In the preceding chapters, we have looked at some of the mechanisms of interference and the disastrous results of ground loops.
In order for audio equipment to be interconnected and to function quietly and efficiently, the following conditions need to be achieved:
"All audio equipment must be grounded via one path only to a single point."
The ideal way of grounding equipment is to connect solid ground wires from each individual item of equipment to a central copper ground stake driven into the ground. The wires should all be as short and direct as possible, therefore implying that the ground stake should be located centrally. Note that the ground wires form a "star" at the ground stake.
There must be no additional way that the equipment is grounded.
In practice, the above situation is difficult to achieve. How many studio owners would like to have a dirty big copper stake sticking up in the middle of their control room? Running longer ground wires to a remote stake can become a problem, since the equipment is no longer bonded closely together. A solution is to create a central ground point in the control room, and then run a single ground wire to the stake. The central ground point becomes the star point.
The point where most of the audio signal wiring comes together is a good location for the central ground point. A ground terminal on the console is often chosen. Alternatively, an external patch bay could be a good location for the central ground point.
In Australia it is usually possible to rely on the mains ground as the technical ground, provided that the studio equipment is run from a separate power circuit. This is because Australian Standards specify that a separate ground for each power circuit is run directly to the switch board, where it is joined to other power circuit grounds, the incoming mains neutral (which is grounded by the electricity authorities in this country), and to either a metal water pipe in old buildings which acts as a ground stake or a ground stake for newer buildings. In other countries, you should check your local mains wiring standards.
Furthermore, it is often possible to use the equipment's normal mains ground as the equipment ground. The switch board ground then becomes the central ground point. This situation can closely resemble configuration described in Section 6.3, The Technical Ground.
There are several arguments for using the mains ground as the technical ground as opposed to installing a separate technical ground:
It is permissible to have more than one star point in a building, provided that the equipment in the studio uses only one of these star points. For example, in large buildings, there might be one main switch board feeding several sub boards. Power circuit grounds from the sub board are starred together at the sub board, and the sub mains circuit grounds from each sub board are then starred together at the main switch board. It is desirable, however, to ensure that a sub board feeding a studio feeds only studio equipment, and not other items such as air conditioners, elevators, fluorescent lighting, heaters, industrial machines, etc. These could inject "rubbish" into the studio central ground. If this is the case, a separate sub board should be installed for the studio equipment. If this is not possible, a separate technical ground should be run either to the main switch board or a ground stake.
For small studios, a power distribution board can act as a sub board and the central ground point for the studio. For studio complexes, there could be a separate sub board for each studio.
In the above examples, ground wires are "starred" to a central point. In many buildings, power points are "daisy chained" together, i.e. a wire runs from one power point to the next and then continues on to the next. This results in a grounding situation as shown below, if the equipment mains grounds are used.
This situation is not desirable, since ground currents from one item of equipment can affect the grounds for all the other equipment downstream on the line, as described in Section 4.1, Common Path Interference. Sometimes, this situation is unavoidable within a rack of equipment fed from a power distribution board, but if separate power lines are run from each rack it is usually tolerable. If not, it becomes necessary to isolate individual items of equipment and run separate ground wires.
Unbalanced equipment grounds must be connected together via the shields of the audio leads interconnecting them. To satisfy The Golden Rule of Grounding, this means that only one item of equipment can be grounded, and all other unbalanced equipment connected to it must be totally isolated from other grounds. The equipment itself can form a star point, as shown below:
The SHIELD connection is never broken in order to resolve a grounding problem with unbalanced configurations. The SHIELD connection is always regarded as equal in importance to the active audio connection. Disconnecting the SHIELD connection never works! Problems encountered when doing this include excessive hum, noise and crosstalk.
This grounding configuration implies that mains grounds (if present) need to be disconnected.
In some cases, there is an alternative to disconnecting the mains ground. Some equipment has provision to isolate the audio ground (accessed through the shields of the audio leads) from the mains and chassis ground by removing a link (often internal). Some equipment connects the two grounds with a low value resistor to permit use either way. Other equipment includes ground compensation circuitry. In these cases, it is usually possible to use metal racks and the mains grounds without ground loop problems.
Some mains-operated equipment is "double insulated" and does not require a mains ground connection. This type of equipment can usually be identified by the use of 2-pin mains plugs. Care should be taken when mounting these in metal racks to ensure that their metal chassis do not come into contact with the rack, possibly via a mounting screw.
In complex installations, it is difficult to avoid loops between equipment caused by cross-connection of signals. For example, the shields of time-code cables linking a multitrack to a computer and a video machine to a computer may form a loop with the shields of cables connecting the multitrack and video machine to the console. The only way to avoid this is to install an isolating transformer or balancing amplifier to break the loop.
We have seen how the effects of electromagnetic interference and common path interference dictate that we must only connect each item of equipment to ground via a single path. This path is usually the third wire mains ground connection or ground wire direct to the central ground point. If we then connect two pieces of equipment together with a standard audio cable, the shield creates a ground loop!
To overcome this problem, we can disconnect one end of the shield. However, this then conflicts with the requirements for reducing electrostatic interference stated in Section 2.5, Electrostatic Interference Summary. Since ground loop based problems tends to produce low frequency interference, whilst small holes in electrostatic shielding tends to produce high frequency interference, we can compromise by connecting a small monolithic ceramic capacitor between 0.01µF and 0.1µF across the break as follows:
This technique can also be used on balanced wiring between balanced outputs and unbalanced inputs or unbalanced outputs and balanced inputs. This technique cannot be used on unbalanced wiring, since the shield is carrying the audio signal.
|© 2005 Colin Abrahams, Studio Connections, Sydney, Australia|