Whoooboy! This is where things begin to get complicated.
I suggest you take a basic electronics course, and learn the formulas that explain these relationships.
In your typical home amplifier system AC and DC circuits coexist together.
If things are not hooked up correctly, dissimilar ground loops can form, corrupting the audio signals, etc.
Electrons are always trying to find an equalibrium.
A charge can build to many volts, then eventually discharges to zero volts.
It always does this through a
load to a "ground".
In our electrical world, there are three basic types of loads:
Resistive load (think: a heater or an incandescent light bulb)
Inductive load (think: motor or transformer)
Capacitive load (think capacitor or battery)
That being said, none of the above loads are considered to be "pure" in their respective forms.
For example, a resistor can have some inductive characteristics that can cause issues in some circuits.
Capacitors can have some resistance which can cause undesirable characteristics in an audio circuit.
Remember this: Every type of load uses up electricity as voltage passes through it.
This can be manifested by the work being done by the device. Examples of work would be a heater puts out heat
through it's resistive winding. A light bulb puts out light and heat through it's resistive element.
A motor creates torque and turns a shaft through the inductive force of it's stator/armature.
If you were to measure the electricity going into a light bulb, it might be 120 volts. When measuring the
electricty leaving the bulb after the work has been done you might only see one or two volts
returning to the ground path.
So in a properly designed circuit, the voltage returning to ground is minimal and is easily absorbed and/or depleted.
All electronic circuits can be distilled into three basic stages.
(although they may have many sub-circuits / building blocks that also adhere to the stages below)
They are:
1)
Source / Input (examples: supply (wall) voltage, radio/audio signal, power supply, computer data)
2)
Load (examples: resistive, inductive, capacitive, amplifier, cpu/memory)
3)
Output (examples: ground, current sink, headphones/speakers, modified computer data)
Ok I've gone a little off the track, but wanted to present some important relationships.
Getting back to the ground thing, the depleted voltage from the
load is seeking to return to it's source.
In the case of your Tread power supply, that would be the ground at the bridge rectifier, or possibly
a grounded center tapped secondary winding of the supply transformer (not shown in the drawing file).
Here are some links below that may help with understanding it.
I've been long winded enough here...
Links:
http://en.wikipedia.org/wiki/Ground_(electricity)
http://en.wikipedia.org/wiki/Ground_loop_(electricity)
http://amasci.com/amateur/whygnd.html
Reason for edit: to repair truncated links.
Hmmm didnt work. After clicking on the above wiki links, when in wiki, click on the item after the red highlighted phrase "Did you mean:"