Adding subwoofer to basic sound system

[mr-fix]

• This is project is under construction. At this moment this is more of a log than tutorial.
• If you are going to follow this guide, you are doing it at your own risk. I'm fully aware that this installation is not in accordance with principles.
• This is not professional installation. It doesn't have much to do with audio quality, sound tuning etc.

Introduction​

My MK7 is equipped with MIB2 VW Discover Media system (Technisat, FW P0478T), without Dynaudio/Fender. It's using internal amplifier, which is 4-channel and it drives 8 speakers. I want to add subwoofer the cheapest way possible.
I challenged myself to add subwoofer to my MK7 with basic sound system and od it for under ~$65 (under 300PLN).
I believe I finally have all the crucial components collected and will finalize the installation in upcoming days.

If you have MIB2 Discover Pro, most likely your main unit is Harman with internal amp that is 6-channel but is using only 4 of them. With proper parameterization it can drive subwoofer as well as front center speaker for total of 10 speakers over 6 channels. That's without any hardware changes other than installing and wiring additional speakers. This audio setup is called 9VD and was available in Audi A3 8V (Audi Sound System) and Seat Leon 5F (Seat Sound System).

Parts​

• Spare wheel subwoofer from Audi Q3/RSQ3 (8U), P/N: 8U0 035 382 - $42 (199PLN delivered): Allegro.pl archive
  The cheapest VAG OEM sub that I found. Fits under the trunk floor even if floor is lowered.
  Came with part of wiring which was handy, since we are on a budget here.
  There's also Bose version with P/N: 8U0 035 382 A. And there's Audi Q7 sub as well with P/N: 4L0 035 382 A/B. Not sure if it's not wider/taller.
• TPA3116 100W Amplifier - $14.93 (~70PLN delivered): AliExpress link
  It's a dedicated amp for subwoofers, it has adjustable low-pass filter, volume control. This means that I don't need separate high-to-low signal converter and crossover.
• Wiring
  I'm using mostly leftovers from previous projects (including wiring tape, cable sleeve...)
  I need to connect 3 things to the amp: ignition-hot power supply, audio input signal, subwoofer output signal.
• Plastic mounting screw
  Since the sub goes under the trunk floor, I still need to secure it. I'm going to do it, eventually.

Connection​

• While testing the amp, I've used front left speaker signal as my input. I don't want to use rear speaker signal since I have In-Car Communication/Voice Enhancer feature in my car.
• Guiding wire though the vehicle was fairly easy. I removed couple plastic elements and on the right side of the vehicle and strapped additional wire to the OEM harness.

Amplifier placement​

• I want to install it in the glovebox, instead of the plastic card holder just above glovebox light.
• Most likely will make a custom holder and 3D print it.

Other info​

• I've tested other amlifiers but they were not powerful enough to drive the sub at desired volume.
• Some amps and high-to-low signal converters were causing MIB to catch audio output fault due to different circuit resistance.

 

[KapnKrump]

Wanted to comment, as I'm interested in following and eventually doing something similar to mine. I've got the PO504D, so mine's a Delphi instead of Technisat; though I don't imagine there's too many differences there. I picked up a couple Tang Band subs, and have plenty of leftovers from pulling my old Alpine set from my old Impala. But getting into the MK7 is evidently a whole new beast...

Unfortunately, everything I've seen and read so far puts this out of the scope of my understanding for a sub install. Every other car I've added an amp to has been straightforward as heck comparatively, but it seems not this. So any advice or input would be incredibly useful! I'll happily post any updates on my end, but I was looking for guidance before getting too far into it...

 

[B3mt1]

@mr-fix how are you pleased with the result? Intersted to do it on my Golf mk7.
I have done similiar but OEM retrofit on my wife's A3 8V with 8v0 035 382 d
and to be honest was expecting more, it seems like it has a frequency gap around 60-70Hz. Really missing bass control on mib2 like it was on mib1, need to play with audio configuration files and VCP but not enough time in the day to learn about it and to do it.

EDIT: now I see you have used Q3 sub and I saw your video on YouTube which you put 6months ago but it is on Polish
It sounds quite good on video. Does the sub fit in your spare wheel?
What is the power of that audi q3 sub, a3 sub is only 40w ?