[msquaredF33]

My 435i came equipped with the H-K (option S688) audio system and it worked well enough for many years until hidden a water leak caused pooling around the amp which caused a short. I tried everything to revive the amp: disassembled, cleaned the board, resoldered a couple joints that looked corroded, sent it in to a Logic7 specialty repair shop, but ultimately the amp was pronounced dead. Given the insane high cost of replacing this H-K (model 65129366164, approx $1200 new or $800 used) I looked at other options and settled on a home built solution that would fit into my tight space (the convertible is way different from the coupe!) and budget.


Adapter MOST-RCA (purchased from Logic7, this device is a DSP logic board that accepts MOST inputs and breaks this out into the Front/Rear/Sub channels with Left/Right RCA outputs.

Dual XPR84D 4 channels x 80Watts drives front L+R + center (I am blending the L+R signals with a resistor)
Dual XPR84D bridged into 2 channels x 120Watts (probably less since it's an 8 Ohm load) drives the L+R underseat woofers
Dual XPR82D 2 channels x 80Watts drives the rear L+R (my convertible does not have any rear deck speakers)
Dual XPR82D bridged into mono 240Watts drives a JL Audio CP108LG-W3v3 that I added in the trunk

My total spend was about $900. This included all the power wiring (2 gauge to from battery terminal to distribution block, 8 gauge from there to amps).

Now before you ask about my choice of amplifiers, please watch this
https://www.youtube.com/watch?v=kFBW9BLZ7RM

Here are pictures of my build. This first pic shows 3 of the amps and the MOST adapter installed into the sidewall where the factory amp was placed.


Next shot is same location slightly different perspective


Again the same area. This amp is powered from the existing 40 amp fused circuit that powered the H-K amp section: you can see the yellow coupling that splice into the factory power leads. But wait you say, this amp has 2*25 amp fuses so how can you provide only 40 amps? Well I am only using 3 of the 4 channels of this amp: the front "center" channel is what makes this an odd number. So I could never draw 50 amps even if I wanted to.


And a final shot of this same area.


Here is my 4th amp which did not fit into the sidewell


Closeup of the MOST adapter. I powered this from the existing 5 amp fused circuit that was powering the factory H-K logic board. The green pair of lines is the factory fiber optic MOST loop. I am splitting the REAR channel and sending that to the underseat woofers. The MOST sub output channel is bridged into my subwoofer amp.


And my J-L audio subwoofer.

[JimmyDee]

Well done. I like the multiple stealth amps. How does it sound?

Here's what I did:
https://f30.bimmerpost.com/forums/sh....php?t=1506949

[msquaredF33]

I like your build. Obviously your budget was several thousand $$ more than mine. My sound is very much improved, I believe due to these 2 factors:
1) Adding a REAL subwoofer.. the stock underseat speakers are a woofer not a subwoofer and cannot hit low notes. This subwoofer was a major sound upgrade.

2) Removal of all the Logic7 digital processing which (for me) adds nothing good: sorry but you cannot extract 7 channels of audio from a source that only has 2 channels (left+right)!!!!

The amps alone would not sound much different than the stock H-K amp. Although Dual is not regarded as a "good" brand, I must say these sound at least as good as the H-K. I am rather picky about sound quality: at home I have a 2 channel music system (separate from my home theater) equipped with Macintosh amplifier and Bowers&Wilkins speakers.

[Murjaani]

Originally Posted by msquaredF33

And my J-L audio subwoofer.

Does the J-L sub fit in the space between backseat and load-through hatch? Or do you need to leave hatch open? If it does, what is the sub type?

[msquaredF33]

Originally Posted by Murjaani

Does the J-L sub fit in the space between backseat and load-through hatch? Or do you need to leave hatch open? If it does, what is the sub type?

No, the sub does not fit inside this "in between" space. I leave the seat upright in place, but the plastic barrier (on the trunk side) I leave down. So the sub lies down inside the trunk, and is (slightly) muffled by the back of the rear seat. But trust me you can still FEEL the sub!!

[Murjaani]

Ok. Thanks.

[335JTSD]

Which MOST adapter is it and where can we get one? Different than a Mobridge or DME? Thanks!

[mdchris]

Nice work, looks like it took a lot of planning

Seeing as you have it all opened up, any chance you could answer my question on the wire gauge for the outputs to the underseat woofers?

https://f30.bimmerpost.com/forums/sh....php?t=1576407

[msquaredF33]

Originally Posted by 335JTSD

Which MOST adapter is it and where can we get one? Different than a Mobridge or DME? Thanks!

The MOST adapter is from some Russian company and I can’t even spell the name, let alone pronounce it!! It is not the typical Mobridge. It cost much less than any other option I could find. I’m quite happy with it. Purchased from
http://www.bmwlogicseven.com

[335JTSD]

Originally Posted by msquaredF33

The MOST adapter is from some Russian company and I can’t even spell the name, let alone pronounce it!! It is not the typical Mobridge. It cost much less than any other option I could find. I’m quite happy with it. Purchased from
http://www.bmwlogicseven.com

Thank you and I'll take a closer look. Didn't see which one right off hand.

[pho604]

Great job on the build and finding the best bang for your buck. Makes me want to buy one of those amps to play around with. I had the same water leak issue in my Audi. I blame that stupid rubber vent flap.

I was about to upgrade my wife's F30 with an audison bit dmi until I saw your post. The thing with the mobridge and bit dmi optical system is that optical output is stereo without front/rear fade.

Do you know if that Russian MOST-RCA 3.0 adapter support F/R fade via the headunit?

Thanks.

[Encanto]

Originally Posted by pho604

Great job on the build and finding the best bang for your buck. Makes me want to buy one of those amps to play around with. I had the same water leak issue in my Audi. I blame that stupid rubber vent flap.

I was about to upgrade my wife's F30 with an audison bit dmi until I saw your post. The thing with the mobridge and bit dmi optical system is that optical output is stereo without front/rear fade.

Do you know if that Russian MOST-RCA 3.0 adapter support F/R fade via the headunit?

Thanks.

MOST is Stereo -no fade as there are no rear channels in Stereo.

[mdchris]

Do you have any pics of the trunk once everything is all finished and liners back in?

[335JTSD]

My local stereo shop says I don't need a Most converter for my hifi 688 HK and that they can just wire into a Jl Audio VXI 800/8. Would this be a mistake to allow them to do that? I'm afraid of them hacking up my wires. I own the car.

[Encanto]

Originally Posted by 335JTSD

My local stereo shop says I don't need a Most converter for my hifi 688 HK and that they can just wire into a Jl Audio VXI 800/8. Would this be a mistake to allow them to do that? I'm afraid of them hacking up my wires. I own the car.

The VX800/8i amp accepts up to 16VRMS/23Vp at its inputs. The 688 amp woofer channels output 32-35Vp. If this shop cannot comprehend the problem with this hack then this is not a shop that you should trust with your car.

This thread shows how this is done. Anything else is a hack.

[cfm56d7b]

I am not an audio engineer but this project is just impressive.

[335JTSD]

Originally Posted by Encanto

The VX800/8i amp accepts up to 16VRMS/23Vp at its inputs. The 688 amp woofer channels output 32-35Vp. If this shop cannot comprehend the problem with this hack then this is not a shop that you should trust with your car.

This thread shows how this is done. Anything else is a hack.

Wow thank you, I won't be going there!