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Discussion Starter #1 (Edited)
OK, so I spent a glorious hot British summer day doing this install to my R172 2019 SLC43 AMG with Audio 20 and standard speakers.

A week or two after I bought the car I posted about disappointing Audio.

I upgraded my car from a 2006 R171 SLK350 Audio 20 with standard speakers which was perfectly fine and punchy, and now had the bad contrast to my new car leaving my music sounding empty and underwhelming.

WELL NOT ANY MORE!

I bought these:





The "under seat" sub does not fit under the seat of an R172 unless you lift the seat right up and / or it set too far forwards - so following another post on this site (@Katiebear00) I installed in the passenger footwell.

Now, I'm in the UK, so I am right hand drive, left side passenger, just to help the rest of you with the image orientation!

The Focal IBUS 20 subwoofer came with all the necessary cabling to connect it to a power source and an ISO audio harness, so I just required the Mercedes Quadlock to ISO harness that I posted a pic of above.
The harness cost £9.95 and means I didn't need to identify and splice any of the audio wires coming out of the Audio 20 towards the speakers. It fits easily behind the Audio 20 and literally took 3 minutes to install between the car and the Audio 20 including moving the Canbus and other connectors into the new Quadlock. (See the empty rectangular boxers in the Quadlock? On the OEM one going into the Audio 20 there is a black and a white connector block that simply unclips and clicks into the new harness plus a single dual optical connector that moves the same)

This part of the install was easy - I simply removed the centre console as per another post on here and put the ISO harness in between the car harness and the Audio 20 of my car and then plugged the Subwoofer into that.
HOWEVER - I could NOT use the power supply to the Audio 20 to power the sub as the Fuse to Mercedes Audio is tiny (<5A) and the sub requires a 15A+ supply. The power wires that came with the sub (including an inline 15A fuse) connect to the ISO connectors and had to be removed if I was to use them.

I cut the power wires from the Focal Subwoofer harness (and taped up the bare wires) and instead ran these wires to the cigarette socket just 10cm below. Here I used two Quick splices (NB - yellow are needed! I initially used the slightly smaller blue ones and they were not tough enough for the thick wire sheaths of both the cigarette cable and the Subwoofer power cable to be cut by the Quick-Splice reliably)



This is the wiring done.
This takes a couple of hours due to dismantling the console and reassembling.
OK! OK! I did the console disassemble several times because firstly I forgot to plug in the ambient lighting cable I temporarily disconnected and another time I dropped the last but one screw inside and so had to dismantle it all again. I also didn't test the electrics before reassembling and found that I had to use Yellow Quick Splices instead of the light blue medium ones.
When once you have done it once, it is easy peasy, but the key is to make sure that none of the metal clips or the fascia touch and scratch any parts of the dash while you are working. Have lots of plastic bags and paper free to use as padding!
Next post is subwoofer positioning:
 

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Discussion Starter #2
It is simple to peel back the passenger footwell carpet (which is foam and polystyrene backed to a thickness of around 45mm)

You then face a beige metal footplate. Removing three plastic nuts exposes a metal cradle that would usually house the Harmon Kardon amp if you have it and maybe other electronics.
Mine is all empty.



I did contemplate removing this and making a custom frame for the subwoofer but took an easier route and decided that the polystyrene and foam on the back of the carpet had to come out anyway to prevent deadening the subwoofer sound and this would make almost enough space for the subwoofer.

Here is the subwoofer sat on the footplate and note the thick foam/polystyrene attached to the carpet.





Here I am cutting away / peeling off this polystyrene and foam (two triangular pieces)





Now the connecting of the wires to the sub, which is simple - two bare wires for power from the cigarette socket in the dash and a plug containing the Left and Right high-level speaker outputs connected in parallel from the Audio 20 harness

Note in this pic below that I have drilled four 3mm holes for the self tapping screws that will secure the feet of the Sub to the foot plate. I did this in the house with the subwoofer as a template. I moved the sub to the right and positioned it so that the controls are on the left so I can access them from the open passenger door.
It also positions the sub speaker in about the centre of the footwell.

(What I have also done when I drilled these holes is to use a hacksaw to shorten the metal bolt-stud on the silver cradle from picture number 1 in this post so that it does not protrude higher than the black plastic nut securing the top of the footplate as otherwise the stud bolt would stick into the back of the Subwoofer)



Next, I am using most of the black felt/foam from the BACK of the footplate to now go on the front of the footplate to cushion and seat the subwoofer on.



The cut-out at the bottom of the felt/foam is where I left some attached to the back of the footplate to cushion between the silver cradle (Pic 1) and the beige footplate.
 

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Discussion Starter #3
Now to position my sub in it's place, using four screws to secure the sub by its feet to the footplate, compressing the felt/foam in order to do it.
The lead on the left side is the remote control to the sub which acts as an attenuator.
It is not necessary as the panel controls all of the settings but when once you have set up the sub, the remote can be used to dial it down.
This might be helpful if I have a passenger who doesn't want a vigorous foot massage!

Again, note that I have positioned the sub to the driver's side of the footplate with the control panel to the passenger door side. This firstly puts the speaker cone in the centre of the footwell and more importantly gives access to the controls! (On your knees, outside the car, through the passenger door with the carpet easily folded back.
You only need to access the controls to set it up once!





