Post by DocSpeed on Sept 12, 2005 14:08:41 GMT -5
Head Unit Basics
Power Connections:
Most head units have 2 power input connections that have to be connected to a 12 volt source. The 'battery' or 'memory' wire has to be connected to a constant source of power (has power on it at all times - even when the ignition switch is in the off position). The other power wire (the 'ignition' wire) is connected to a power source that's controlled by the ignition switch and therefore will only have power on it when the ignition switch is in the 'on' or 'acc' position.
Speaker Outputs
Most head units have outputs that are designed for driving 2 to 4 speakers. These outputs provide the audio output power to drive the speakers. The power output from a high power head unit is limited (by battery voltage) to approximately 20 watts per 4 ohm speaker.
Note:
On "high power" units, the internal audio amplifier will be instantly damaged if a speaker output wire is allowed to contact anything except a speaker terminal, even if the volume is not turned up. Be sure that there is no power to the unit when making any connections. This link explains why the outputs will be damaged if they contact ground.
Preamp Outputs
Preamp level outputs are audio outputs that are used to drive an amplifier (or equalizer or crossover...). Preamp level outputs do not have enough output current to allow them to drive speakers directly. They are generally included on higher end equipment. In virtually all cases, they will have RCA type connectors. Some units will have only one set of preamp outputs (would likely be a 'rear' output). Other units have multiple pairs of outputs for front, rear and subwoofer signals.
Remote or Power Antenna Output
There may be one or more switched 12 volt outputs on your head unit. These outputs are usually called remote outputs. They are used to control power amplifiers or power antennas. If your unit has only one remote output, 12 volts will be sent through it when any audio source is being used (tuner, CD, tape, aux...). It will be 'hot' (have 12 volts on it) when the head unit is on. It will have no voltage when the head unit is off.
If the unit has two remotes, one wire (generally a dark blue wire with a white stripe* for aftermarket radios) will be for your power antenna and will only have 12 volts on it when you are using the tuner. It will have no voltage on it when the head unit is switched to CD, tape, aux or is switched off. This is so that your power antenna will go down when it is not needed. The second output (generally a dark blue wire* for aftermarket radios) will be marked amp remote (or amp turn-on) and will have 12 volts switched to it when any source is in use. The diagram below only shows a single remote wire. There is a diagram on the test light page that shows both types of outputs.
Please note that the remote wires for stock (OEM) radios will be of a different color (probably not blue).
*Some radios use blue with a white stripe for the power antenna and others use it for the amp turn on wire. Just keep in mind that if it has 2 dark blue wires and one has a white stripe and the other is solid blue, those wires are for amp/power antenna control. You'll have to refer to the deck's owner's manual for the color code.
Again with the fuse thing...
Most remote outputs are switched through a very small transistor. This means that they are EASILY damaged by excessive current flow. They should be fused close to the head unit with a .5 amp fuse. A 1 amp fuse may not protect the switching transistor, which supplies power to the remote output. Do not try to use the remote output to power fans directly.
Stock/Factory/OEM Head Units:
If you have a stock head unit and don't want to replace it, you'll likely need to use a Line Output Converter (LOC). It will convert the speaker level signal to a preamp level signal. Many times, the LOC is connected to the speaker wires that feed the speakers in the rear deck (parcel shelf) of the vehicle (especially when the LOC is going to be used to drive an amplifier which is going to drive a subwoofer). If the system is going to use multiple aftermarket amplifiers for front/rear and subs, you'll use 2 LOCs and they'll be installed behind the head unit. If your head unit doesn't have a built in amplifier (it would have amplifiers elsewhere in the vehicle), you'll have to connect the LOCs to speaker wires (after tha amplifiers) wherever you can get to them. For vehicles with special 'systems' like the Bose© system, you may need a special adapter. PAC makes both LOCs and system-specific adapters.
Speaker Output Clipping:
If you use an LOC, you will have to set the gains on your amplifier so that the amplifier reaches full power before your head unit's volume control reaches 1/2 volume. Most radios' speaker outputs will start to clip at about 1/2 volume. This is far earlier than the point where the preamp output signal starts to clip on most radios. The only time that I've seen a preamp out start to clip along with the speaker output is when the preamp output signal is derived from the speaker output via a voltage divider network (sort of a cheap internal LOC). This is usually only done on the bottom_of_the_line head units.
If you blow a fuse when installing your head unit, consult your owner's manual for the proper replacement fuse size. By proper size I don't mean physical size. I mean amp rating. If an electronics manufacturer specifies a certain size fuse, they do it for a good reason. They know what the equipment and internal electronic devices can handle. More than half of the head units that I serviced would not have been damaged if the proper fuse would have been used.
TECH TIP:
Speaker Connections
If you are not using your speaker output wires, cut them off so that there are no copper strands sticking out of the insulation AND tape up or otherwise insulate the end of the wire. You may be able to pull on the insulation so that it stretches over the end of the cut off copper wire.
REASON: If the speaker wires on a high powered radio are allowed to touch to any grounded piece of metal, the head unit's internal amplifier will be instantly damaged (even if the volume is not turned up).
RCA Cables
Do not allow any source of power (12 volts) to come in contact with the shield (outside metal part) of the RCA cables.
REASON: The shield is grounded to the case of the head unit. It is the reference for the preamp audio signal. If it comes in contact with a source of power, the ground connection inside of the head unit will be damaged. This may cause lots of problems including blown tweeters, engine noise, lack of bass and many more.
Aftermarket Wiring Harnesses:
When installing an aftermarket head unit, you should not cut the factory plug from the vehicle's factory wiring harness. Instead, you should buy an aftermarket wiring harness that would plug into the factory harness. These harnesses (such as those made by Metra) will have each wire marked with it's function (ignition, battery/constant, speakers...).
You would connect the harness to your radio with butt connectors (or other connectors) and then simply plug the harness into the factory harness. If you want to reinstall your factory head unit at a later date, you'd be able to plug it right back into the factory harness.