Avoiding Earth Loops
Earth loops, also known as ground loops and hum loops, are the bane of every studio installer's life. Due to the sheer number of signal sources and destinations, earth loops are easy to create and, without thought and planning, almost impossible to rectify. Ea
Figure 6. A method for avoiding earth loops with multi-channel unbalanced equipment.
rth loops are created when the ground leg of a signal travels, via other wiring and equipment, back to its source. This loop picks up mains hum, digital noise, clicks and pops from any available source. Noise is introduced when the unwanted interference sources induce a current in the loop. In the real world, the loop cannot have zero resistance and therefore (according to Ohm's law) a noise voltage is developed across the loop resistance. This noise voltage is added to the genuine audio signal. From this description it should be easy to understand that all sorts of noise can be introduced in this way, as long as the source is able to induce a current in the loop. Because of this, I prefer to refer to earth or ground loops, rather than 'hum' loops.
Although balanced systems are more tolerant of interference, an input's Common Mode Rejection Ratio (CMRR) will dictate how well the induced noise is attenuated. Often, in balanced systems, earth loops are not immediately obvious, as hum may not be apparent. Because CMRR will worsen with increasing frequency, low-frequency noise, such as mains hum, may be effectively removed, whereas noise with a higher frequency content may still be evident. Timecode and PCs are particular offenders in this scenario, and may often be heard 'screaming' away at low level in the background. Unbalanced wiring can be hugely affected by ground loops. Any induced noise appears directly at the input with none of the balanced system's cancellation improvements.
The only sure way to avoid earth loops is to avoid any connection scheme which allows the signal earth path to connect back to itself. A typical earth loop is illustrated in Figure 4, and the path will often take a route via the mains earth of the equipment concerned. With balanced equipment, breaking the loop is very straightforward. There are a number of possible earthing schemes in balanced systems and each scheme will dictate where the earth loop is broken. My preferred method is to connect cable shields at equipment outputs and not at equipment inputs. If ground loops are experienced when introducing hire equipment into a system, 'ground lift' switches may also help. However, it is vitally important that the loop is not broken by removing a mains earth connect
If all else fails, a resistor connected in series with the signal ground in the cable can often reduce earth loop hum to acceptable levels.
ion, because this may result in the equipment becoming dangerous. Some installers choose to connect grounds at inputs rather than outputs, but this will give rise to problems at patch panels — balanced outputs will not be unbalanced correctly when connected to an unbalanced input (the balanced signal pair and their ground will not both available on the output socket), and therefore the signal level may be compromised.
For unbalanced systems or part-balanced, part-unbalanced systems, the problem is much worse. Fortunately, many units sporting unbalanced connectors, typically phonos, do not require mains power grounds and therefore only have two-conductor power cable. This means that, with a single signal connection, no ground loop can arise. However, if you make a stereo connection, two phono cables will create a loop around themselves, as shown in Figure 5. Although this is not normally a problem if the cables are short, the kinds of lengths found in typical studio installations may well induce noise. This is simply cured by removing one shield connection at the equipment input.
Unbalanced loops can become a real headache with multi-channel equipment such as many soundcards. If each cable had its shield connected at both ends, a large number of loops would be created. A simple trick to avoid this is shown in Figure 6. Here, none of the shields are connected at the unbalanced inputs. Instead, a single thick wire is connected between one output ground and one input ground. In this way, each signal is shielded and the two units have the same ground reference, but no loop is created. I have also used this method when constructing patch panels for mixing consoles. In many situations, particularly where the unbalanced equipment uses a power ground, the only sure-fire cure is by using a signal transformer (as shown in Figure 7) or a balanced interface unit.
Finally, in some situations, noise can be reduced to workable levels by inserting a resistor (100(omega), 0.25W) between the cable shield and the signal ground. This causes the noise voltage to be developed mainly across the resistor instead of in the wiring, thus reducing the resulting noise signal. This is a last-ditch solution, but is sometimes helpful. To avoid ground loops in complex systems, some or all of the above may be necessary. Also some experimentation may be required. The only hard-and-fast rule is: do not cure loops by removing the power ground!
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