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Old 03-13-2006, 02:23 PM
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Location: San Francisco Bay area
Join Date: Jan 2005
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Mein Auto: '05 545i
Improving Aux Input Sound Quality & Volume

Some people find that the sound quality (particularly the bass) & volume of the aux input isn't as good as it could be. My experience is if you rip your MP3 files using a high quality encoder at a reasonably high bit rate, and normalize volume (see my post above for details), this isn't a big issue, although the volume is noticeably lower. I recently saw a post on another forum that contained a mod that addresses this issue, and am happy with the results, so I thought I'd provide a DIY.

The aux input wiring harness contains (2) 220nF capacitors and a 300K-ohm resistor, which filter out low frequency and degrade sound output. Apparently, the reasons they are there is to reduce gain coming from headphone jacks (as opposed to the line out), and filter out low frequency. The second reason is the side effect of the first one.

Since I'm using a line out (highly recommended, for reasons in my post above), I decided to remove these components. It took about 45 minutes, and is well worth the effort. The sound quality is improved (not by a huge amount, but every bit helps!) and the volume is higher, which is a benefit when switching from, for example, aux input to CD, where before the volume difference was so great that when switching to CD, the volume was far too loud.

First, you need to remove the glove box door--instructions here:
click here

To access the aux input wiring harness, you need to remove a small (2" x 2") panel, which is located on a horizontal piece of the glove box liner, just below & in back of the aux input (see arrows in pic below--pic is before glove box door is removed):

There is a tab at the front of the panel--insert a screw driver and pry it open, then pull it down and out of the way (there's a cable attached to it--don't know what that's for). Then unscrew the washer that surrounds the aux input jack (needle nose pliers work best) and through the hole that the small panel exposed, pull the aux input through and down.

A few inches away from the aux input jack, you'll see a section of the harness that bulges, which contains the offending items--see pic (the section I removed is 5"):

Cut the wire on either side of the bulge, then remove the friction tape, and splice and solder the 3 wires (color coded--be sure to match). Wrap with electrical tape, test sound with iPod to ensure connections are correct, and reassemble.

Naturally, if you're about to have the aux input installed, it's a lot easier to perform this surgery before the harness is installed!

The sound quality is cleaner, with more bass response, and louder--all of which contribute to cleaner sound.
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2005 545i - sold
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