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The following sections will try to give you a more complete idea of the different ways designers isolate the RCA shields from the chassis ground. Remember that this is necessary because, even though the shields of the radio's RCA connectors are connected to the chassis (through the ground connection for the radio), the voltage drop through the resistance of the vehicle's chassis would create a ground loop and introduce noise into the signal path if the input of the next device were referenced to it's physical mounting point.
Isolated Power Supply:
If the power supply shown below was designed to produce ±35 volts DC and the secondary ground were connected to chassis ground (which is common in many amplifiers), the output voltage would read ±35 volts if the chassis ground is the reference for the volt meter (the black lead on chassis ground). If the secondary ground were connected to +12 volts (instead of chassis ground), the output voltage would read +47 and -23 volts if the black meter lead is again on chassis ground. The reason that I've shown the secondary ground at different voltages is twofold.
In the following demo, you can change the voltage applied to the isolated secondary ground and see how the voltage changes in various parts of the circuit and with different points of reference (the references are either chassis or secondary ground in this case).
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To adjust the voltage of the variable power supply below, click on the yellow slider and then move the mouse up or down. Click the slider again to lock its position. The top terminal of the power supply changes color to represent polarity (referenced to chassis ground). Red is positive. Yellow is negative.
Click to make this applet fill this window.
The diagram below shows the signal output configuration of a head unit with unbalanced outputs. The signal is on the center terminal of the RCA cable and the shield is connected to ground.
This diagram shows a balanced output signal. The center conductor has one signal and the shield conductor has an inverted version of the center conductor's signal. This type of output is capable of producing twice the effective output voltage of the unbalanced configuration. The head units that say they can either produce 4 volts unbalanced or 8 volts balanced have these 2 types of outputs (shown here and above). They can be selected by either a switch or they may have different sets of RCAs for the 2 types of outputs.