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Formation of Ground Loops Key for Figure 4-5 through 4-10 Page 4-6 Figure 4-6. Single-Point Grounding 4.3.3 Basic Grounding Techniques We will discuss four basic approaches to handling grounds within audio systems: single point, multiple point, floating, and telescoping shield. Each has specific advantages in different types of systems. Figure 4-6 illustrates the single-point grounding principle. Chassis ground in each individual component is connected to earth; signal ground is carried between components and connected to earth at one central point. This configuration is very effective in eliminating line frequency hum and switching noise, but is most easily implemented in systems (or subsystems) that remain relatively fixed. Single point grounding is very often used in recording studio installations. It is also effective in the wiring of individual equipment racks. It is almost impossible to implement in complex, portable sound reinforcement systems. Multiple point grounding is shown in Figure 4-7. This situation is common in systems that use unbalanced equipment having the chassis connected to signal ground. It has the advantage of being very simple in practice, but it is not very reliable – particularly if the connection configuration of the system is changed frequently. Multiple point grounding systems which include unbalanced equipment are inherently rife with ground loops. Hum and noise problems can appear and Figure 4-7. Multiple-Point Grounding disappear unpredictably as pieces of equipment are inserted or removed. When they appear, problems are very difficult to isolate and fix. Multiple point ground systems that employ balanced circuits with properly designed equipment may present no special noise problems. Figure 4-8. Floating Ground Connections Figure 4-8 shows the floating ground principle. Note that signal ground is completely isolated from earth. This scheme is useful when the earth ground system carries significant noise, but it relies on the equipment input stages to reject interference induced in cable shields. The principle of telescoping shields is illustrated in Figure 4-9. This scheme is very effective in eliminating ground loops. If shields are connected only to earth, unwanted signals that are induced in them can never enter the signal path. Balanced lines and transformers Figure 4-9. Telescoping Shield Connections Page 4-7 Figure 4-10 illustrates a typical audio system in which various grounding techniques are combined. The basic rules that guide the choice of grounding schemes may be sum-marized as: 1) Identity separate subsystems (or equipment environments) that may be contained within an electrostatic shield which drains to earth. Equipment Rack 2) Connect signal ground within each separate sub system to earth at one point only. 3) Provide maximum isolation in connections between subsystems by using transformer coupled floating balanced connections. 4.3.4 Balanced Lines and Ground Lift Switches By using balanced signal lines between two pieces of sound equipment, you can lift (disconnect) the shield at one end (usually at the output) of an audio cable and thus eliminate the most likely path that carries ground loop currents. In a balanced line, the shield does not carry audio signals, but only serves to protect against static and RFI, so you can disconnect the shield at one Mix Console Figure 4-10. Combining Grounding Techniques in a Practical System Page 4-8 CAUTION: Microphone cases typically are connected to the shield of the cable, and the shield is tied to the console chassis via pin 1 of the XLR connector. If there is any electrical potential on any external equipment, such as a guitar amp chassis, then a performer who holds the mic and touches the other equipment may be subject to a lethal electrical shock! This is why you should avoid “ground lift” adaptors on AC power connections if there is any other way to eliminate a ground loop. In those audio devices which anticipate ground loops by providing “ground lift” switches next to XLRs or three-wire phone jacks, the ground lift switch makes and breaks the connection between the connector’s shield and the chassis of the particular device. Ground lift switches are usually found on “direct boxes”, which are used when an electric musical instrument is to be plugged directly into a console whose inputs are not designed to accommodate direct connection of such instruments (a direct box also includes a transformer and/or isolation amplifier, as discussed in Section 4.5). One of the best ways to exclude noise from a microphone input is to use a high-quality, low-impedance microphone and to connect it to the console’s low- impedance, balanced (or “floating”) input. Use high- quality microphone cables fitted with XLR connectors, and keep microphone cables as short as possible. Also, physically separate mic cables from line-level (console output) cables, speaker cables and AC cables. 4.4 Audio Connectors and Cables The signal-carrying...

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