500W Subwoofer System

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 first-gen 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, especially at that price! 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 LC2i signal converter. The LC2i accepts up to 400 watts of speaker-level input, restores any bass that might be suppressed by the factory stereo, and then outputs line-level signals to my Rockford Fosgate Power T500X1BR  500-watt monoblock amplifier. I think Helix’s output is optimized for the six-inch subwoofers. So, adjusting the amplifier’s low-pass crossover frequency to block peaky mid-bass helps a lot.

I use a Rockford Fosgate Power T1S1-10 10-inch high performance shallow-mount subwoofer. 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 and with a mounting depth of just three inches. 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%, leaving plenty of room to provide much more bass, if desired. In fact, 500 watts through this subwoofer is enough to produce ridiculous amounts of bass in a tiny car. I’ve shared a photo of the remote control in the album at the bottom of this page.

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. Those with the Fender system can duplicate my efforts using the parts and method I shared here. I have shared more photos of my project below. See a YouTube video about this system here.

I have addressed the physical attributes of the electronics panel, the amateur radio system, and a sophisticated 12V power distribution network on separate pages. All are complex enough to warrant their own pages. Feel free to ask any questions about my rather unusual setup.

Thumpin’ Away,



28 Responses to 500W Subwoofer System

  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.

      • Marko Jerman says:

        Helix DSP amplifier has 4 separate outputs: 4 x 40W RMS @ 2 Ohm. Each 17cm woofer is 2×2 Ohm, so 4×2 Ohm in total.

      • Scott says:

        Thanks for sharing that info. It seemed unlikely to me that the small Helix box has four separate amps for the sub, but I suppose it would run cooler than one large amp.

  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

  6. Matt says:

    Thanks for the great write up. Just bought my first GTI (2016 with fender) and have always had full aftermarket systems from top to bottom but that is becoming near impossible with infotainment displays. The Fender Front stage sounds very good for factory but wished they would have used the same MIdrange and tweeters in the Rear. Planning to add a bigger sub of the low line input in the back but wondered of you have had any luck upgrading the High’s while keeping the factory amp and if so what did you go with. Also debating sub size – have always ran dual 12’s in cars but the GTI is such a small hatch i was thinking 3 8’s , 2 10’s or a single 12.

    • Scott says:

      Matt – Thanks for visiting! I have not changed my door speakers for more vivid highs. Helix addressed some of that. I’m also not convinced I have the hearing range to need it. I think my hearing cuts off around 12-13kHz. Someone I know to hear above 16kHz commented that my soundstage sounds very bright and crisp. So, I’ll take his word for it and not try “fixing” issues that I’ll never hear. 😉 As for subwoofer size, my 10″ with 500W sounds very good. A single 12″ may sound better. I guess it depends on the type of music you like. Regardless, I cannot stuff a 12 into my hidden area without removing the extra electronics. If I was “stereo only,” I’d have plenty of room. I think I’d try a single 12 before dual 10s just because the 12 requires less space. Good luck! -Scott

  7. Alan says:

    Hey i was wondering if you had specs on your subwoofer box dimensions? It practically what i would consider the perfect size and setup.

  8. Simon Poole says:

    Which pins did you tap into to feed LC2i from helix amp connector?
    I want to fit larger sub amp and sub

  9. Chris says:

    Can you elaborate more on the eql. I installed a 12” Rockford sub and 600 rms alpine amp off of helix and an lc2i. There are times when it sounds good others not, I think there should be lower bass notes on some songs, would replacing the lc2i with eql correct this? It’s a sealed box facing hatch. Do you have a picture of what the settings looked like afterwards on eql, granted it would change per system and person, but as a reference? Thanks for your help and this page it helped me keep my helix setup and add some much needed bass!

    • Scott says:

      Chris – I removed my EQL to make room for other equipment. But I was able to get the LC2i tuned and working properly. I don’t recall the specifics, but I essentially increased any bass restoration settings, but then set the amp itself for 80-Hz cutoff frequency to eliminate the “boomy” output from the Helix amp. I’m considering a new setup that will eliminate Helix’s handling of bass altogether. But I’m in no rush. Feel free to contact me via my “Contact” page if you need more help. Take Care!

  10. Jason Labovitz says:

    Scott, do you know the part number for the plastic piece below the ebrake? I want to put my JL Audio bass knob in the same spot but would like to make sure I can order a replacement if I need to!

    • Scott says:

      Thanks for visiting, Jason! The part number is in the description beneath the photo in the album below.

      • Jason Labovitz says:

        Scott, thanks so much for letting me know. I’m sorry I didn’t see it at first, I didn’t realize there was a picture of you literally holding that part. I’ll be installing a JL Audio bass knob in my ’21 and I’ve been trying to find a good spot. When removing that piece from the ebrake, it looks like it only clips in? What is your recommendation on the side to start with to remove that specific piece?

        I really appreciate the documentation and quality content. From one teacher to another thank you for helping us out!

      • Scott says:

        If I recall, the piece comes out best if you pull up the parking brake boot first. Do that from the front edge. Tug on the leather, the rest will follow. Once it’s up, then the part is easier to pluck out from the rear. I think the front has a hook. So, don’t pull from there.

  11. Jason Labovitz says:

    Sounds good! Thanks again!

Leave a Reply