I thought that covering the sub with carpet would deaden it, but the effect is minimal. Two carpets actually, as I have a floor mat!





Note that I have left the remote control in the passenger footwell cargo net. Seems a good place for it and I don't think I will move it. It has a 2m cable so could go just about anywhere you care to run the cable!

Now! I know the concern! The subwoofer is unprotected, it just has carpet over it!
Yes, but honestly, you could stand on the grille over the speaker, it is so tough! By this, I meant that you could place the subwoofer on the floor, speaker grill facing the sky and and literally stand ON IT!
I don't plan to do this but with only the minimum of care and common sense, it will be absolutely fine!

I have not yet put foam or padding behind the footplate to insulate the cabin or around the sub to fill in the gaps that feel a bit spongy around the edges of the carpet, but I will.
 

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Discussion Starter #4
I lost just over an inch of footwell space, that's all!

Here are the two foam enclosed polystyrene pieces I peeled off:



The height of the subwoofer is 71.5mm / 2.8"

The polystyrene is 42mm thick



And for those non metric Statesiders, this is for you!

 

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Discussion Starter #5
So how does it sound?

PHENOMENAL!

It really is night and day.

I don't like thumpy bass, but I do like a bit of punch and for the music to be balanced.
It is not all that noticeable - music just sounds like it ought to.
Then you turn the sub off by dialing the remote all the way down and you get that soul-destroying lacklustre sound again, and realise just what the new sub is adding. It becomes noticeable by it's absence.

I could not be happier.

The R172 Audio 20 with standard speakers, to me, felt hollow; empty; uninvolving;
I had the bass turned up to 6 in the equaliser and the treble down 2 and mids on zero. All to try and make it sound "OK"
Not any more. The Audio 20 equaliser is on zero for Bass, Mids and Treble and with the Sub it sounds fabulous with the already reasonable (but lacking bass) audio bolstered by the missing bottom end with the subwoofer.

My settings on the subwoofer control panel:

Auto Power On: Yes. Button depressed. I am using the ignition switched power from the cigarette lighter but also the sub detects audio coming in on the audio cables and auto-powers up. Mercedes does not have a remote power output from the Audio 20 so that you cant use that as a trigger.

Phase: 180 degrees inverted. Button depressed. This surprised me as I am taking audio straight from the speaker feed so I thought in phase would be correct. However, the bass is definitely punchier and in sync with the phase reversed, but muddy and less intense with 0 degree phase setting on the sub.

Bass boost: Maximum (Use the remote to dial it down if needed) (Use the Gain setting to dial down the maximum output you want from the Subwoofer. Read 5 lines down, and also see the manufacturer's instructions)
Subsonic: All the way down - the Sub is amplifying and playing 20Hz upwards - why lose the low frequencies? An SLC shouldn't rattle apart! :surprise:
Low Pass: Set at about half way / 100Hz. The dial allows from 50-150Hz but normal home theatre subs have a crossover set at 80Hz (Large speakers) -100Hz (Small speakers) so I decided setting it at 100Hz unloads the subwoofer somewhat rather than making it play sound frequencies up to 150Hz that will be deadened by the carpet

The final Control to set:
Gain: After setting the above three, I turned the gain down until the bass was the maximum bass that I wanted from the sub. Keeping Gain to a minimum is important to decrease noise and distortion. We are using high level feeds from the Audio 20 amp!

AFTER this, I plugged in the remote.
The remote control seems to function as an attenuator and will only DECREASE bass from the level you set on the bass boost on the sub itself. It does not turn the volume up higher that you have set on the bass boost dial.

My advice to anyone with an Audio 20 and non Harmon Kardon:

If you have a little DIY knowledge and confidence, JFDI!
 

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Has anyone done anything like this on a R172 with a COMAND Head Unit?

Thanks to @MarkChat and @katiebear00) I've got all of the way from the info in the above thread and others up to but not completing splicing in the speaker wires to the COMAND unit but have now run into a snake's wedding of wires. Is there an aftermarket connector I can use to piggyback into the existing harness or is it literally identifying the right wires? I'd rather splice in behind the head unit rather than trace back from the doors.

Another thread says that Right Side is Orange +ve & Orange/White -ve and Left Side is Blue +ve and Blue (Black) -ve but I cant see these out the back of the COMAND unit. So close.....
 

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Has anyone done anything like this on a R172 with a COMAND Head Unit?

Thanks to @MarkChat and @katiebear00) I've got all of the way from the info in the above thread and others up to but not completing splicing in the speaker wires to the COMAND unit but have now run into a snake's wedding of wires. Is there an aftermarket connector I can use to piggyback into the existing harness or is it literally identifying the right wires? I'd rather splice in behind the head unit rather than trace back from the doors.

Another thread says that Right Side is Orange +ve & Orange/White -ve and Left Side is Blue +ve and Blue (Black) -ve but I cant see these out the back of the COMAND unit. So close.....
I don't know of an easy way to tap the signal coming from the back of the head unit because is uses a fiber optic link connected in a loop arrangement. A simple adapter harness won't work. The good news is that it is super easy to tap the signal coming out of the stock amp in the footwell. No need to trace anything from the doors or touch the door panels. See the wiring diagram I posted here: https://www.slkworld.com/4545128-post24.html I also posted the procedure for accessing the footwell amp in that thread.
 
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