TravisinFlorida said:
two questions.
1. how can a balanced cable solve a ground loop? (maybe three questions) ok, i know that in an unbalanced cable, the ground is also the shield. in a balanced cable, the ground is isolated and the two hot wires, or hot and cold, however you like to think of it, carry an equal signal of opposite voltages. ..........damn, another question still yet, how exactly does a negative voltage exist? .........ok, back on track. is a ground loop caused by a difference in voltage? what exactly is a ground, technically? a voltage of zero?
ok, i know i sound really ignorant here. i have read, multiple times, the basics of electricity and electronics over the years. i understand electron flow (i think i do anyway), ohm's law, inductance, the basics of electronic components and their functions in a circuit, but I really do not know how both a negative and a positive voltage can exist.
2. is something lost when an extra signal is "created" or "received"? how exactly is an exact opposite signal created?
sorry, that's alot more than 2 questions.
disclaimer: damnit, my name is not Walters!
wow, you're asking a lot here
Voltage measurements are always referenced to some other location in the circuit (i.e. a reference point). Probably most of the time it's to ground. So Ground is assumed to be Zero Voltage reference. Now based on the circuit design, voltages can be above the reference or below it. Remember that voltage measurements are always relative to some other point. If you flip the leads on your VOM, the Voltage reading will read the negative.
Ground loops (GL)...
A GL is when two devices that operate similarly have different ground references and are connected. Here's an example that you can prove...
Power one piece of equipment (cassette deck) directly from your wall socket...
Power another piece of equipment (the amp) using a long extension cable. Now connect these two... 60Hz buzzolla...
The reason is significant 60Hz current is flowing between the two devices because they do not share a common ground but expect too. The cassette deck to amp interconnect is trying to bring the two devices to a common ground point. Not a good thing...

The extension cable acts like a resistor and you have a significant voltage drop in the extension cable which changes the ground reference point for the amp.
How can a balanced design help this...
First, the balanced cable ground lead is connected only at one end in the cable, so no current flows in the ground lead. There's a preference to connect the ground at the Xmitter or Receiver, but I don't recall
Next the balanced receiver only looks at the difference between the two signal carriers and not the difference between the signal and ground... thus the ground connection is taken out of the circuit... voila no ground loop
How is a balanced signal created...
It's common to use two opamps with one inverting and the other not. But the design requires very high component tolerance (i.e. resisitors, caps and layout.) The goal is to create two signals with
exactly the same gain and output impedance. This goal is never totally realized... but a very well designed Xmitter presents a better common-mode signal to the receiver.
What is Common-Mode Rejection...
It's the ability of the receiver to reject signals that are common on both conductors. This is the key... Remember, the receiver develops a signal that is the
difference of the two input signals, so assuming the induced noise signal is the
same on both conductors, (i.e. it is common on both conductors), the receiver will reject it by subtracting the two (not summing). Now, since the Xmitted signal is differential (i.e. one is positive and the other is negative) subtracting these two we end up with two times the signal level.