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Thread: Speaker Interference Following Aftermarket Head Unit Install

  1. #1
    Join Date
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    2001 BMW 530i

    Speaker Interference Following Aftermarket Head Unit Install

    Hello Everyone,

    I recently installed an Alpine CDE-HD149BT into my '01 530i and I now have interference coming from, apparently, all speakers. I used a Metra 40-EU10 antenna adapter and a Metra 70-9003 loom to connect the head unit to the factory harness and antenna.

    I would describe the sound as "quiet" interference, similar to a white noise machine, or a light "hissing" noise. The noise does not change in tone, frequency, or pitch with any engine noise or electrical components. The noise does not remain on when the car is turned off. This head unit was previously installed in my prior vehicle, and I experienced no interference. Aside from the new head unit, all other audio components are stock. All speaker wires are connected with crimped butt splices and all other wires are connected with plastic splice caps.

    Interestingly, the interference noise stops after about 2 seconds when I place the head unit on CD mode! It returns as soon as I switch to any other mode.

    I've attempted a number of the usual fixes, with no luck, including:

    - checking the ground cable for a good splice between the loom and the head unit; it's currently connected with a splice cap

    - unplugging the antenna and antenna adapter, both at the head unit and adapter, no change in noise observed

    - checking the remote on cable for correct connection to the BMW adapter loom, and connecting it with a splice cap

    - connecting the required 12v fuse tap to the head unit with a splice cap

    Any help is greatly appreciated!

  2. #2
    Join Date
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    Iím betting that the amplifier is shut off while a cd is slipped into the slot in the head unit. Hence, a short period of no noise. Iím betting the amp is bad.

  3. #3
    Join Date
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    Are you running the factory amp or an aftermarket amp in addition to the HU? I had the same problem, also with an HD149BT. I ended-up needing to ground it at the same location as the amp, rather than using the ground wire in the factory harness. Now no more hiss or alt noise.

  4. #4
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    Hi guys, thanks for the replies!

    Regarding the CD mode, the noise disappears in CD mode even though no actual CD is placed inside the head unit.

    Regarding the amp, I am running the factory amp with my Alpine head unit. The factory amp worked without any interference with the factory BMW Business head unit right before the Alpine install.

    Twistytee - did you end up running a wire all the way to the trunk? I'm planning to install a dedicated sub amp, and will need to pull up panels to run the RCA cable back into the trunk.

  5. #5
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    Quote Originally Posted by MRX9989 View Post

    Twistytee - did you end up running a wire all the way to the trunk? I'm planning to install a dedicated sub amp, and will need to pull up panels to run the RCA cable back into the trunk.
    Yes. I left my factory wiring intact but ran all new speaker wire and RCAs to the trunk and connected the HD149BT to an Alpine PDXV9 back there. I took a gamble and bought all the components used on eBay. I trouble shot this issue for several weeks and thought I might have gotten a defective HU or amp. First I tried a separate ground at the shifter off the dash. Still had the noise. Then I read something on one of the car audio forums about grounding the HU, Amp and battery all at the same location. So I ran a ground wire (away from the RCAs) back to the battery area and grounded the amp and HU adjacent to where the factory battery ground bolt is and bingo - no more noise. Normally they say keep your ground wire runs as short as possible but this did the trick for me. YMMV but maybe you have a similar ground loop issue.

    I previously had the factory HU and factory amp on the original harness with no interference so maybe it's something specific to the Alpine HU.

  6. #6
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    I highly recommend that you donít ground any audio component at the battery. Thatís a very noisy point of the carís electrical system.

  7. #7
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    Thank you both again for your input! Happy New Year!

    Since I have to run an RCA cable to the trunk anyway, I'm thinking of putting a tap splice on my existing HU ground cable and running a ground cable to the amp ground location. If the interference gets worse I'll just take it off, and there won't really be any lost time for the effort.

    Do you guys have any experience with ground loop isolators? Do you think such a device would help in my situation?

  8. #8
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    Iíd respectfully disagree about not grounding near the battery. Thereís an old adage in the car audio industry about running all your component grounds to a common location and then running a single ground from there to the best ground, which is the car battery negative terminal. The car chassis can do this, but it picks up a lot of noise between the dash area and the battery.

    Mine is grounded not to the terminal itself but to the same body brace that the battery box is grounded to. Works well with no introduced noise.

    I have no experience with ground loop isolators but they are reported to degrade your sound quality.

  9. #9
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    Quote Originally Posted by Twistytee View Post
    I’d respectfully disagree about not grounding near the battery. There’s an old adage in the car audio industry about running all your component grounds to a common location and then running a single ground from there to the best ground, which is the car battery negative terminal. The car chassis can do this, but it picks up a lot of noise between the dash area and the battery.
    Second this. This is a common practice when using star grounds and would apply this way in a car. I also second connecting the head and any amps to the same ground point.

    A few thoughts: It is peculiar that the noise goes away in CD mode. Does this unit do any kind of accessory switching? Have you tried isolating the accessory +V? Have you tested every single other mode the head might have to see if there is noise, including modes you might not normally use like line / Aux? If you have a very uncommon issue like a head unit that is new but slightly out of acceptable noise limits, it could be that an IC inside the head unit is bad and is noisier than intended. When changing modes there are a number of circuit paths that will change just depending on how it is designed.

    Also, Ground Loop Isolators: They are usually 1:1 transformers that allow AC signal to pass, but not DC, which is usually where a lot of noise traverses... over the DC ground loops. That being said most of these transformers are cheap and non-linear. This means they will usually translate signal poorly across them and there will be a noticeable degradation in quality unless you want to get a decently high end Isolator that may cost you over $50 or even much more. In case you are interested (but make sure you check to see what the impedance of the RCA signal is, if you are using RCA connections!) https://www.bhphotovideo.com/c/produ...ansformer.html

    I don't recommend jumping for an Isolator until you can... isolate the problem.
    Last edited by Aeko; 01-06-2018 at 11:35 PM.

  10. #10
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    Hey Everyone,

    Here's an update on my situation. Based on the collective advice above, I ran a 14 gauge ground wire to the trunk, and grounded the head unit, battery, and amp in the same location. I completely disconnected the factory ground wire/location. I also installed a "powered" antenna adapter, which required me to also tap into the 12v power.

    Unfortunately, my situation remains the same. The static-like noise remains the same, no better or worse than before.

    My gut tells me that this is some issue related to using an Alpine HU with the BMW factory amp and wiring. Regrettably, I don't have the time in my personal life to diagnose the issue any deeper right now. Fortunately, the interference is unnoticeable when the vehicle is running, and I was able to install a sub and dedicated amp without any additional interference or sound degradation.

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