Many shield-wiring practices exist in the audio industry today. The majority of available literature on the subject prescribes clear solutions to any wiring problem, yet problems still run rampant due to inconsistent variations on the existing model. Two different-minded groups have formed on either side of the issue: those in favor of the balanced world and those of the unbalanced world.
The source of the problem is actually caused by the consumer: as professional audio prices have decreased over the years, more and more home studio owners have begun supplementing pro-audio into their existing unbalanced arsenal, and have run into massive connectivity issues as well as the dreaded hum or buzz we so often receive when utilizing audio.
Hum vs. Hiss
The AES has specific categorizations for two sounds that are often heard if any of the above problems occur in the audio signal chain. Hum is a continuous low frequency sound, similar to the whirring of a low-pitched motor. It has a typical frequency of 60 Hz. Hum is generally caused by electrical interferences or when the recording equipment is not properly grounded.
Hiss is described by the AES as a noise often caused when random electrons deviate from their intended path under the influence of heat. These deviated electrons manipulate the output signal voltage and thus an audio noise is created. Although noise often exists in audio due to electronics – and is near impossible to completely remove when dealing with analog audio gear, when it becomes extremely present is when one knows that the electronic circuitry that the audio signal is flowing through is wired incorrectly.
Balanced vs. Unbalanced
Now that the noises that will be heard when a problem arises have been stated, one can begin to look into what causes these noises to develop.
To fulfill their users' desires to have the highest quality of audio, designers and manufacturers started upgrading equipment to balanced connections. A balanced cable has 3 wires running through the cable, with 3 conductors at the connecter: two signal wires and a ground wire.
The two signal wires each carry a copy of the original signal, but exactly out of phase with each other. This allows the receiving end to revert the signal back in phase with itself at the end, effectively phase cancelling any noise that was picked up along the cable’s length.
More and more home studio owners have begun supplementing pro-audio into their existing unbalanced arsenal, and have run into massive connectivity issues as well as the dreaded hum or buzz…
Conversely, an unbalanced cable contains 2 wires, signal and ground, and has no way to remove inherited noise along the way. The old unbalanced “shield” (really the return signal conductor, not a true shield) is already grounded.
Combining balanced and unbalanced systems may not add a chassis-grounded shield around the existing 2-conductor cable. This redefines the old return conductor as a new “negative” signal carrier, not as a shield.
Many older buildings, electrical systems, and audio gear now face this dilemma but happen to work satisfactorily even with improperly grounded shields. However, one can easily see how noise is added to the system when crossing balanced with unbalanced systems. If the unbalanced cable’s ground is redefined as the negative signal carrier, suddenly all the AC noise that was being carried along the ground enters the system in the audio pathways and is heard in the output at the end of the line.
With multiple companies upgrading to balanced interconnections, many have realized that by connecting the shield to signal ground, interfacing to unbalanced equipment is made simpler since signal ground (needed for unbalanced interconnection) will be available on the cable.
This unfortunately also allows easy use of 1/4″ mono connectors, which still creates the same problem of signal-grounded balanced shields. Signal-grounded shields on balanced equipment create ground loops in the audio path and modulate the audio signal ground, distorting most systems. This practice penalizes those who want to realize the superior performance of balanced interconnections and has given balancing a bad reputation.
A third possible reason for a signal-grounded balanced shield arises if designers change phantom powered (+48 volts) microphone inputs to balanced line-level inputs, and do not use caution. The phantom power return currents travel through the shield, requiring shield connection to the signal ground. When changing this layout to line-level balanced inputs, the designer may not think to change the shield connection to chassis ground.
Similarly, manufacturers who incorporate ground-lift switches in their products further complicate this issue. These switches disconnect chassis and signal ground. Thus care should be taken to ensure that phantom power return currents always have a return path to their power supply, regardless of the ground-lift switch position or else noise and/or damage to equipment could occur in the system.
Many manufacturers have their roots in balanced fields, such as the telephone and broadcast industries, and used chassis-grounded shields due to having to send audio signals across vast distances. Perhaps users from these balanced fields assumed that all balanced equipment had chassis-grounded shields. When improperly wired manufacturer's equipment was installed, they discovered hum and buzz problems. This they solved in one of three ways: by installing isolation transformers into the system, by disconnecting one end of the shield, or by simply not using that manufacturer's equipment.
Actual feedback to the designers who were wiring these devices incorrectly never seemed to happen. Manufacturers may have suggested isolation transformers or cable rewiring solutions instead of addressing the cause of the problem: signal-grounded balanced shields. Again, some systems with signal-grounded shields work acceptably even with all the above mentioned happening, further showing us that we don’t really know what is happening with electronics still!
There is a distinct difference in terms of grounding audio devices, one called chassis and the other signal grounding. Chassis grounding is generally considered as any conductor branching off of the circuit path, which is connected to a unit's metal box or chassis. The term chassis ground may have come about since units with 3-conductor line wires connect the chassis (generally a metal box around the inner-wirings of a unit) of the system to earth ground when plugged in to a properly wired AC outlet. In units with a 2-conductor line cord (consumer equipment), the chassis does not connect to earth ground, though the chassis is normally connected to the signal ground in the box in both unbalanced and in balanced electronics.
Signal ground is the internal conductor used as the 0 V reference potential for the internal electronics and is sometimes further split into digital and analog ground sections. Further signal ground splits are also possible, though it is important to remember that all divisions of signal ground connect together in one place. This is usually called a star-grounding scheme.
It is easy to confuse chassis ground and signal ground since they are usually connected together – either directly or through one of several passive schemes. Some of these schemes are shown below. The key to keeping an audio device immune from external noise sources is knowing where and how to connect signal ground to the chassis.
The above picture shows how a correctly wired star-grounding schematic should look for connecting chassis to signal ground.
In conclusion, balanced and unbalanced are two completely different entities. The incompatibility between these two configurations, whether using analog or digital audio, must be considered when designing, specifying, installing or upgrading equipment and systems. Literature on the subject of grounding and shielding audio devices directs the user towards chassis-grounding balanced shields. However, most manufacturers signal ground their balanced shields.
Although many solutions exist to removing the noise induced by these discrepancies in the electronic circuitry we find everywhere, the only foolproof way to never deal with these problems are to utilize a single type of shielding, a single way of grounding, and a single method of connecting (balanced or unbalanced). Everything else provided is just attempting to patch the problem’s holes up, not fix it at the source
Sources:
- Macatee, S. (Ed.). (1995). Grounding and Shielding Audio Devices.
- Grounding and Shielding Audio Devices [Fact sheet]. (2002).
- Wilkinson, S. R. (n.d.). Anatomy of a Home Studio: How Everything Really Works, from Microphones to MIDI.
