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2016 BMW 550i
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Discussion Starter #1
Hi, I have a 2016 550i with the Harman Kardon (TopHiFi) 688 system. It’s not doing it for me, and I want to build a proper system for the car. The car has too many speakers, and I want to simplify and run an active setup with front tweeters, front mids, underseat woofers, and a single subwoofer in the trunk.

I’ve researched and found that I can’t tap the line level signal to the factory amp because it’s a proprietary signal (if this is incorrect, please let me know). Since the factory amp processes the sound, I figured I need something to recreate the original signal. Was thinking about using the JL Audio Fix-82, taking speaker level inputs from the front tweeters, front mids, and existing underseat woofers.

Here are my main questions -

1. Will the Fix-82 do a good job at recreating the original signal?

2. Will I be able to use my car’s volume control without impacting the recreated signal, or will I have to use an aftermarket volume knob that adjusts volume of the flat signal? I’ve never used the Fix and don’t know if it can dynamically produce a flat signal when the processing from factory amp changes as volume changes.

3. Are there any better options out there to recreate the original signal?

My plan is to use optical cable to feed the flat signal to a JL Audio VX800/8i and use 4 channels for tweeters and mids, and the remaining 4 channels bridged to feed the two underseat woofers. Will use the VX for time alignment and EQ. And then use the digital out on the VX to feed an amp for the sub.

Also plan to replace the front tweeters, mids, underseat woofers, and add a 10” subwoofer.

Does anyone have any additional suggestions? I’m by no means an expert on car audio. Have done a few builds before, but always looking to learn more.

Thanks!
 

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I think you would be happier with a MOST25 pre-amplifier. I use the HELIX SDMI25 with a HELIX DSP in my 2016 X6. I upgraded from HK 688 as well.

mObridge also has a few MOST25 options.

“MOST25 interfaces seamlessly with the controls of the vehicle. Volume, fade, balance, bass, and treble remain completely controllable via the existing vehicle controls, and all factory audio such as parking sensors, navigation, voice, and telephone are integrated to behave exactly as per the original system.”

With a MOST25 device you can connect from the head unit to the MOST25, then from the MOST25 to many DSP's and amps, including the JL Audio VXi line I think.
 

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If you are only going to run the 3-way front stage, plus a subwoofer, then you want this:
And this:
And this:
Then just choose speakers / subwoofer of your preference.
 

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If you decide you want to include the rear door speakers (rear fill), then you will need a different DSP with a few more output channels.
(rear fill is totally subjective - but my signature below shows a system that incorporates rear fill)

You will need this:
And this:
Or this:

Amplifiers and speakers are whatever your preference might be. You'll need minimum of 9 amplifier channels.
  • 6 channels for the active 3-way front stage.
  • 2 channels for the rear fill.
  • 1 subwoofer channel.
 

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As you can see; the common denominator is the SDMI25. This is STEP 1.

The SDMI25 taps into your vehicle's MOST fiber optic system (which is what the factory stereo runs off).
It completely replaces your factory amplifier, and acts as a pre-amplifier with a pure flat digital output signal.
It retains all factory controls (volume, balance, navigation, chimes, bluetooth, park distance tones, etc).
Everything works as it would with your factory system... flawlessly.

From the SDMI25; you run a Toslink fiber optic cable into a DSP.
You must use a DSP with this unit. The DSP can be anything you want / any brand you want.
I suggested a few Helix options above, because that's what I've used (with perfect success).
But if you already own the JL Audio VX800/8i, then the SDMI25 will work fine with that too. Just Toslink straight into it.
 

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2016 BMW 550i
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Discussion Starter #6
@Midway @jimmydee thanks for the responses! Had no idea there were pre-amps available that can work on the MOST bus. This is excellent info and most definitely going this route (no pun intended).

I was going to start off with no rear fill, but I definitely do want a DSP that has the extra channel capacity so I can add it down the line for sound and also for passengers through a preset.

In both of your experiences, what are some of the issues (if any) with the SDMI25? Or has it been a pretty much seamless transition as far as maintaining all factory functions?

And what is the main benefit using the SDMI over the moBridge DA-G2.PRO? Is it just the ability to use any DSP vs. using the built in DSP for the moBridge? I haven’t seen any reviews of the moBridge DSP, but it looks like a robust product with a good UI.

Thanks!
 

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@Midway @jimmydee thanks for the responses! Had no idea there were pre-amps available that can work on the MOST bus. This is excellent info and most definitely going this route (no pun intended).

I was going to start off with no rear fill, but I definitely do want a DSP that has the extra channel capacity so I can add it down the line for sound and also for passengers through a preset.

In both of your experiences, what are some of the issues (if any) with the SDMI25? Or has it been a pretty much seamless transition as far as maintaining all factory functions?

And what is the main benefit using the SDMI over the moBridge DA-G2.PRO? Is it just the ability to use any DSP vs. using the built in DSP for the moBridge? I haven’t seen any reviews of the moBridge DSP, but it looks like a robust product with a good UI.

Thanks!
@jimmydee has used mObridge with success, both as a pre-amp and as a MOST25 with DSP but moved to the same Helix SDMI and Helix Ultra DSP I choose for my X6. I’m sure he can add a lot but be careful, his influence has cost me a lot of money :).

Helix is highly regarded for good reason but mObridge is a viable option. While my vote is Helix, a good tuner is equally important. Do you plan on tuning yourself or having a shop do it for you?
 

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I'll send you a PM.
 

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Here's a link to my build, for reference:
 
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