Amplifier/Sub installation on 98 328i
I just completed the installation and integration of amplifiers/subwoofer into my existing Harmon Kardon system on my 98 328i and would like to share my setup experience with all the Bimmer enthusiasts.
Rockford Fosgate 301M Mono amplifier
Two 12” Rockford Fosgate P112S4 subwoofers (configured for 2 ohm)
Kenwood 4 channel amplifier
Amplifier wiring kit (8 gauge)
One 4 way distribution block
Scosche 4 channel Speaker Level Converter
Three 6’ RCA interconnects
Two “Y” interconnects - Single Male to Double female
My project will be handled in 3 phases:
Phase 1: Installation of Amplifiers and Subwoofer
Phase 2: Setup 4 channel amp with existing H/K speakers (to determine if I need to replace H/K speakers).
Phase 3 (if required): Replace speakers
I have included a Kenwood 4 channel amplifier on planning to replace the existing HK amplifier and retaining the existing HK speakers
I’ve decided to retain the Alpine head unit since the combination of radio/cd system work well together. I also enjoy the weather band which I frequently use, the switch-able sound mode, and volume/speed sensor.
Before proceeding, you’ll find it easier to remove the truck liner. It’ll make it a cleaner installation since you can twist tie the cables and tuck the cables nicely behind the liner. You’ll have to remove the rear speakers to remove the liner.
4-Channel Speaker Level Converter
The Scosche speaker level converter has 8 speaker line inputs which connect to the radio wire harness.
I used standard speaker wires to connect between the converter and the BMW wiring harness. Below are the wiring colours to look for. I did not want to cut into the existing BMW wire harness so I tapped directly to the wires instead.
BMW wire harness
Front Right (+) Blue/red
Front Right (-) Brown/orange
Front Left (+) Yellow/red
Front Left (-) Brown/orange
Rear Right (+) Blue/black
Rear Right (-) Brown/orange
Rear Left (+) Yellow/black
Rear Left (-) Brown/orange
Remote turn on White
Having the battery in the truck simplifier the installation where a single wiring kit can be used for both amps. I mounted the amplifiers directly to the rear seat backing. I have the Fosgate amp mounted on the right side and the Kenwood on the left side. You’ll need 2.75” screws to hold the amps since the seat backing isn’t flat. You’ll find that there are plenty of grooves hidden behind which made it difficult for the screws to catch.
I connected the main power line between the battery to the distribution block. From the distribution block I connected one power line to the Rockford amp and another power line to the Kenwood amp.
The ground for the Rockford can be screwed to the body/chassis. I had removed my rear seat and discovered an existing ground screw used for the interior lights. I decided to use that instead. For the Kenwood, there was also another ground screw situated on the left side of the car (again on the backing of the rear seat).
The amplifier kit came with wires for the remote turn on. I basically connected/tapped both amplifiers to the “White” wire on the BMW wire harness.
Connecting between Line Converter to Amplifiers:
I connected one 6’ RCA interconnect between the Front Output (Converter) to the Front Input (Kenwood).
I connected the “Y” interconnects to the Rear Output (converter); one “Y” interconnect to the rear right and one “Y” interconnect to the rear left. The “Y” interconnects allow the sharing of multiple amplifiers from a single source.
I connected the 6’ RCA interconnect from the Rear Left/Right output (converter) to the Rear input on the Kenwood.
I connected the 6’ RCA interconnect from the second Rear Left/Right Output (converter) to the Rear input on the Rockford.
Connectivity Test (Very Important):
Testing connectivity will require spare speakers which you use to test the amplifier channels. I have spare Pioneer speakers sitting in the basement which I was finally able to utilize. I took an RCA interconnect and connected between the Converter (front) and the Kenwood (Front). Next I connected the speakers to the amplifier front channel and listened to the radio. Pay attention to the sound quality. If the sound is a distorted, check the connection. I had to re-tap a couple of time because they were not tapped properly. Once connectivity is tested and music sound great with the fronts, you’ll need to do the same for the rear channels and then finally the subs amplifier. Once all amplifier channels are tested and you’re satisfied with the sound quality, you can proceed to put the liner back to the trunk.
This concludes the amplifiers installation.
The two subwoofers (4 ohms each) are connected in parallel (+ from one speaker to + on the second speaker and - from one speaker to - on the second speaker) for 2 ohms. I used 12 gauge speaker wires throughout. The speakers are enclosed into a ported enclosure which provided amazing lows. I am not a bass-head or teenager who bops down the neighborhood, stereo blaring; waking the neighbors in the middle of the night …… you get the point. Instead I tuned the bass just right for my music taste …. Top 40’s and Retro. Surprising, my wife enjoys the additional lows and has asked me to increase it. We’ll see ...
Before I proceed to tackle Phase 2 – connecting to existing H/K speakers, I will give it a week or so of listening. I’ll need to do a little investigation on x-over for these speakers.
So far, I am satisfied with the sound quality from this project.