Audio off ground

EDN magazine has a very good article series on audio electronics related to ground loop issues (issue I have written myself in this blog and my ground loop documents). The article series is based on material that has originally appeared in Linear Audio, a book-format audio magazine published half-yearly by Jan Didden.

The G word: How to get your audio off the ground article starts by describing the grounding issues in the audio devices. According to GND Gurus the root cause of all hum and buzz problems is current flowing through “the same ground” as that used as voltage reference. So, they suggest, we use “different grounds.” The supposed solution is called a “star ground,” a common point where “different grounds” connect. It looks nice at first glance and its practitioners defend it as though it were a fundamental truth. Practically speaking though it’s a nonstarter.

The problem in star grounding is that it only works at all when it’s rigorously done and on any circuits larger than tiny amplifiers you’ll run into stability problems (try switching circuits and all assumptions go out the window). Stars are a sticking-plaster to try to make flawed assumptions work. Fortunately there is solution on balanced signals. Sadly enough, this too is riddled with confusing semantics: Balanced, differential, symmetrical, what shall it be?

The G word: How to get your audio off the ground (Part 2) article tells that the ideal differential input/output would use a transformer (but audio transformers have their own set of challenges elsewhere). This article describes basics input and output circuits using active electronic components (opamps).

The G word: How to get your audio off the ground (Part 3) article goes to more details on differential interfacing circuits. It shows the problems on some traditional designs, and gives details of circuits with improved performance. The article has also details on cabling, cable shielding and cable shield connections to equipment.

The G word: How to get your audio off the ground (Part 3) article claims that the XLR connector has been something of a missed chance: It should have been a round shell with just two pins in it. Nobody would have doubted that the shell should connect at the chassis. But now it’s got the third pin which has misled people into thinking that it was some kind of “audio ground” connection that should connect somewhere other than the shell. What happened is that a lot of people connected pin 1 to their internal zero volt reference (infelicitously called GND): Instead of shunting away circulating currents into the chassis, this actually invites them in to have an all night party inside sensitive audio circuits.

Pin 1 problems drive users mad! I have personally used countless of hours to track down humming problems cause by pin 1 issues on many audio systems. Tracking down those problems is frustrating even when you have all the needed tools in hand (including sensitive clamp multimeter to measure currents flowing on audio cable shields).

At some point the problem became so prevalent that the AES had to enshrine the obvious into a standard. Called AES48, it patiently explains that the shield should be connected to the chassis via the shortest possible route and that connections between the PCB ground and the chassis should be made elsewhere.

5 Comments

  1. Tomi Engdahl says:

    The G word: How to get your audio off the ground (Part 4): Demo project – A balanced volume controller
    http://www.edn-europe.com/en/the-g-word-how-to-get-your-audio-off-the-ground-part-4-demo-project-a-balanced-volume-controller.html?cmp_id=7&news_id=10005147&vID=209#.VFs-m8lsUik

    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

    Reply
  2. Tomi Engdahl says:

    The myth called “ground”
    http://www.edn.com/electronics-blogs/test-voices/4437985/The-myth-called–ground-?_mc=NL_EDN_EDT_EDN_today_20141218&cid=NL_EDN_EDT_EDN_today_20141218&elq=2c0040928de34be4b9f2bf91178fe23a&elqCampaignId=20789

    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.

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  3. Tomi Engdahl says:

    Ground Loops and Hum
    https://www.youtube.com/watch?v=GS-6jBk9YPM

    How Ground Loops happen and how to avoid them.

    Reply
  4. Tomi Engdahl says:

    WTF are Ground Loops?
    http://hackaday.com/2017/03/09/wtf-are-ground-loops/

    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.

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