Adding an amplifier to a preexisting head unit is probably your best bet to pump up the volume and clarity of your car’s sound system. You can install an amp whether your radio is old or new, and best of all: it is a relatively easy process.
Before you begin, check the back of your head unit for pre-amp outs. These are RCA stereo connections that should come in pairs. If you do not have pre-amp capability (no RCA out jacks), then make sure that the amp you choose is able to accept speaker or line-level connections. In addition, make sure that the rated RMS power of the amp is less than or equal to the RMS power of the speakers you plan to connect it to. When you buy the amp, be sure to buy a power connection set and a patch cord to match salesperson should be able to help you with this.
Now that you have the amp, the first step is to choose a location inside your car to mount it. Typically, this means in the trunk, but it is possible to put it in the cabin if you have enough room. Be careful not to choose a location that will expose the amp or wires to direct heat, and be sure that the amp will not be covered by carpet. Your amp will get hot in the course of use, and it is essential that it has enough space for air to circulate over and around it.
Once the amp is secured in a good location, it is time to start wiring. Before you even think about where to run the wires, disconnect the negative terminal of your battery. Speaking of the battery, this would be a good time to take out the power connection kit and connect the long, (in most cases) red wire to the positive terminal of your battery. If you do not see an easy way to do this, it might be a good idea to invest in some inexpensive gold-plated terminals that will allow multiple connections from your positive terminal.
Once the positive wire is connected directly to the battery, run it back toward the cabin of the car. Once again, avoid sources of heat, and try to keep the cable away from places that are likely to be splashed by water from the tires. If the cable already has a fuse on it, make sure the fuse stays in the engine compartment. If the fuse came separately from the main power cable, you are going to have to cut the cable and patch the fuse in using crimps or a soldering gun. Make sure to use the heat-shrink tubing that probably came with the kit to seal off the connection so that water does not get inside.
Now you should be able to run the rest of the cable through the firewall. Make sure to use a rubber grommet if there is any chance the insulated wiring might rub on sharp metal. Once inside the car, run the cable underneath the carpeting and back to wherever you have put your amp.
You can now connect both the long positive cable and the short negative cable to the amp. Find a grounding point within the range of the negative cable. If none is available, you can make one by drilling a hole in the floor panel/chassis (be absolutely certain about what lies beneath) and using a nut and bolt to connect the ground wire directly to bare metal. If you can, scrape paint off of the metal before you connect it. Make sure you have a good ground, as this will directly affect the quality of your sound. At this point, the amp is wired for power. Now it is time for the sound.
If you are using the RCA pre-outs this is as simple as running a patch cable from the back of the head unit to the amp. If you are using a line-level connection, you need to run speaker wire from the outputs on the back of the head unit to the amp. A tip: if you are hooking up your rear speakers and your amp is in the trunk, you can just use the wires that are already there. This is a simple matter of moving them or lengthening them so that they will reach the amp.
Whether you are using a line-level or RCA connection, plan the paths of the sound wires so that they do not run parallel to the power wires. It is best to run power wires on one side of the car, and sound wires on the other. Remember to run the remote amp control (if your head unit or amp supports this) at the same time as the sound wires. Because this wire carries very low current, it shouldn’t be a problem if it is parallel to the sound wires.
Finally, run speaker wire of 16 gauge or less from the speaker outputs on the amp to each speaker you wish to power. Connect the speakers, paying attention to the polarity. Now fire up the stereo and test everything out. Sound alright? If you hear distortion, it is probably caused by interference with the sound wires, so make some adjustments to get the best sound quality.
If you hear your engine through your speakers as you rev it, this is caused by a bad ground. Once everything sounds good, set the gain control on the amp so that all of your speakers sound even and balanced. Enjoy your new amp!
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