Ground loops and guitar pedals

Yesterday, I received an email from a beginner that decided to make his first guitar pedal. I always enjoy this kind of emails and answering questions is part of the game. This time, he asked me a question that I had several times: "my circuit is noisy, could it be a ground loop?"

Ground loops are part of the legends and myths around DIY guitar pedals. When asking about noise in a setup, it is the most common answer, and is supposed to be the main cause for hum, hiss or other noises that you can have on your first circuits.

Montagne Tremolo PCB

So I decided to write a post about it, starting from the begining:

What is the ground?

The ground connection is the reference point of the circuit, with a 0V potential. On schematics, it is represented by these symbols:

Ground schematic

It is very important that the reference point is the same in all the circuit, so all the ground connections should be connected together!

To make it easier, lazy PCB designers like me usually add what is called a "ground plane". It is a large conductive surface that is connected to ground and allows easy connections of grounds together.

On this picture, you can see the ground plane between the components and the tracks. I circled a pad of R21 that is connected to the ground plane:
Ground plane connection

You can clearly see the cross-shaped pad that connect it to the ground plane.

So what happens if you do not connect the ground connections together? By doing that, the reference point is not the same in every ground connection: you create a potential difference that is to say a voltage! Most of the times your circuits will not work, so pay attention to it!

So, what is a ground loop then?

Most of the times, ground is connected to "earth". This is the third plug that is usually on your power outlets. It avoid electrocutions by connecting metallic parts of the device to ground.

However, it sometimes happens that earth connections are not connected to each other. This can cause a slight difference between the two grounds reference point, creating a small voltage between the two grounds!
Ground Loop

Even if this difference is generally quite low, it is sufficient to generate a current if you connect these two grounds with a jack! This current will generate noise, usually modulated by the frequency of your outlets (50 or 60Hz depending where you live): it is a ground loop!

Ground loop guitar pedals

In this case, the ground loop would be caused by your electrical setup. This happens quite on lot on crowded pedalboards in houses with old electricity installations.

However, the same thing can happen between your effects pedals! Sometimes, each pedal can have a reference point that is slightly different and create current in the ground! This is why it is generally better to use power supplies with isolated outputs.
Ground loop guitar pedals
However, even with the best power supply available, ground loops can still happen between the grounds of the jacks. You understand that it is no simple solution! 

This is why it is important in to design pedals that do not have this kind of problem.
But how to do it?

Ground loop and star wiring

Ideally, multiple path to connect two different grounds should be avoided. If many paths connect ground together, it can create potential differences that will generate noise! That is why a ground plane should not be divided in several parts.

A good practice that I always recommend for your DIY pedals is the "star wiring": connect all ground connections in one point, most of the times at the negative pin of the power supply jack.
This is what I recommend in my post about guitar pedal wiring.

Ground and antenna

As you see, everything is very simple... So let's add one more level of complexity: did you know that the ground connection can behave like an antenna?

That is why it is very important to connect the enclosure to ground. By doing so, you create a Faraday cage that prevents the circuit from being parasited by outside electromagnetic fields. A metallic enclosure is thus a must to make guitar pedals. Avoid plastic enclosure and use aluminum enclosures like Hammond enclosures for instance.

This can also be a huge problem when mixing analog and digital circuits. Indeed, the high frequencies used by digital chips can be received by the analog ground and create noise. It is thus very important to separate analog ground from digital ground physically and connect them only in one point.

This was one of the main difficulty that I encountered when I designed the Montagne Tremolo circuit board. It was quite a lot of work to solve it out!

There it is, I hope that everything is clear for you now! If not, post a comment!

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Icons used to desig the diagrams in this post are from the Noun Project and are protecte under the Creative Commons License.
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