Guitar Pedal Development Board

Guitar Pedal Development Board

Project Overview

The guitar pedal development board is another project for the Audio Engineering Club. I wanted an easy way to protoype guitar pedal (or other audio FX) circuits without having the mess of breadboarded quarter inch jacks and knobs. You can check out the complete project in the image below. I will hopefully be using it in the future to do some YouTube tutorial videos, which I will post here on this page.



Guitar Pedal Development Board

If you're curious, you can check out the current schematic for the board here. Right now it is just a voltage regulator and a volume control circuit. I will probably switch out the regulator chip that I used for a cheaper one in a future revision. And also hopefully redesign the enclosure so that it can be 3D printed. If you are interested in the story behind this project and the design process, read on.

Inspiration

I was inspired by a product I saw on eBay, the Beavis-Board-Guitar-Effect-Pedal-Prototyping-Kit. I've also put a picture of it below. It's a guitar pedal with connections from the input/output jacks that can be attached to a breadboard, where the FX circuit can be built.



Beavis Board Prototyping Kit

The thing that I wanted to do differently from the Beavis Board, though, is that I wanted to make the whole thing a guitar pedal, much like how the ADDAC210 Open Heart Surgery is itself a synth module that happens to have a breadboard on it, allowing you to make it your own module. Pic below.



ADDAC210 Open Heart Surgery


Design

So I came up with a quick and dirty design for how I would want my pedal to look. I knew I wanted three breadboards, input and output jacks, DC power, a footswitch, volume control, and voltage control for voltage starvation. I also wanted the input/output jacks as well as the power jack to be directly connected to the breadboard, instead of having the user connect it up themself. I've included a picture of that original design drawing below.



Intial Design for the Guitar Pedal Development Board


Circuits

I didn't initially think that there would be much circuitry for this project, but as it turns out, both the voltage control and the volume control require some circuitry, so I decided to build a PCB for the project. For the voltage control, I decided to use a linear voltage regulator for simplicity of design. The regulator that I ended up using was the LT3081. The image below shows the final circuit that I designed around this chip.



LT3081 Voltage Regulation Circuit

The resistors and potentiometers in this circuit are set up so that the voltage can be controller between ~1.4 to ~9 volts (with 9 as the maximum because that's the voltage that guitar pedals use generally). I was on the fence about using a 12V supply because that would mean that the dev board couldn't be powered by a regular guitar pedal supply, but I needed that headroom to get smooth voltage regulation all the way up to 9V, so I went for it. It's not like you will be putting this thing on your pedal board with the rest of your pedals anyway. The circuit also allows for current control, so I added a screen to the project which can display the voltage and current.

The other circuit that had to be designed was the volume control. Usually, guitar pedals just use a 500k or so potentiomer for volume control, but the size of these varies for different pedals. With no idea of what kind of pedal would be built on the development board, I needed something more uniform. So I went with a circuit that Gabe Schafman recommended to me, two unity gain buffers with a volume potentiometer in between them. I ended up using an opamp called the NE5534. The circuit I designed is shown in the picture below.



NE5534 Volume Control Circuit

Surrounding each of the NE5534 opamps are the circuitry that give each of them a unity gain. In between the two circuits you can see a potentiometer that will act as the volume knob. It's worth noting that the opamp input has to be biased at 6V. The small circuit underneath the opamp circuitry is used to achieve this.


PCB

With those circuits finished, I went ahead and designed a PCB. I thought I would use SMD components, since I had never made a PCB with SMD components before. It went well. I realized though, that SMD components don't automatically connect to the ground plane, because they don't go through the board. So it was a bit more difficult to route the ground nets. You can check out a pic of the PCB below.



PCB for the Guitar Pedal Dev Board

I have yet to fully assemble the PCB, but I have found one large issue. I bought the wrong LT3081 package from the footprint on the PCB. I will solder some jumpers to the board for testing, but I will need to order new PCBs or buy a different LT3081 in the future.


Enclosure

I wanted the enclosure for this project to be a large box, like a guitar pedal but bigger. My inspiration for the form factor is from those old EHX pedals like the original Memory Man (pictured below).



EHX Memory Man

Obviously it would be a lot harder to make the angled edges of the Memory Man enclosure, so I knew I should probably just go with a rectangular enclosure. I ended up deciding to buy an enclosure from Hammond Manufacturing, one big enough to fit three breadboards, the voltage display, and all the controls I want. This one isn't the exact size, but here's a picture of a very similar enclosure.



Dev Board Enclosure


Circuit ISSUES

I got the enclosure all drilled and the circuit all soldered and I realized that none of it worked. Something was very wrong with my circuit or the PCB. I talked to my coworkers at Biamp and they told me that the issue was with my volume ciruit. I didn't need two opamps, just one. I didn't want to wait for another PCB turnaround so I decided to just protoboard the whole circuit and I managed to get it working! The results of that are shown in the image below.



Dev Board ProtoBoard


Finishing Up

I redesigned the circuit in KiCAD to match the working protoboard circuit. You can check it out here. Then I redesigned the PCB, this time with through hole components for easy troubleshooting. The new PCB is shown in the image below.



Dev Board PCB V2


When I first soldered it together, the PCB did not work. After hours of troubleshooting I realized I forgot to put a load resistor on the power supply. After I put that, it worked perfectly, except for the current control knob. I still have not gotten that working, but it is not totally necessary. I put everything together in the box and created the final product you see in the image below. I'm really happy with how it turned out and I'm excited to design guitar pedal circuits on it. I'm also excited to redesign this pedal and make it a little more streamlined. Hopefully I can make a few and sell them to folks on etsy or something.



Guitar Pedal Development Board