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Solving Ground Loop Problems - Star Grounding

 

What is Ground

Ground means a common reference potential, usually determined to be "zero volts". Ground is chosen as the common return path from the power supply to all circuits in a device. Most electrical and electronic devices have also common ground connected to the cabinet/chassis. There is also a "Safety Ground", which is usually done with a three prong power plug. The center pin of that plug, is connected through the wall-outlet with the main power ground/earth.
A green wire (green/yellow in Europe) is used for safety ground connection with the chassis. That means, the chassis (cabinet) is kept at ground potential at all times. Some consumer devices are required by code to have a safety ground.

Power Line Hum

Power line hum is a 60/120Hz hum coming from the speakers. It can have several causes. From poorly designed power supply to bad amplifier layout. The first cause can be fixed by sufficient filtering and proper layout. The second cause is more diverse. Most common source is magnetic coupling between power transformer, output transformer, input tubes and circuits. Another cause could stem from eddy currents in the chassis. An eddy (circular) current is a local electric current induced in metal by the AC magnetic field. Steel, is a ferromagnetic material but a poor conductor. If the chassis is made out of steel and is used as the main ground conductor, eddy currents mix with ground return (signal) currents. This results in hum and noise. The solution here is star grounding (see below) and proper transformer (shielding) placement.

What is a Ground Loop?

Ground Loop can be described as unwanted signal pickup from shared-ground connections. In audio equipment it is evident as hum or other interference in the speakers. Ground Loop is best explained in Figure 1. below.


Fig.1

The microphone and the loudspeaker share a common line between node N1 and N2. A small signal voltage is generated due the loudspeaker relatively high current and some resistance in the line (wire). The voltage equals i * Rl. The sensitive microphone input will pick-up this unwanted signal and amplify it further in the amplifier stages. In the worst case, there could be a positive feedback, resulting in spontaneous oscillations. Damage to speakers and/or amplifier may occur. At best, feedback signal will modify the overall frequency response of the audio equipment.
Fig.2 shows a correct common ground wiring of the same device.


Fig.2

Solving Ground Loop Problems

In audio equipment, it is strongly recommended to have only one connection to the chassis, and this should be made close to the power supply. In equipment with a "safety ground", the chassis connection point becomes the main and (only) spot for common ground.
The most effective means of avoiding ground loops is to provide a separate ground return to the power supply for each grounded point in the system, thus STAR GROUNDING.
Star grounding shown in Fig.3


Fig.3

If there are to many points to be grounded, we may compromise and supply only a separate ground connection for particularly long ground leads and/or carry relatively large currents. Other points can be connected to a common busbar. A busbar is a piece of bare copper wire, which runs elevated trough the under-chassis of an audio device and is terminated at the center ground connection. In tube equipment, center terminals of the tube sockets provide a convenient support for it. All low current components to be grounded are connected to the busbar. Utilizing a busbar is shown in Fig.4


Fig.4

One external connection to power line Ground/Earth must be made, where required by code. When two or more units are interconnected, the chassis of each unit should be daisy chain grounded. Connection to power-line ground should not be duplicated. When more than one unit has a three-prong power-plug, the audio patch-cable shield must have only one (side) ground connection. The shield on the other end of the cable must not be grounded.

Grounding and Shielding Highlights

More Reading on this Topic
http://www.equitech.com/articles/rep1.html
http://www.audioholics.com/tweaks/connecting-your-system/ground-loops-eliminating-system-hum-and-buzz/
http://www.dself.dsl.pipex.com/ampins/groundloops/grndloop.htm


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