This video talks about a lesser known form of ground loop noise that can occur with audio equipment and how to eliminate it.

These magical creatures crop up out of nowhere and fry your electronics or annoy your ear holes. Understanding them will doubtless save you money and hassle. The ground loop in a nutshell is what happens when two separate devices (A and B) are connected to ground separately, and then also connected to each other through some kind of communication cable with a ground, creating a loop. This provides two separate paths to ground (B can go through its own connection to ground or it can go through the ground of the cable to A and then to A’s ground), and means that current may start flowing in unanticipated ways. This is particularly noticeable in analog AV setups, where the result is audio hum or visible bars in a picture, but is also sometimes the cause of unexplained equipment failures.

The Solution

Now that you’re an expert, solving the problem (or avoiding it entirely), is pretty straightforward. The most certain way is to cut the loop, which means removing the cable, or replacing it with something that isn’t a wire. You could switch to a wireless communication, like Bluetooth or WiFi. Some wired protocols use differential signals instead of single-ended signaling so that there isn’t a need for a common ground for reference. Move plugs around so that they are plugged into the same outlet, making your loop as small as possible. Another option is to use an isolator, which you could purchase for your cable of choice or design into your project with an optoisolator or isolation transformer. Do not use a cheater plug or remove the ground pin, as that just eliminates a safety feature and could create a dangerous situation with a chassis at live voltage.

But if it’s connected to mains and has an earth pin (even indirectly, like a device powered by USB through a computer power supply), there is the potential to create a ground loop, because you’re connecting your grounded scope to another grounded device via the probe.

So to sum up: ground isn’t just ground. For measurement noise purposes, it’s best for each device to have one and only one path to a single ground point. When there are two or more paths to ground, they can form a loop that will pick up all sorts of environmental electrical and magentic interference. Fixing a ground loop is as simple as breaking it open, but to do so you have to have a good mental picture of all of the ground paths in play.

How Ground Loops happen and how to avoid them.

A friend of mine told me years ago that there’s no such thing as voltage. This was a real shock to me, because I had a voltmeter in the lab. He explained that Maxwell’s equations, which are accepted as the basis for all electromagnetic (and therefore circuit) theory, include current, electric field, and magnetic field, but no voltage.

Most PCB designers talk about things such as DC supply voltage or a signal voltage on traces. If I bring up the idea of current flow, they will accept it. Then we discuss how the return current always flows back to its source on ground.

The term “ground” is probably the most misunderstood and misused term in electrical engineering. I blame the universities. They start their electrical engineering instruction with DC circuits and then progress to AC circuits with resistors, inductors, and capacitors. But the ideas of parasitic and nonschematic effects are seldom discussed in classes. Usually, lab assignments are relatively low-frequency projects, probably designed to ensure parasitic effects aren’t encountered.

We learn to read schematics with this magical return current path called ground. At low frequencies, the physical distance between the ground connections is electrically small. This concept of having all ground nodes connected at the same point is reasonable.

In the real world of high-speed circuit boards, the physical distance between ground node connections isn’t electrically small, so the distance between the nodes becomes meaningful. Current must travel some distance to return to its starting point. This distance can adds losses that make ground something else entirely.

According to a quick scan of professional audio fora, a perennial question is how to build a purist balanced volume controller. Two recurring themes are H-pad attenuators and dual-gang pots.

I have a double agenda in presenting this demonstration project. Firstly just to demonstrate how the “new” design methodology works in practice, but secondly to invite doubters to discover for themselves how a bit of rational engineering can produce staggeringly good sonics without resorting to boutique parts or boutique thinking. This is going to be the cheapest and best-sounding preamplifier you’ve ever built