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Line output converters (LOCs) are used to convert a head unit's speaker outputs or the outputs of an OEM amplifier into a signal suitable to drive the preamp inputs of an amplifier. Most line output converters (LOCs) used in the past few years are designed for high output head units.
The speaker output of a high powered head unit is designed to operate in a configuration. The speaker wires from the head unit each have half of the DC battery voltage on them when the head unit is on. When the head unit is off, the voltage on the output wires is 0 volts DC. When the head unit is switched on, the DC voltage instantly changes from 0 volts DC to 1/2 of the battery voltage.
A high powered line output converter consists, primarily, of a transformer and often a high power resistor (commonly a10 watt resistor). The resistor is used to load the amplifier in the head unit. Some amplifiers will exhibit less than ideal manners when they are unloaded. Remember that we said that the head unit's outputs had a large quick increase in DC voltage when the head unit is switched on. The transformer will pass the difference in voltage between the two input terminals. Since the DC voltage increases simultaneously on both of the input terminals of the line output converter (and therefore the transformers in the converter), the pulse of DC is canceled out. If the line output converter simply used resistors as a voltage divider (with no transformer), the turn on pop would be severe for most amplifiers.
Another benefit of using transformers in the converter is that it allows the shield ground on the output of the converter to float. This means that it can be grounded to any point in the vehicle and there will be no chance of creating a ground loop between the output shield ground and the head unit's ground.
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Shield ground wires:
The following image shows what the inside of a basic line output converter looks like.
This next line output converter is a bit different than the previous one. This one adds the ability to produce a remote turn-on output. When the converter receives an input with DC bias, it drives 12v to the remote output wire (blue). The B+ and ground wires must be connected for the remote output and the LED power indicator to work. The two potentiometers near the RCA jacks are level controls for the preamp outputs. This converter doesn't have the loading resistors or the shield ground wires that the previous converter had.
This is the label from the converter above.