shaping: Sand the bridge feet as you normally would and use a pencil to mark the grooves on the edge of the PCB for smooth string sliding.

Further Customization

For those looking to tailor the bridge pickup to different sizes or heights, an editable STEP file is available. This file allows you to make precise adjustments to the bridge to perfectly fit your instrument's unique specifications.

Additionally, if your setup requires a pickup for five or more strings, you can take advantage of the fully editable KiCad project. This project includes a DXF outline, enabling you to design and customize your PCB to accommodate the specific needs of multi-string electric violins.

Copyright and License

Copyright (c) 2024 Penk Chen. All rights reserved.

All files are licensed under MIT license, see the LICENSE for more information.