Electric Hum: Causes & Solutions

Today we will tackle a problem that can be very bothering on stage: The “Buzz” or “Hum“. I am sure that many of you will have had bad moments fighting with this annoying and unpredictable phenomenon. In this article we will analyze the aspects and nature of this phenomenon and give some practical solutions hoping to help you and thus be more prepared and faster in solving the problem.

What is Hum?
Hum can occur when multiple audio devices are connected to each other, and is a low frequency noise (typically 50 Hz or multiple).
It generally occurs due to two types of causes:
1 – Cables that pick up the (electro)magnetic or electrostatic fields generated by the electrical system or other equipment
2 – Ground loops

What are ground loops?

Ground loops are formed when the earth voltage of one of the devices (equipped with grounding) connected to each other, is slightly different from the others thus creating a difference in potential and therefore a circulation of current through the audio cables that connect them.

The ground loop is more frequent in case of connections with unbalanced cables: the unbalanced cable, in fact, has only one conductor plus the ground while the balanced cable has two conductors plus the ground.

Obviously the problem becomes even more evident when using an amplification unit.

By connecting two devices (A – B) with an audio cable, these two points are joined together also by the braid (ground), thus forming a loop. This loop behaves like an antenna picking up electromagnetic noise in the surrounding environment which, when added to the audio signal, is amplified and reproduced.

 

What to do?

Well, there is no universal solution, but there are some strategies or tricks that we can play out trying to eliminate (or at least considerably attenuate) the electrical noise:

– check the quality and correct operation of the cables and connectors used for the connections.
– move the audio cables away from the power cables (in particular from 63A, 125A or especially from the cords of the generating set)
– if possible, use balanced audio cables.
– avoid using multiple power sockets shared with household appliances
– connect the audio equipment to a single power strip
– eliminate the ground of one of the two devices (a little more extreme and dangerous but often effective solution)

The ground lift is the first of the available solutions to reduce or eliminate the noise generated by ground loops.
If there is a D.I. in our system (highly recommended choice in case of unbalanced cables such as keyboards, etc.) just activate the ground lift button which opens the connection between the grounding of the instrument or device and that of the cable, preventing the flow of current along the shielding while maintaining the independent ground of the individual devices, thus making them safe to use.
In the most extreme cases, if there is no D.I. box in use, it is also possible to eliminate the ground physically by disconnecting it from one end of the cable that connects the two devices.

Other ways to isolate the cable shield are isolation transformers or a Hum Eliminator.

 

A transformer is generally used to raise or lower the available voltage. There is, however, the isolation transformer which has a different function. In fact, the input voltage can be the same as the output voltage (1: 1 transformer), since its main task is to separate the input power circuit from the output circuit.

How does it work?

Signal isolators provide electrical (galvanic) isolation between the input and output circuits and couple the signal to the output through a transformer.
They also interrupt the direct electrical galvanic path between two or more loop points and protect against dangerous voltages and increase protection from surges and peaks.
The isolation transformer can be used for both unbalanced and balanced signals by effectively interrupting any current flow in the cable.
Transformers also have the advantage of being passive (therefore without the need of power).

I hope you got a clearer picture of the situation without going into too much detail and without filling you with technical terms of electronics that you can read more easily on the web if you want.
Now you know what to do the next time the buzz will show up in your setup!

Nicola Trapassi
ZioGiorgio Editor
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