What is a ground loop? (Updated 3.4.2024)

Many guitarists want to use two amplifiers simultaneously and keep them on all the time. However, using two amplifiers usually causes hum in one of the amplifiers, which needs to be eliminated. The hum is caused by a ground loop, but fortunately, there is a solution for it, called an isolation transformer. The best way to break the ground loop is with the Lehle P-Split.

These devices are passive (Passive Split), meaning they do not require electricity to operate. Inside these boxes, you'll find high-quality isolation transformers designed specifically for the sensitive signal of the guitar.

Lehle P-Split

Lehle P-ISO

Lehle P-Split Stereo

The Lehle P-Splits effectively break ground loops while keeping the sound completely unchanged. Additionally, with the larger P-Split, the signal can be split directly to two amplifiers, whereas the Lehle P-ISO is used with only one amplifier, and any splitting to multiple amplifiers should be done beforehand. On the other hand, the routing options of the Lehle P-Split Stereo box are extremely versatile.

What does grounding mean?

Ground appears in guitar equipment in three different forms.

1) In the mains supply - Earth Ground
2) In the equipment chassis - Chassis Ground
3) In the guitar cable - Signal Ground

Tube amplifiers use so much current that practically every device needs to be grounded. Grounding helps prevent airborne interference from entering the amplifier.

Grounding also serves as protection for the player. If there is a fault in the amplifier and electricity starts to move inside it, it travels directly from the device's chassis to the ground along the mains power cable. If the device has a fuse, it is placed in between, so when the fuse blows, it protects both the amplifier's components and the player.

The guitar and the guitar cable are also grounded. They function as part of this chain. Grounding protects the guitar's electronics from interference, just like in the case of the amplifier.

What forms a ground loop and what causes it?

A ground loop is created when a device is grounded to electrical ground from multiple points. In practice, this means that the device is grounded from its chassis to the electrical ground. Additionally, as the input jack is connected to the chassis by default, the ground connection extends to another device, such as an effects pedal, via the signal cable.

Instrument cables start acting as antennas, picking up interference from the environment. This antenna needs to be disconnected. Additionally, when grounding occurs through cables of different lengths, a phenomenon easily arises, known to electricians as a ground potential rise (GPR). We don't need to know more about this, but rather focus solely on how to safely eliminate the ground loop.

What does an isolation transformer mean and what is it used for?

An isolation transformer (ISO) is a 1:1 transformer that physically separates different devices from each other. The signal is transferred through induction between the input and output of the transformer. Since the signal passes through them, it is important that the transformer is carefully designed.

The isolation transformer is placed in a housing and jacks are connected to it for input and output. This box is placed in the signal path where noise occurs. Usually, this place is just before the amplifier.

How do I know if the hum in my guitar system is due to a ground loop?

1) Plug in your guitar and then turn the instrument's own volume all the way down.
2) If the hum stops, it's due to interference picked up by your guitar's pickups, which is a different issue than a ground loop.
3) If the hum continues, it's coming from the amplifier, and if the amplifier is fine, you are experiencing a ground loop.

Examples of ways how a ground loop forms and how it can be neutralized

The pictures below are drawn with different colors, representing different parts of the electrical network and the signal path. These help to better understand how and why a ground loop is formed.

Brown - 230-volt alternating current, commonly known as electricity. Brown electricity runs through the wall and the extension cables provided by the venue's equipment supplier. This is the big and bad, mysterious electricity that comes from the house's wall and over which we have no control.
Blue - 230-volt alternating current, the same electricity as the brown one. In the pictures, these are our own power cords, which connect us to the venues’s electrical network.
Green - Ground path in the mains supply, devices, and signal.
Yellow - Guitar signal.

We use different abbreviations to describe different parts of the chain as follows:

GTR - Electric guitar.
AMP 1 & AMP 2 - Amplifier/cabinet combination.
A - Main power supply point on the venue's wall, from which electricity is distributed for musicians' equipment.
B - Electricity brought to backline (usually) with a white 3-way Schuko socket, from which you are expected to power your amplifier.
C - Backline power brought to the front of the stage, where you are expected to connect your pedalboard or other devices in the front of the stage.
ISO - Isolation transformer.
PEDALBOARD - Collection of pedals before the amplifier's preamp.

1) Guitar and amplifier

The first combination is the guitar and amplifier. In this example, the ground travels from the guitar strings to the instrument jack's sleeve, then along the cable's shielding (signal ground) to the amplifier's chassis (chassis ground), and from there to the wall (earth ground). Hum does not occur because there is no ground loop.

2) Guitar and two amplifiers

Problems arise when adding another amplifier. The example below is the most classic one, where two Marshall Plexis are linked together through their input jacks. The signal travels from Amp 1 to Amp 2, causing a ground loop. In the image, the loop is clearly visible as a continuous path highlighted in green.

