Using aftermarket HU preamp outs with Bose Amplifier - Mercedes Benz SLK Forum

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#1 Old 07-23-2015
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Using aftermarket HU preamp outs with Bose Amplifier

Hi everyone,

A couple of weeks ago I got a replacement head unit for my car so I could connect my iPhone. I ordered from Crutchfield, I highly recommend them because they do provide all (well typically) you need for an installation.

After I installed the new head unit, I had noise in the system when the key was on. It did not matter if the car was running or not so it was not alternator whine. Crutchfield at first suggested a ground loop isolator but I also was not getting the same sound level out of it as I was before so then I tried a line out converter to use the speaker outs. This worked okay but I was not satisfied with using a bandage on the issue.

Finally I found that the Bose Amplifier wants a balanced input while the head units puts out unbalanced outputs. Check this link to see what I am talking about Balanced Line Drivers
Using unbalanced outputs cuts your output in half and using balanced outputs eliminates noise.

Searching online I could not find a reasonable balanced line driver (unbalanced to balanced converter) so I made my own! TI has a chip I used http://www.ti.com/lit/ds/symlink/drv134.pdf

To power it I used a DC/DC converter and a couple of linear voltage regulators.

It was a fun project and it sounds great.

Here is the PCB (not my cleanest work and it is before I cleaned the flux off, but it works).

Front

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Last edited by ichris93; 09-25-2015 at 09:07 AM.
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#2 Old 07-24-2015
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Glad you understand all of this. Pretty foreign to me. A few questions:

About how much did it cost to make the board?

How long did it take?

Where did you mount it?
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#3 Old 07-24-2015
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Originally Posted by El_Toro_33809 View Post
Glad you understand all of this. Pretty foreign to me. A few questions:

About how much did it cost to make the board?

How long did it take?

Where did you mount it?
It may seem esoteric, but if you studied electrical engineering it would be more familiar.

I would say the cost is right around $20. The most expensive component being the DC/DC converter (about $5) to get the negative voltage needed.

From design to PCB I would say it took about 3 days. I had trouble making the PCB because I had not used the toner transfer method in a long time and there were a few design issues.

To produce more PCBs, it would not take as much time and I could have it made professionally but that would increase the cost.

It is mounted directly behind the radio. It is best to be mounted as close to the radio as possible to eliminate noise most effectively. A 3D printed enclosure can be made to insulate it from shorting somewhere.
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#4 Old 09-20-2015
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Great project. Exciting to see more enhancements / hacks / and mods showing up here on SLKWorld.
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#5 Old 09-20-2015
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Great project. Exciting to see more enhancements / hacks / and mods showing up here on SLKWorld.
Thanks! It was exciting getting this working properly.
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