Mobile Electronics Australia

If you already own an aftermarket source unit with auxiliary RCA outputs then installing a 4-channel amplifier is one of the easiest and most rewarding upgrades you can do. Better still, it only takes around an hour to complete.

A single 4-channel amplifier can be used in various ways depending on your goals for the system. By paralleling the front and rear speakers together you can power both pairs from two channels while powering a subwoofer with the other two channels – though this way you lose the ability to fade the levels between the front and rear speakers. A more typical arrangement would be to have two channels powering just the front speakers and the other two channels powering a subwoofer. The source unit’s internal amplifier channels can then be used for the rear speakers thereby retaining the fading capabilities.

Adding an amplifier provides three to four times the power and headroom that the small internal amplifier inside the source unit can provide, increasing SPL and dynamics in the process. The second thing an external amplifier provides is a crossover. Crossovers are very important devices that ensure that each speaker in the system is kept playing within the frequency ranges that they can handle. For subwoofers this requires a low pass filter to only allow bass to be reproduced, while for the main speakers you want to remove low bass below around 100Hz to allow them to handle more power without flapping about madly.

The amplifier I chose for this upgrade was an MTX X-Thunder 704X 4-channel rated at 70WRMS per channel or 210WRMS when two channels were bridged - all at 4ohms. At $369 this amplifier has some of the best crossovers money can buy, being suitable for virtually any situation. The 704X is also quite compact and, in its black and chrome finish, looks pretty darn sweet too. For this installation, however, the owner of this car wanted a stealthier appearance, so the 704X slipped under the driver’s seat.

List of parts needed:

• 4-channel amplifier
• 8-awg red power cable – 5 meters
• 8-awg black power cable – 2 meters
• 8-awg ring terminals
• Blue single strand wire – 3 meters
• RCA cables – 2 pairs
• Speaker wire – 6 meters

List of tools:

• Socket set
• Screw driver set
• Soldering iron / Solder
• Cable ties
• Stanley knife
• Large handle pliers
• Sand paper

Step 1: Plotting the path for the main power cable

Begin by disconnecting the earth connection to the battery, which is essential whenever you are working on the electrical system of your car. Inspect the firewall to locate a grommet through the firewall that will provide enough room for an additional 8-awg power cable to pass through. Plot the shortest possible path between the grommet and the battery’s positive terminal that avoids hot or moving parts and that also allows the cable to be secured using cable ties. If you have worm tube protective covering it’s a good idea to use this over the cable when it runs through the engine bay.

Step 2: Running the main power cable through the firewall

Cut a 10 centimeter long piece of coat hanger wire and tape this tightly to the end of the red 8-awg power cable. If the existing grommet has sufficient space then poke the coat hanger wire through the hole carefully, being aware of obstructions behind the firewall. You may need to slice a small hole in an existing grommet with a Stanley knife, but be careful not to cut any wires. Once the cable is feeding through, use gentle but firm pressure to pull most of the cable through into the cabin leaving enough cable in the engine bay to reach the battery. Do not connect the cable or fuse holder just yet!

Step 3: Preparing the interior

With the main power cable now pulled through inside the car it’s time to prepare the interior for the remainder of the installation. Remove the seat that the amplifier will be mounted under, as well as the scuff panel to the adjacent door and the centre console. You will also be running new wires from the source unit to the amplifier so remove the source unit from its mounting positions to access the wiring behind it.

Step 4: Running the main power cable

The main power cable needs to be run through the car’s interior following the shortest possible route while also staying clear of things like your pedals and their linkages. You also want to find a path that stays as far away from any ECUs in the interior as well as the RCA music cables that will eventually arrive at the amplifier. I clipped the cable into an existing row of wiring clips conveniently provided by the factory for a neat finish, or you could cable tie it neatly along the route. The end of the power cable finished where the amplifier will sit ready for connection.

Step 5: Finding an earth point

Begin by attaching the ring terminal to the end of the black 8-awg power cable. This is done by crimping the terminal over the exposed wires then soldering the frayed ends of the wire to the ring terminal for a secure connection. Many cars will have various 10mm or 12mm bolts around the interior that can be used as earth points, but ideally you want a position that connects directly to the floor pan or sill panel. Do not use the seat mounting bolts! I found a suitable 12mm bolt under the centre console that was suitable and fastened it down over the earth cable ring terminal end. If the contact surface under the ring terminal is painted then use some sand paper to sand back the surface and expose the bare metal of the chassis. Once the bolt is secured you can paint over the connection to avoid any corrosion.

Step 6: Running the RCAs, speaker wires and trigger wire

Bundle the RCA cables, blue trigger wire and one pair of speaker wires and run these down the centre of the car under the centre console and through to the amplifier’s mounting positions. Leave enough length on either end of the cables for easy connection behind the dash and under the seat and cable tie the wires in place so they don’t foul any moving parts like the gear shift mechanism. If there aren’t any holes in the carpet where the amplifier is mounted you can slice a neat hole in the carpet with a Stanley knife.

Step 7: Connect the wires at the source unit

Your aftermarket source will have white, grey, green and purple wiring pairs denoting the front and rear outputs to your speakers. For this installation you will disconnect the white and grey front pairs from the source and connect the wires leading to the front speakers to the pair of new wires that now run down to the amplifier. Tape up the left over white and grey wires attached to the head unit to avoid shorts. Connect the blue trigger wire to the blue wire with the white stripe that is the trigger output from your source unit. Solder all these wires and tape them up to avoid shorts. Connect the RCAs to the back of the source unit and then reconnect the other looms so the system is ready to test. At this stage do not reinstall the source unit.

Step 8: Running the speaker wire for the sub-woofer

The last wire to run is the speaker wire for the subwoofer that will be mounted in the rear of the car. Find a neat path free from obstructions that runs under the rear seat and down to the amplifier’s position, which will probably be along the sill panel under the scuff plate.

Step 9: Connecting the wires to the amplifier

With the amplifier in position you can begin to connect all the wires, beginning with the earth cable first. Now connect the main power cable, trigger wire and RCA cables. Lastly, connect the speaker wires for the front pair of speakers, while the single pair of wires for the subwoofer is likely to be bridged across the two remaining channels of the amplifier. It is common on most amplifiers to use the left positive output and right negative output from the amplifier to derive a bridged connection, while the remaining two terminals are left unused. Check with your amplifier’s owner’s manual to confirm this.

Step 10: Connecting the fuse

Find a suitable and safe mounting position for the fuse holder that is within 20 centimeters of the battery’s positive terminal and mount it there using screws or cable ties. Cut a small length of red 8-awg cable that will reach from one side of the fuse holder to the positive terminal. Crimp and solder a ring terminal like you did on the earth cable, and then connect one side of the fuse holder’s input connection to the other end of the cable. Screw this end into the fuse holder before connecting the ring terminal to the battery’s positive terminal. Insert the end of the main power cable to the other terminal of the fuse holder and screw this is with the fuse attached. If there’s a small arc or pop don’t be alarmed as this is normal when an amplifier is first connected to power and its internal capacitors charge up.

Step 11: Finishing the amplifier installation

With the wiring completed you are ready to test the system and do your initial set up for crossovers and input level controls. You can also check that the system is functioning correctly with fade and balance and that the correct RCAs are connected to the correct inputs on the amplifier. See our ‘System Set Up Story’ for how to conclude the installation. Click Here