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Line Output Converters
Function:
Line output converters are used to convert a head unit's (HU) speaker outputs 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 HUs.
Background:
The speaker output of a high powered HU is designed to operate in a bridged configuration. The output speaker wires each have half of the DC battery voltage on them when the HU is on. When the HU is off, the voltage on the output wires is 0 volts DC. When the HU is switched on, the DC voltage instantly changes from 0 volts DC to 1/2 of the battery voltage.
Construction:
A high powered LOC consists, primarily, of a transformer and a high power (10 to 20 watts) resistor. The resistor is used to load the amplifier in the HU. Some amplifiers will exhibit less than ideal manners when they are unloaded. Remember that we said that the HU's outputs had a large quick increase in DC voltage when the HU 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, the pulse of DC is canceled out. If the LOC simply used resistors as a voltage divider (with no transformer), the turn on pop would be severe.
Note. The transformer also gives you a clean ground which wasn't available from the bridged outputs of the head unit.
Shield ground wires:
There are usually 1 or 2 extra wires on a LOC. These wires are used to ground the shield of the RCA output cables to the HU. Some amplifiers work fine without them, others don't. If the LOC is installed behind the radio, I would recommend connecting the shield ground wires to the case of the radio.
NOTE:
The above diagram is simply a generic circuit. An actual LOC would likely have a few more resistors and a level control on the output. This is simply to give you an idea of the basic structure of an LOC.
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