The main's plug on an amplifier should never be
Earlier this week, we covered dirty power and the irritating line noise that it creates. In that article, we touched on ground loops but felt like it deserved a story of its own in order to fully understand the crucial concept. Getting right to it, a ground loop occurs when there is more than one ground path between two individual electrical equipment. Normally, one path is the screen of an audio cable connecting two pieces of equipment and the other path is through their chassis safety earths in the mains plugs. Inside of the equipment, the audio screen earth is normally connected directly to the chassis earth, therefore creating the possibility of a ground loop. If the two parts of the equipment are plugged into the same mains socket, their chassis safety earths are then effectively linked together at the same potential, meaning there shouldn’t be any circulating ground current despite the apparent ground loop. However, if one of these items is plugged into a different mains socket, its chassis safety earth might be grounded pretty far from the other equipment’s earth which can then create a small difference in potential voltage between them.
As counter intuitive as it may seem, earth does not have the same potential everywhere and this voltage difference between their two chassis earths can cause a small current to flow. And since the earth is there to provide a reference for the audio electronics, that small flowing current causes the earth reference to vary slightly. This is small flow is what creates that annoying hum or buzz. The ideal situation would be to make sure that everything is earthed at one central point so that everything shares the same common earth reference point. The easiest way to avoid this problem is to plug everything into a star arrangement of plug-boards connected to a single socket that has a suitable power capacity. But sometimes, this simply can’t be done. In that case, the next best thing – as well as the safest solution – would be to break the look by isolating the audio cable screens at one end. This way, the cable is still screened but there is no longer any possibility of creating a loop because the small current that creates that annoying buzz can’t flow around it. Another way of eliminating ground loops is by inserting a transformer in the signal path so that it can break the loop (this is a common solution in outside broadcast and live sound equipment rigs). There are even DI boxes that include transformers for this specific purpose.
But it has to be said that messing with equipment with high voltage potential such as an amplifier can be a serious and even life threatening endeavor, especially when people attempting this don’t know exactly what they’re doing and are uninformed of the risks. A common and dangerous mistake is made when people who simply don’t know any better decide to break the loop by removing the safety earth in the mains plug instead of the techniques mentioned above. Obviously, removing the safety earth from the mains plug effectively breaks the loop – and thereby eliminating the hum – but this also means that the equipment is no longer earth. You don’t have to be a professional electrician to realize that if any fault were to occur in this equipment, the results could easily be life threatening.
Even worse with this scenario, you don’t even need a fault to cause very dangerous problems. Most equipment has filtering on the mains input to stop main-related noise getting in or out. If you happen to disconnect the mains earth in the plug, the way the filtering mechanism works makes it so that the previously earthed chassis, along with everything connected to it, lifts in voltage. This lift is enough that even all of the exposed metalwork – including the pickups and guitar strings – now carries a potentially lethal amount of voltage.
To further elaborate, the strings on a guitar are supposed to be earthed through the guitar lead to the socket on an amplifier. The amp’s socket meanwhile is usually connected to the amp’s chassis earth, and thereby through the mains plug on the mains safety earth. Metal radiators are also connected to the mains safety earth too, as is all the inside electronics. Therefore, if you remove the mains safety earth – which everything is earthed to – the chassis is likely to rise in voltage and everything that would otherwise be earthed will rise in voltage as well. If you were to rest your guitar strings on anything that is earthed, such as a mic stand holding a mic that is earthed through its cable to a mixer, and you have mains power now flowing directly through the equipment trying to find a real ground, you will almost certainly cause severe damage or destroy the amp, guitar and possibly yourself as well.
Unfortunately, death due to the amp’s mains safety earth being disconnected – in order to stop the low end buzz – is more common than most might think. All someone has to do in this situation is rest one hand on their guitar strings and then use the other hand to grab a properly earthed microphone and they can effectively send over 100 volts straight into their heart. It should also be said that sweaty hands make excellent conductors and in only takes a few milliamps of current flow to stop a human heart.
By now, you should fully realize that you should never under any circumstances remove the earth from the mains plug. If you have humming problems, break the screens on audio cables, use isolating transformers in DI boxes or simply take a look at this week’s story on that very subject.