Audio Technology in General: Hum and Interference
First and foremost musicians want to play music. Plug in, turn on, and then – you hear hum like a buzz saw. It is not very easy to explain how this hum, mostly caused by so-called ground loops, is created. For musicians and roadies it is important to get some idea what causes this annoying by-product in audio systems and what can be done about it.
Wherever we go in our civilized world, dangerous forces lurk in the shadows, ready to disturb the peace of our pure, innocent sound signals. Running motors, transformers, mobile phones, electric tooth brushes – in short, a whole army of possible sources of interference. Every cable is being threatened by all kinds of external interference. Some of these hidden forces travel through air in form of radio waves, while others sneak through the normal electrical system in our house, for example when we switch on fluorescent lights, dimmers or the power supply of an amplifier.
With PA systems sometimes you have no other choice but to use long cable runs between stage and mixer. Interference caused by fluorescent lights, for example, can be picked up by these cables and cause “humming“, “buzzing“ or “crackling“.
Typical causes for these problems are the power supplies that are required for most audio equipment.
All electronics, from effect units to instrument amplifiers, require a power source of 115 or 230 V. If you have ever taken a look at a power cable you have noticed that it has three wires, usually black, white, and green: Phase = carries the voltage, zero = grounding at the generating plant, and the protective earth (PE) or equipment grounding conductor = grounding at the house to the foundation earth or ground.
On most units the conductive parts, surfaces and ground connection for the cable are internally connected directly to the PE contact, and thus to the protective earth conductor at the power outlet. The purpose of the protective conductor is to protect. It discharges the voltage if there is a defect in the unit or the phase touches the housing. This should normally trip a safety breaker.
Even though all we want to do is make music, (un)fortunately the subject of required power supply safety standards result from a number of legal guidelines that were not randomly created by some officials just to make our lives more difficult. The opposite is the case, because it could be a matter of life or death!
The voltage of 115 V with 60 Hz in the US, and even more so the 230 V with 50 Hz commonly used in Germany/Europe will endanger your life if you come in contact with it. This makes the operation of amplifier and PA equipment, and even more so lighting systems that are powered by a so-called three-phase current, dangerous. Here we are dealing with currents of up to 380 V !!! They are guaranteed to be lethal, a fact that is supported by accident statistics.
Everyone who makes music would like to show off his or her talents at public events. To do this, in most cases you need technical equipment. And here the challenges begin. Even a performance at the town hall in front of only 100 – 150 people is a public event.
Such events are governed by conditions and regulations which have only one purpose: to protect the public and performers from injury or worse. The organizer of the event is mostly responsible for ensuring that escape routes and emergency exits are available, the stage is built safely and that appropriate power sources and wiring are used. And now we have come full circle – back to supplying the electrical power.