Star ground?

[mossen]

Hello everyone.

Hope this is the right forum to ask this question. I'm new to amps, and relatively new to electronics in general. I built a chip amp based on an LM1875. But it was made from a schematic and instructions that someone else put together. Although I got it working, there was one question that never got answered.

Author kept referring to a "star ground": Each component needs it own path to ground. This is confusing to me, because isn't ground just a common point? How could each component possibly not have its own path to ground, yet still be grounded? Also why is such a design useful?

Thanks for any advice!
mossen

[johnr66]

In the real world the conductive traces have some resistance. If the grounding scheme was poor, say the output was returned to a point on the ground half way between the power ground and input ground, the current in the output circuit will make a small voltage drop across the ground trace which will be picked up by the input. This can make the amp unstable and oscillate or cause distortion. In my experience, grounding would have to be really bad with thin traces to cause a problem.

[thaumaturge]

Star ground refers to one single physical point in your amp. Such as a large circuit pad area, a metal tophat with a bunch of solder holes in it, or just a short length of heavy guage wire. The purpose of them is for all circuit components to have the same reference electrically as well as virtually eliminating any ground loops. To make a simple star ground, just bend a piece of #12 bare copper houshold wiring into an arc and mount it in a central location in your chassis. Then run all grounds to that point.
Doc

[drowranger]

Originally Posted by mossen

Hello everyone.

Hope this is the right forum to ask this question. I'm new to amps, and relatively new to electronics in general. I built a chip amp based on an LM1875. But it was made from a schematic and instructions that someone else put together. Although I got it working, there was one question that never got answered.

Author kept referring to a "star ground": Each component needs it own path to ground. This is confusing to me, because isn't ground just a common point? How could each component possibly not have its own path to ground, yet still be grounded? Also why is such a design useful?

Thanks for any advice!
mossen

it means that not all components in a circuit connected to ground can be joined together in one track.it should have a dedicated tracks and intersects or meet at one point.and this point should have an execellent contact in your chassis.if you cannot avoid wires as connection between your pcb and your chassis make it as short as possible or close to your board.and if you are designing your own pcb do not flood it with gnd.it is same as connecting your components in a single track.

[benb]

Originally Posted by johnr66

In the real world the conductive traces have some resistance.

To expand on this, every conductor, whether a trace on a PCB, a wire between two points or a lead coming out of a resistor/capacitor/coil/semiconductor/other device, has both resistance and inductance (in series), and there is also capacitance between it and every other conductor. In many or most cases the values of these are small enough to be ignored, but sometimes they're not. A power amplifier has several places where high currents travel, and the non-zero resistance to "ground" cannot be ignored.

Here's the grounding article, "Audio Component Grounding and Interconnection"
Audio Component Grounding and Interconnection
There's even more to read in the references at the end of the article.

[marce]

What you are better doing is creating seperate grounds for low level signal, high power ground returns etc and connecting these at a star point, you will get much better RF immunity amonst other benefits. The idea of the return path (or GND) is to create as near as possible and equipotential reference. Also dont forget as the frequency of the signal increases the return path (always path if least impedance) start to move from path of least resistance (DC) to path of least inductance.
Current and electrons do not travel in a loop as such, as often illustrated by the ligh bulb and switch, but return current starts flowing at the same time as the driving voltage, the loop being formed by the h and e fields coupling.

[marce]

To illustrate the above on return currents:
Printed Circuit Design & Fab Magazine Online

[magdrop]

Originally Posted by mossen

Author kept referring to a "star ground": Each component needs it own path to ground. This is confusing to me, because isn't ground just a common point? How could each component possibly not have its own path to ground, yet still be grounded? Also why is such a design useful?

mossen

The alternative to using a "star ground" would be to "daisy-chain" all of your grounds. When the grounds are daisy-chained together, the different return currents (from the individual components in the chain) can interact with one another because of the individual voltage drops across the conductor. That's why separate conductors arranged in a star pattern are recommended. In actual practice, some ground return signals can be daisy-chained without much impact on the circuit (such as bypass capacitors, for example) but it's always best to use separate ground traces if you're not sure.

[benb]

Here's another resource, the (in)famous LM3886 datasheet. The relevant descriptions are on pages 19 and 20:
http://www.national.com/ds/LM/LM3886.pdf
It uses the phrase single-point ground system, which is just a fancy name for a star ground.

[zepple]

The LM4780 datasheet has a good example schematic and PCB layout of a single point ground. All of the signal grounds (minus output grounds) are connected in a star pattern on the PCB, which then connects to the power ground through a 2.7 Ohm resistor to limit large power return currents from affecting the signal ground.