I have tried to get this info too no luck (archive)

but I finally got BMW AG to reply to my emails that there are adjustments which had been factory set and could not be done by an end user without test gear and special equipment. Of course, I believe there are adjustment potentiometers inside the DSP amp for each of the 14 outputs, which may be able to be re-adjusted to balance the frequency spectrum, but if BMW are not willing to share the schematics or adjustment procedures, then it is unlikely we can force them to. Cracking open the DSP amp may prove to be fruitless, if some or all of the levels are software adjustments... In the end, so many owners of the BMW HI-FI Premium sound system have complained about the lack of low-end that you should be completely firm in your opinion that this is the case. I have set the LD button and added low end with the graphic EQ to compensate for this but there is something else missing that we cannot get at from the user interface provided. There is a really interesting web site where an owner measured and anaylsed the DSP amp and system outputs and posted his findings. Reading this only reinforces what our ears already told us, there is no bottom-end in these systems.

The only real solution I have read about is to replace the entire system with something that can be balanced. Some have added a sub to the factory system which preserves the DSP controller and original Radio/Cassette player, check Bavarian Auto for the Infinity Basslink sub kit with direct retrofit wiring. This is a bit costly compared to what you could buy the Infinity unit for elsewhere, but then you have to fabricate your own wiring harness, and Bav Auto will not sell it separately, others have already tried this approach.

Here is a post you may find helpful if you decide to remove the DSP...

Posted by Henry on February 22, 2001 at 22:46:53:

Below is the wiring identification for the three connectors at the DSP amp on the driver's side of the trunk. You will find the amp behind the CD changer. Although I have this diagram, but as some people here, I do not want to alter the car originality. I agree that the low frequency of the Premium sound system is really lackluster. I want to add a sub and an amp but I do not want to cut the rear deck. Probably the best way is to find a way to channel the sub output thru the opening of the four factory 5 1/4" on the rear deck. I still try to design the add-in in my head, no work has been done to my car yet.
Here, the pinouts for the 15 pin connector are:
PIN - CONNECTION
1- Sub 1 +
2- Sub 2 +
3- LH Rear loudspeaker + (Touring LH rear speaker ground)
4- Radio on/antenna signal
5- RH rear loudspeaker + (Touring RH rear +)
6- Sub 4 -
7- Sub 3 -
8- Terminal 30 connects to Fuse 56
9- Sub 1 -
10- Sub 2 -
11- Touring LH rear +
12- Touring RH rear -
13- Sub 4 +
14- Sub 3 +
15- Terminal 31 connects to Ground Point

The pinouts for the 18 pin connector are:
PIN - CONNECTION
1- Not used
2- Low freq. negative (-) signal - Transceiver/charging electronics
3- Not used
4- Not used
5- I-bus Signal Line - Splice X18344
6- Not used
7- Not used
8- RH front speaker - Connects to Radio
9- LH front speaker - Connects to Radio
10-Not used
11 Low freq. voltage (+) signal - Transceiver/charging electronics
12-Not used
13-Telephone mute signal - Transceiver/charging electronics
14-Not used
15-Telephone ON signal - Transceiver/charging electronics
16-Not used
17-RH front speaker + - Connects to Radio
18-LH front speaker + - Connects to Radio

The pinouts for the 26 pin connector are:
PIN - CONNECTION
1- LH front woofer +
2- LH front woofer -
3- Not used
4- RH front mid-range -
5- LH front mid-range +
6- Not used
7- Not used
8- Not used
9- RH front mid-range +
10-LH front mid-range -
11-Right rear mid-range speaker/tweeter +
12-RH front tweeter +
13-RH front woofer +
14-LH front tweeter +
15 LH front tweeter -
16-Left rear mid-range speaker/tweeter -
17-Not used
18-Not used
19-Left rear mid-range speaker/tweeter +
20-Not used
21-Not used
22-Not used
23-Not used
24-Right rear mid-range speaker/tweeter -
25-RH front tweeter -
26 RH front woofer -

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