As a "network"? Not sure what you mean by that...

Ground is ground and hence there is continuity between all ground points. Sometimes the ground taps on transformers are isolated but the potential should be the same.

In many cases in arcade systems, the negative wire of the speaker is DC ground.

Take a look at the diagram of a typical audio amplifier. For current to flow there must be a "path", and usually that is from positive to ground. No different with speakers.

non-inverting-op-amp-speaker-driver.png

Ground means different things to different people.

In electronics, ground is a common point on the power supply that measurements are taken from. For instance, in a typical computer power supply, you have 3 different voltages. The negative side of the meter is attached to ground, +5 volts is measured on one supply, -5 volts is measured on another, and +12 volts is measured from the third, IIRC. It is a base point for all measurements, whether DC supplies, or AC signals like in Wayout's schematic.

To an electrician, ground (aka "earth" as it is know to our friends across the pond) is the third wire in the power outlet. It is tied to the chassis of the machine in question, and is intended to drain away any harmful voltage should raw power come in contact with the chassis. The hope is to trip the circuit breaker and avoid shocking the purple hell out of whoever comes in contact with it.

As far as I know, there is nothing that says that the power supply ground has to be tied to the chassis ground, but I think most times it is. And I've seen systems where signal ground was kept seperate from power ground, but I doubt that would ever apply here.

On a side note, do not assume one side of the speaker is always tied to ground. I don't know if they have any bridged amplifiers (LONG story) in pinball machines, but neither side of the speaker is ground. To ground one side would let out the magic smoke.

HTH's

As far as a pinball machine goes you will usually just be dealing with "earth ground" as a common point for all current to flow and as a pioint to measure voltage potential. Note that you can have a portable device such as a cell phone that has no physical connection to the earth, but will still have a common point of reference called ground in the device.

Grounding in electronics is often much more than just audio ground or earth "personnel safety" ground. There's ways it is to be utilized for noise immunity, there are different schemes of design for the types of signals it is being used with (such as low or high frequency). Complex devices may need other types of arrangements as well. Improper grounding can introduce noise or alternate ground paths a.k.a. "ground loops"

In some of our commercial audio gear we use a multipoint ground, or a star topology ground to solve certain problems.

http://www.celectronics.com/seminar/sample/IEEE11-9-05.pdf

And when you work around tube video monitors, don't ever ground your scope probe on the logic chassis.

Big ba da boom.