Electronics Suite

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. biggrin

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.

Clockwise from bottom: VHF/UHF digital transceiver, “Super Booster,” low current fuse box, APO3 (x2), 120A relay, fuse block, ground block.

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.

Thumpin’ Away,



13 Responses to Electronics Suite

  1. George says:

    Hi, love your build! you inspired me to do something similar with my GTI. I just finished building my sub box and I’m starting to install everything, but I wanted to make sure that I had the polarity of the Helix subwoofer out-did you wire it with the yellow wires on that ATX connector being + and the black being-?



    • Scott says:

      George – Thanks for visiting! Yes, as luck would have it, the yellow turns out to be the “+” lead while black is “-“. Even if you got them backwards, many amps allow you to reverse the phase that’s output to the speaker. Just be consistent and everything should work well. Good Luck! -Scott


  2. Patrick says:

    Hey scott, excellent write up. I have a 2017 tiguan and am interested in getting the helix sub system to get the increased power and better DSP for the doors, but using the sub outs to power a standard sub mounted in an enclosure like http://www.uberstealthaudio.com/store/p10/VW_Tiguan.html or https://www.ebay.com/itm/MDF-closed-subwoofer-enclosure-for-VW-Tiguan/323154654901?hash=item4b3d84b6b5:g:Sz8AAOSwdBZar3sl&vxp=mtr so that I do not have to give up my spare tire. Do you foresee any issues with this setup?


    • Scott says:

      Those boxes are cool. The UberStealth box looks like the better product… just need to know its internal volume to be sure. I’d have some reservations about using the eBay box: 1) The 15L volume converts to ~0.56 cubic feet. That’s probably enough, but make sure your speaker can work in a small space; 2) Make sure the box has sufficient mounting depth. The eBay box says 84mm, which is just 0.1″ more than my T1S1’s depth. Mechanically, it would fit, but the vent in the magnet needs room to flow air, too. I think 1/4″ is a minimum. Just a thought. Thanks for visiting! -Scott


    • Scott says:

      Almost forgot: Be careful when adding a subwoofer directly to the Helix amp. The specs says it’s 160W at 2-ohms. I’m not sure how they got there with four voicecoils. It can be done with four 8-ohm voicecoils in parallel or four 2-ohm voicecoils wired in a series/parallel fashion, but it’s not clear where the connections happen. There’s eight wires coming from the amp. It’s easy to assume that each pair gets an 8-ohm voicecoil and that the balancing act happens in the amp. But I’m not sure. The question then becomes which subwoofer to buy and how to wire it to eight wires. Would a 2-ohm subwoofer work with all four positive wires attached to the (+) terminal and all four negative wires attached to the (-) terminal? That’s why I chose to convert the Helix subwoofer output to a high-level amp input. It allowed me to regulate the power and impedance matching on my own.


  3. afman917 says:

    Scott – great build, looking to do something similar myself here over the next few weeks.

    If you had to distill this build down to the basics component-wise (i.e. bang for the buck) what would you recommend?


    • Scott says:

      Thanks for writing!

      Going for a subwoofer upgrade only, it depends on whether I’m starting with Fender or the base MIB2. Starting with the base MIB2, I still think the upgrade to Helix is a good start. Let’s assume that Helix and Fender present an equal starting point for a subwoofer. There’s debate about which system is better, but let’s start with either system.

      My barebones upgrade from there would be to take the subwoofer output of either system and feed it into an amplifier that can accept a speaker-level input. My T500X1br is such an amplifier, as is the P500X1BD, which is also a 500-watt amp. I would power it directly from the battery and use the 12V outlet in the trunk as a turn-on wire.

      If you build the box and install everything yourself, you can probably build this system with a T1S1 subwoofer for around $1000 or less. That’s a steep price tag for someone who’s never built a system before, but it would be a really solid system. By way of comparison, my original 300-watt system (a P300x1 amp and a P3SD4-10 subwoofer) could be barebones-built for around $400; however, the P300x1 does not accept high-level input (LC2i or equivalent required). That’s an easier pill to swallow, but would be barely adequate for a system running 50W/channel in the doors. Just my opinion. The 300W system could be an ideal complement to the 35W/ch base MIB2.

      That’s where I’d start. I might find flaws in this plan as I build, but not many. I hope that’s helpful. Thanks again for taking the time to post here. -Scott


      • afman917 says:

        Thanks Scott – I have the Fender sound system, so I’m wondering if the EQL would be necessary or if an LCi2 would suffice? Read another one of your posts where you mentioned the reason behind switching to the EQL was because of an abundance in bass between 50-70 HZ, but that you thought the Helix amp was the culprit. Not sure if the Fender system has a different amp. I’m new to all of this, so learning as I go!


      • Scott says:

        The Helix setup was the culprit that led to my choice to use the EQL. Fender works fine with the LC2i. In fact, you might not even need the LC2i if you use an amp that accepts high-level input. The LC2i’s claim to fame is that it corrects “bass roll-off” at high volumes. I don’t recall having that problem in my Mk6 with Fender, which is the first place I used some of my current setup. Try a setup without it and go from there. Worst case is you’ll need to add one later. Some details of my Mk6 setup are still on my old website, with a info about the connector P/Ns at http://www.stealthtdi.com/Albums/NewStealthElectronics/subboxconnectorparts.html. Good Luck! -Scott


  4. Adrian Saidac says:

    I’ve got the B version of the Helix subwoofer (dealer instead)
    My question is related to the DSP files available for the “dongle” (I’ve purchased one)
    The file 57 (GTI) sounds boomy (roll over frequency too high?) Any suggestions?
    Thank you!


    • Scott says:

      Adrian – I left the Helix unit installed only long enough for a functional test, then went straight to being a mad scientist by replacing the dual 6-inch units with a 10-incher (retained the rest of the Helix system, though). 😉 I read about person who experimented with other files who seemed to like file 97. Give it shot. Worst case is you have to go back to 57. Good Luck! -Scott


  5. Derek says:

    This is awesome! Great detailed writeups. Ive also been one that has never been content with the stock audio systems of any of my vehicles. Just recently purchased a 2016 R, and while the Fender system sounds pretty good, the low end leaves a little to be desired. I’ve been doing some research about under-floor stealth boxes already, but this is the best write-up, and best designed and looking system I’ve seen to-date. Definitely saving your post for my own future reference.


    • Scott says:

      Derek – Thanks for stopping by! And thanks for your comments. I didn’t include details about converting a Fender system since I don’t have one in my GTI. However, I did a conversion in my previous car with Fender that you might find useful. I wanted to mimic the factory subwoofer connector so that I could connect the factory subwoofer wiring to an amplifier without cutting anything. To create a factory-like fit/finish, I purchased the Fender subwoofer connector housing, VW P/N: 8E0-972-714, and the appropriate pre-crimped wires, VW P/N: 000-979-134-EA (x2). This allowed me to mimic the subwoofer connector and plug directly into the car’s harness. I’m not sure how photos work in this comments section, but I have some hosted at https://stealthgti.files.wordpress.com/2018/08/subboxconnectorparts.jpg and https://stealthgti.files.wordpress.com/2018/08/factorysubwooferharness.jpg. I may update my post to assist Fender owners. Good Luck! -Scott


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