Ground loop and interference with USB audio devices

What is a ground loop

A ground loop can occur when the grounding of multiple devices is interconnected. In such a setup, if a voltage difference arises, for example, because one of the devices consumes higher power, it can cause variations in the ground level for other devices. As a result, from the perspective of a single device, there will be an alternating reference voltage, leading to the appearance of an analog signal. Devices such as mixers, amplifiers, and audio interfaces, being precise measurement equipment, will reproduce this signal as it is, making it easily audible to our ears.

How does ground loop sound like

Speaker output recorded with a ground loop (DAC connected straight to the PC):

Speaker output recorded without a ground loop (A2circuits isoUSB+ installed):

Depending on the cause of a ground loop, you might also hear 50 Hz hum or anything that is closely related in time or activity to higher power consumption devices like PC graphic card (GPU).

Digital ground loop noise solution

This kind of higher frequency hiss is strictly related to digital source of the ground loop and popular solutions like common power outlet might not solve the issue.

A solution that is strictly unrecommended is to lift your protective earth pin. This exposes yourself to the risk of electrical shock in case of accidental short of the high voltage to the chassis.

Thus, using an USB isolator is a recommended solution, as ground pin remains connected and protective earth can ground the chassis etc. Additionally, it provides 1500 V or more of the isolation voltage that can help you in case of electrical failure or surges.

Before and after ground isolation

Following plot denotes the noise spectrum before installing the USB isolator onto audio interface (blue color) and after (yellow color).

Over the 80 kHz frequency range measured, you can see there up to 32 dB noise reduction and hear only natural noise that is always present.

Keep in mind that for audio interfaces, a High Speed isolator that supports 480 Mbps is required. Most of cheap isolators support only 12 Mbps USB Full Speed.

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