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.
The balanced input uses both the center conductor and shield for signal input (both connections may have audio). For a device with balanced inputs, the shield grounds between the input of the device and the output of the device are isolated from each other. In this type of system, the output reference ground for the audio is normally connected to the chassis ground of the vehicle through the ground wire of the component. Any noise that would normally be created due to a difference of ground potential between the audio source and this device is cancelled out at the balanced input. If you measure resistance from the input shield to the output shield, you will read a very high resistance (thousands of ohms).
Isolated Power Supply:
This type of system is commonly used in signal processors and involves a switching power supply. As I said earlier the primary and secondary windings of the transformer do not have to have any common electrical connections. Therefore the output of the switching power supply can be a source of power with no electrical connection to the battery or chassis ground. This means that the ground of the switching power supply's output may be connected to the audio source shield which accompanies the input signal and there will be no connection to chassis ground through the ground wire of the device containing the switching power supply. In this type of system the audio reference ground connection will pass straight through the device and into the next device with no connection to chassis ground. If you measure continuity from the input shield of the device to the output shield of the device, you will see that there is a direct connection. The resistance from the input shield to the output shield will be less than 1 ohm.
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.
First, it shows that it's not connected to chassis ground. This means that the audio circuit can't be influenced by the chassis ground. This helps break any ground loops and prevents noise from entering the system. It is fairly common for signal processors to use an isolated secondary configuration. For the signal processors that use the isolated secondary ground, they connect the RCA shields to the center tap of the transformer (the same place the power supply below is connected). Amplifiers generally have the secondary ground connected to chassis ground. Most amplifiers will use some sort of balanced input to avoid ground loops and other noise problems.
The other reason that I show the secondary ground (center tap) at various voltages is because some amplifiers actually have the negative output connected to chassis ground (as with the power supply below set to 35 volts). This means that the supply will produce +35 volts and +70 volts (with reference to chassis ground). This is fairly common in the class D amplifiers that use the Intersil HIP4080 driver chip.
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).
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.
The previous example dealt with balanced inputs. In this section we will discuss balanced outputs.
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.
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