3) Blocking the ground loop from the mains supply

The traditional and at the same time dangerous method of removing the ground loop is to place something between Amp 2 and the wall socket. There are many options, and they are all equally bad. By taping the earth contacts of the wall socket or using different adapters, the passage of the ground is prevented. Electricity is fundamentally lazy. In case of a fault, it goes wherever it can reach the fastest and easiest. If the natural flow of electricity to the ground (earth ground) is blocked, it will then move to the guitar cable next and comes directly to your fingers. Not a good idea.

4) Disassembling the ground loop with an isolation transformer

By placing an isolation transformer in front of the input of the second amplifier, we effectively and safely break the ground loop. Both amplifiers are grounded for safety in case of faults, and the signal remains undisturbed. The isolation transformer passes the guitar signal through unchanged but separates the signal grounds from each other.

5) Guitar, battery-powered pedalboard, and amplifier

Now, a pedalboard is included. Similar to example one, the ground travels with the signal to the amplifier and then to the mains supply. The path of the ground is simple and clear. As long as the pedals are used with batteries, a ground loop cannot form around the pedalboard.

6) Pedalboard plugged into mains power in front of the stage

This is an easy way to create a ground loop. The pedalboard is plugged into the power supply in front of the stage (socket C), where it may travel through the wall for tens of meters before physically reaching the same point as your amplifier (socket B). This creates a difference in grounding and generates a ground loop, resulting in hum.

An isolation transformer can help with this problem too, but it is the wrong way to handle it. If we add anything else to the chain, such as another amplifier (Amp 2), our ace card is already used, and we are in trouble.

If the problem is not eliminated from the outset, we enter a spiral where troubleshooting becomes significantly more difficult. Moreover, you are at the mercy of local technicians. If they change their connections during the day, the ground changes, and the ground loop may disappear. Now you have separated the ground with an isolation transformer, and your system hums because it is not grounded. It's best not to get caught in this vicious circle.

7) Pedalboard power is sourced from the same location as the amplifier.

Instead of socket C, we connect the pedalboard directly to socket B, where we also get power for the amplifier. This shortens the runs of the cables to the extent that no hum occurs. We have a small ground loop, but it's so small that it doesn't become a problem.
In other words, if the power cord from the amplifier to the wall outlet is 2 meters long and the power cord from our pedalboard is 6 meters long, the difference is only four meters. Through the structures from socket B to socket C, there could be tens of meters of cable. In this situation, we are closer to star grounding, which means that all devices are connected to the venue's power from the same socket.

8) Two amplifiers and a pedalboard with batteries

Now we're using batteries in the pedals. We use a splitter without an isolation transformer and connect the pedalboard to two amplifiers. The result is a situation similar to point two. This time, the splitter box connects the grounds of the amplifier chassis together. The ground loop is ready, the familiar green circular line is visible in the picture.

9) Two amplifiers and a pedalboard plugged into power in front of the stage

If you want to make the previous situation worse, connect the pedalboard to power in front of the stage (socket C). This is the so-called "worst case" scenario, where you have two different ground loops in your system. This connection buzzes so much that no respectable sound engineer would agree to work with such a setup. In the picture, there are clearly two green loops, indicating ground loops.

10) Two amplifiers and a pedalboard plugged into the same power outlet as the amplifier

Here, we resolve the horror scenario from point nine. First, we connect the pedalboard to the same power outlet as the amplifier. This way, the second ground loop is eliminated.

11) Correct connection of two amplifiers and a pedalboard

Now, we place another amplifier in front of the isolation transformer, thus eliminating the remaining ground loop with Lehle's P-Split isolation transformer. In the illustration below, we route the signal from the pedalboard directly to Amp 1 and to Amp 2 via the isolation transformer of the P-Split. Alternatively, we can route both amplifiers directly from the P-Split. We take power to the pedalboard directly from the back of the amplifier at point B. This way, we form a so-called star grounding, where all devices are grounded through the same path before being connected to the main (brown) power grid. The result is a quiet guitar rig without hum.

12) Three amplifiers and a pedalboard without hum

We can continue this same idea endlessly. In the final example, we connect three amplifiers to play simultaneously. Only the signal ground of one amplifier's input is connected; the others are disconnected. The amplifiers are grounded to the mains (earth ground) through their chassis (chassis ground). The system is safe to use, and no ground loops occur.

Read more about Lehle P-Splits

*****

If you have purchased all the parts and components but get a feeling that you might not be up to the task after all, we can make your pedalboard for you, using the components you have bought from us. Don’t worry, we won’t let anything go to waste.