Upgraded under-seat HK subwoofers with aftermarket amp -- Here comes the BOOM

[DrRoboto]

I just wanted to let everyone know that it is the stock amp that is the issue with the subs under the seat. This weekend I took on the task of upgrading the amplifier that powers those speakers in my 2021 GT1 with the HK system. I can attest that this is a true upgrade to those subs. Is it worth the effort? Well, for most people, not likely. For someone like me that enjoys this kind of stuff, then absolutely. Let’s be clear, no one is going to think that you all of a sudden dropped a 12” 500W sub in your trunk. It does, however, make a clear difference in the low end and with them under the seat you get that extra physical impact. My wife even noted that she can feel it in her chest when the bass starts thumping. I agree, I loved it drving to work and back today. Too much power/volume and you can hear them distort before they really thump, but for something that is approximately a 7” sub, it does really good. So far, I am very happy. I am not even sure I will add a dedicated 12" sub anymore.

My install is not the prettiest, maybe even a little hacky. I was mostly focused on just seeing if it would work. I could absolutely do a better job, but it works and is hidden out of site, so works for me.

Here is the basic setup that I used. First, I uploaded the pinout for the HK system. I assume it is the same for all years, but don’t know for sure. I have heard mention that maybe the wire colors changed. I chose to use both the subwoofer and the rear door speaker (woofers) and combine them together using an AudioControl LC7i 6-Channel Line Output Converter (LOC) (Amazon for $129). This allows you to sum/combine the subwoofer and speaker channels into a single “full range” signal. This ensures that I get full control over the frequency and Low Pass filter on my amplifier.

Second, was tapping the speaker wires for their signal using 14–18 AWG splice connectors (quick wire connects from Walmart).

Third, cutting and splicing in the aftermarket amp to the subwoofers. I used 14 AWG butt splice connectors. One set for the output signals from the stock amp to the LC7i and another set for the aftermarket amp output to the subs. I purchased an MB Quart NA-400.2 400W amplifier (Amazon for $88). Can’t beat that price and it is compact and easy to install (made for UTVs and boats). I originally purchased a Soundstream Stealth Double Shot ST2.500D, but it was taking too long to ship and arrive (Amazon $83), but that amp is about half the size!

I ended up using double-sided tape to mount the LC7i to the air pressure vent in the back corner of the trunk. Not perfect, but high-quality double-sided tape is pretty amazing (right up there with duct tape).

I mounted the MB Quart amp near the rear well. I had to drill two small holes and used some self-tapping screws (Home Depot). I attached it to the interlayer of the body, so no harm done.

Hooked up power, ground, etc., and tuned the LOC and amp. I ended up setting the LP filter to around 100 Hz or so. This was a full Saturday job for all of the little stuff and a trip to Walmart. Didn't help it was about 90 degrees in my garage.

Not the prettiest, but believe it or not, it all fit behind the trunk side panel without any fuss!!!

Hope this helps.

 

[Landshark]

appreciate the effort, but its a lot of effort lol.

given the ease of just adding a powered sub to the Stinger (easy battery-in-the-trunk connections), i just threw in the Kicker truck-style box from my previous car.
after a while, i wanted something better visually (or hidden), and to allow more trunk space. i replaced it with a "spare tire sub". of course, it stuck up higher than the spare tire did, so i had to raise the entire trunk floor using slices of pool noodle as pillars.

looking back, if i would have known the spare tire sub was too tall, i would have just left the Kicker box. oh well, its done now

 

[Volfy]

If i remember correctly, those OEM subs are 1 ohm, so make sure your amp are 1 ohm stable, or else you'll have a different- albeit very brief - boom.

If your sub amp is 2 ohm stable bridged, you can always wire the two OEM subs in series and driven them in mono 1-ch mode.

 

[Thomby]

So do you think the improvement came more from the added power, or raising the crossover to 100hz? I seem to remember a post about how the under seat subs are low passed at something like 50hz with nothing much above 70hz getting through, which in addition to leaving very little of the spectrum, seems particularly low for such small subs.

 

[DrRoboto]

So do you think the improvement came more from the added power, or raising the crossover to 100hz? I seem to remember a post about how the under seat subs are low passed at something like 50hz with nothing much above 70hz getting through, which in addition to leaving very little of the spectrum, seems particularly low for such small subs.

I would say it is both. I merged the sub and rear door signals because I was concerned about the LP filter. I did turn the LPF down with my amp and they still played well. I think they are something like 40Hz, which is way too low for those little subs. The higher LPF and the extra power just make a great combo. Another day under my belt with this setup and I am truly amazed. I am very happy with the setup.

 

[DrRoboto]

If i remember correctly, those OEM subs are 1 ohm, so make sure your amp are 1 ohm stable, or else you'll have a different- albeit very brief - boom.

If your sub amp is 2 ohm stable bridged, you can always wire the two OEM subs in series and driven them in mono 1-ch mode.

I didn't measure them, but I read that they were 2 Ohms. The amp I bought is 2 Ohm stable and is two channel.

 

[DrRoboto]

appreciate the effort, but its a lot of effort lol.

given the ease of just adding a powered sub to the Stinger (easy battery-in-the-trunk connections), i just threw in the Kicker truck-style box from my previous car.
after a while, i wanted something better visually (or hidden), and to allow more trunk space. i replaced it with a "spare tire sub". of course, it stuck up higher than the spare tire did, so i had to raise the entire trunk floor using slices of pool noodle as pillars.

looking back, if i would have known the spare tire sub was too tall, i would have just left the Kicker box. oh well, its done now

I think you can do it with just an amp with speaker level inputs. You might be able to get away with just the rear door signals. I need to test that when I get some time. You could also just use one sub channel and one rear door channel. I think that would work also. It would be a lot less work and you don't need the LC7i.

I am super happy with the final result. The bass hits hard enough and loud enough that I actually had to turn down the bass level on the audio. I have run it at +10 since I bought it. I now turned it down to 6 maybe 7 depending on the song. Again, I 500W+ 12" sub will hit harder and deeper, but this modified stock setup is enough for 95% of people.

It was just a fun project with my teenage son. Likely cheaper or more efficient ways, but it was the exploration of would it work that was fun.

 

[Landshark]

I think you can do it with just an amp with speaker level inputs. You might be able to get away with just the rear door signals. I need to test that when I get some time. You could also just use one sub channel and one rear door channel. I think that would work also. It would be a lot less work and you don't need the LC7i.

I am super happy with the final result. The bass hits hard enough and loud enough that I actually had to turn down the bass level on the audio. I have run it at +10 since I bought it. I now turned it down to 6 maybe 7 depending on the song. Again, I 500W+ 12" sub will hit harder and deeper, but this modified stock setup is enough for 95% of people.

It was just a fun project with my teenage son. Likely cheaper or more efficient ways, but it was the exploration of would it work that was fun.

yeah, no LOC needed ... both powered subs accepted high-level inputs. i didn't even need a remote turn-on for the Kicker because it had auto turn-on. super easy install. i did have to run a switched REM wire for the spare tire sub ...