Posted 20 July 2011 - 10:15 AM
You could have a failing internal amplifier on the oem head unit audio setup - by that, I don't mean a separate amplifier like you were initially looking for, but the basic inbuilt amplifier that drives the standard speakers in your car. As you also mention, could be a dodgy line output converter, so change that to a known working one. Take the capacitor out and te-test.
There's quite a few London based car audio installers and dealers, so you could always go in to ask those guys.
Finally, the line output converter method will 'work' to an extent, but it a very basic method. As you increase the volume using the oem head unit (and it's little in built amplifier) then as you further increase, distortion will quickly be introduced. That distortion will be passed by the LOC to your amplifier and speakers. Doing it this way increases the chances of you damaging either or both the amplifier you use and the subwoofer. The reason this happens is that as you've massively amplified the bass notes, the cabin normal speakers handling the mid/treble will get drowned out as you turn the volume up - i.e a mismatch. If you leave the volume low, then you'll not get the output from the subwoofer, so naturally you'll turn the volume up, and that's when damage is likely to occur.
In the 2005 and onwards BMW's, many high quality setups will use a dedicated signal processor which takes all the speaker outputs of the weedy oem amplifier, cleans the signal, boosts it's voltage and outputs an as pure and acoustically flat signal as possible. You then connect amplifier(s) to drive subs, mids and tweeters for a much better and clear sound with much less or negligible distortion.
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