Audio Earth Loops and How To Fix Them

April 2006 – Article by Justin Hurt

Earth loops or improper grounding can be difficult to trace at the best of times, there are many factors which can contribute to the problem.

Noises and buzz in an audio system can frequently be traced to earth loops.

Each piece of equipment in a sound system has it`s own internal grounded circuit (signal ground), the loop occurs when the grounds of two units are connected together somewhere in the external circuit such as the ground wire in the power cable or by the mounting rails in a rack system, and these devices are also connected together through their signal inputs and outputs.

This creates a circuit between connected devices through which current can flow, this is not a problem in itself, (in fact it is impossible to completely eliminate voltage from this circuit) and only becomes a problem when the induced current interferes with the audio signal.

Fig1: Diagram of an Earth Loop and potential points to break the loop.

Balanced vs Unbalanced Connections

Consisting of a three conductor cable the balanced interconnection standard was initially created to solve the issue of hum and noise introduced over phone lines. The three conductors are made up of two signal lines i.e. ‘hot’ (or ‘positive’), ‘cold’ (or ‘negative’) and a shield. Hot and cold wires are twisted together as a pair. Induced noise is picked up equally by the hot and cold wires. At the input of the destination equipment a clever circuit rejects any signal that is the same in the two wires (referred to ‘common mode’ ie: the induced noise), the circuit only accepts signals that are opposite between the two wires (ie: the intended audio signal from the source device). This principle is called ‘common mode rejection’.

The circuits at the source and destination equipment can either involve transformers (with the advantage of complete isolation, but impedance matching is particularly important, and frequency response can be an issue unless quite expensive transformers are used), alternatively active circuits can achieve a similar result (they don’t offer complete isolation, but are accommodating for impedance matching and offer a good frequency response for comparatively lower cost).

When properly utilized a balanced system can greatly reduce hum and noise.

Unbalanced connections are most common in consumer equipment, typically in the form of RCA connectors such as on your home stereo. They utilize 2 conductors, simply signal and ground. A lot of 'semi-pro' equipment which is common in the industry comes with these types of connections and as they are not technically compatible with the balanced spec, they can be the cause of a lot of problems.

There are methods of wiring that can be used to connect unbalanced to balanced equipment but results vary depending on the specifications of devices. It’s also possible to add external ‘balancing boxes’ to unbalanced equipment, and we would recommend doing this, particularly where patch panels and larger systems are involved.

Fixing the problem

Changing unbalanced systems to balanced: By the use of external ‘balancing boxes’.

Transformer coupling: By inserting an audio transformer between a signal and it’s destination you can effectively break the earth loop as the transformer provides complete isolation.

Keep cable runs to a minimum: Resistance increases with the length of cable used hence increasing voltage drop across the cable and subsequently noise.

Keeping signal cabling away from mains cabling and other sources of interference: In addition to earth loop buzz, noise can be picked up from mains cable, fluorescent lights, dimmer packs and other items that transmit a magnetic field.

Breaking the shield connection at one end: As shown in the Figure 1 it can sometimes help to disconnect the signal shield at one end of a cable run. This prevents current from flowing along the shield (and inducing hum in the associated signal wires at the same time). There are some wider issues though, and in a well planned professional installation it would pay to study the first of the URL links below.

We don’t recommend disconnecting mains earth as this can create safety issues!

Good Planning pre installation: With good planning pre installation you can identify and eliminate potential problems. The ideal installation involves a very clean ‘technical earth point’ which is used for all audio equipment, and not for lights, air conditioners, motors, and other (electrically noisy) parts of the plant. Efforts should be made to minimise the resistance (electrical potential difference) in the technical earth between pieces of audio equipment.

It is also a good idea to run all technical equipment off the same electrical phase, and to try to keep other electrically noisy equipment off this phase.

Using digital cabling: Digital cabling greatly simplifies dealing with earthloops. Copper digital cabling may still have an earth current flowing through it, but the signal will generally be completely unaffected. Fibre Optic cabling is even better as no electrical current can flow through it at all.

URLs

There has been a lot of very good technical research done on this subject with recommendations for equipment designers and system installers.

· Audio Video Wholesalers have some good reference links www.avw.co.nz/

· This link had a good article www.epanorama.net/documents/groundloop/

· A thorough article written with unbalanced audio in mind can be found at www.jaycar.com.au/images_uploaded/humloop.pdf

Ways we may be able to assist

• Unbalanced to balanced conversion.
• Advice re earth system design
• Quantifiable measurement of buzz and noise level in audio signals, trouble-shooting, and optimal calibration.

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The information in our articles is given sincerely with best intentions to be helpful. Details often include our experience, research we’ve done, and our professional opinion, however CPR doesn’t guarantee absolute accuracy and can’t be held responsible for the implications (unless of course you’ve hired our services).