I’ve never been one who’s able to leave “good enough” alone, especially when it comes to electronics. I like deep bass in my music. I outgrew my desire for twin 12-inch subwoofers decades ago, but I still like a deeper sound than what most factory stereo upgrades can give. I’ve been satisfied by various single 10-inch subwoofer systems for over 20 years. So it was a natural pursuit when I decided I wanted to upgrade my GTI. Of course, I “had” to add a ham radio and make the entire installation more complicated than necessary.
I had been mostly pleased with Volkswagen’s MIB II base stereo. It’s no powerhouse, but it’s more than adequate. I had originally planned to simply tap into the rear speaker wires to feed a signal converter, an amplifier, and then a subwoofer. I think I would have been satisfied with such a simple setup. Then VW decided to discontinue its Helix subwoofer accessory, meaning they slashed the pricing by more than 50% to clear their inventory. I had already purchased supplies for adding a subwoofer, so I already knew I wasn’t going to keep the Helix box after the initial installation and testing. But the super-clean wiring harness and 5-channel amp with DSP were too good to pass up. Helix is a great starting point for a system upgrade.
After installing the Helix system, I routed two pairs of the Helix amp’s subwoofer output into an Audio Control EQL graphic equalizer with subsonic filter. The EQL accepts the speaker-level input, shapes the signal according to my settings, and then outputs line-level signals for my Rockford Fosgate Power T500X1BR 500-watt monoblock amplifier. I use a Rockford Fosgate Power T1S1-10 10-inch shallow-mount subwoofer since my goal was to keep my spare tire and still have room for my other electronics. This subwoofer works in an enclosure as small as 0.4 cubic feet. I use a PLC2 Remote Punch Level Control to fine-tune the subwoofer levels from the driver’s seat. It’s mounted just inches from the parking brake handle and is usually set to just 35-50%, leaving plenty of room to provide much more bass, if desired. I’ve shared a photo at the bottom of this page. For the record, I could have done this without the EQL since my amplifier accepts high-level inputs. That’s one layer of complexity that can be eliminated, if desired. Those with the Fender system can duplicate my efforts using the parts and method I shared here.
The box itself is unique, mostly because it’s more than just a box. I wanted everything to be concealed beneath the removable floor in the trunk. “Everything” includes the subwoofer, power distribution and regulation, amplifiers, ham radio, and a pure sine-wave voltage inverter. On top of that, I wanted it to be easily removable in case I ever decide to autocross or track the car. With that in mind, I made a full panel with integrated sub box and quick-connections for the electrical parts. The 3.25″ tall box has ~0.55 cubic feet remaining with the subwoofer mounted. I used Acousta-Stuf polyfill to fake more volume. The 68-lb panel can be lifted to reach the spare tire and whatever is left of the storage area; or it can be removed altogether. Getting the carpet applied evenly was my greatest challenge. It’s not perfect, but I think it turned out well. I will very likely build a replacement box and finish it with truck bed liner.
I ran 12V directly from the battery via 4-gauge wire to a 120-ampere relay on the driver’s side of the panel. The relay is triggered by a voltage controlled switch, the APO3. This device allows battery power to feed the rest of the panel so long as its voltage is above 13V, basically any time the car is running, or for about 30 minutes after shutdown. Once the battery voltage falls below a preset threshold (I can set it for 13.05, 12.7, 12.1, or 11.8 volts) for a preset time (0, 5, 10, or 20 minutes), the 120A relay loses its trigger until the car is restarted and has run for a few seconds. The APO3 assures good battery health and no surprises as a result of a dead battery (been there, done that). From the relay, 12V travels to a fuse block with circuits for the amplifier, a “Super Booster,” an Icom ID-5100A VHF/UHF digital amateur radio transceiver, a Raspberry Pi3 mini-computer w/DV-Mega (not pictured), a 400W pure sine wave voltage inverter, and a second APO3 (which is set to power the EQL and switch the amp only when the car is running). It’s overkill for my application, but it should never act as a weak link in the system.
In short, I replaced the Helix subwoofer with a system that features double the driver size, triple the power, deeper bass response, and retention of the spare tire. Bonus: a 10-inch subwoofer in a hatchback provides even better bass than what I had previously enjoyed in sedans since there’s no “trunk” acting as a sound barrier. I have shared more photos of my project below and I have addressed the amateur radio installation on a separate page. Feel free to ask any questions about my rather unusual setup.