I am fitting a new soundsystem in my bmw (e46). There's an amplifier in the boot (mb quart onx 4.80/500). A line out converter converts the high level inputs from the OEM headunit, into the low level inputs required for the amp. And a bluetooth module (GROM BT-3) which acts as a CD changer.

Setup: Bluetooth -> Headunit -> Line Out Converter -> Amplifier -> Speakers

Everything is great when I am using the CD player in the Headunit. No interference what so ever.

But when I use my bluetooth module for playing music, there is so much interference. A constant static sound which increases in volume as I turn up the volume on the headunit, with/without the engine being on.

It's clear my bluetooth module is causing the interference, but how can I get rid it and still use bluetooth?

Note: I had another OEM bluetooth module(hands free) installed in the car, that was also giving me similar interference and I have removed that which helped a bit. I have also tried moving the module from the boot, into the front of the car away from the amp. It made no difference. Using a different device connected to the bluetooth makes no difference either.

  • This sounds like a power line or ground is causing the distortion/interference... Recheck your connections and make sure you have them tight and covered. I like shrink wrap because I can identify colors easy.
    – Dee
    Jan 17, 2016 at 1:46
  • @Dee Looks like this question died out of the gates. I would support you expanding on why a bad ground would cause interference and posting it as an answer - if only to get it off the 'unanswered questions' list. After you see this comment please flag it as obsolete c: Thanks! Jun 22, 2016 at 22:25
  • I think you need to get a proper headunit with built in BT. If noise changes with volume adjustment, it is introduced before the volume potentiometer. That leaves the BT-module or the insides of the HU.
    – Daniel
    Nov 8, 2017 at 15:05

1 Answer 1


There are three possibilities here, and the final solution may be a combination of all three.

  1. The Head unit has a noise problem on the input port.

  2. The input device has poor power filtering/regulation.

  3. The two BT modules have noise issues because the environment is extremely active with 2.4GHz wireless signals.

For option 1, Test the head unit with a wired input from a different source. It isn't clear that the BT unit is causing the noise but that will tell you if there is a problem with the head unit.

For option 2, Alternator caused AC noise on DC lines is really common. Noise caused by "High Performance Wires" which means low shielding for some unknown reason, is another biggie. You can purchase DC filtering Supplies which mostly consist of a transformer and some largish capacitors to try an filter some of it out. you want it as close to the BT module as possible.

For option 3, there is no fix really. You could try putting the BT module someplace low in the car near the front in a corner to block as much environmental RF as possible. It would be coming in the windows. If you have in car wifi, that may not be successful. Make sure your BT module fully supports a very new spec which is better adapted to communicating in noisy environments. Unless you live in the woods getting away from 2.4GHz noise sources is going to be basically impossible.

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