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Stock CM5908 HU with LC7i and Alpine MRV-F300

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    Stock CM5908 HU with LC7i and Alpine MRV-F300

    Hi folks,

    Wrapping my head around the stock CM5908 in my 91 318is.

    The car seems to have the premium sound package (speaker pods, external boxes on hat rack) but does not seem to have the stock amp. PO had run a cheap Sony unit with new wiring (thankfully left existing wiring in place). I'm bringing it back to "stock" but with new life.

    I know the CM5908 has high level speaker outputs which makes it special, and a pain in the arse.

    I have to still verify existing wiring. With the CM5908 plugged into existing harness I have sound coming out but it's fairly low. Could be the radio but also could be that the amp isn't there or isn't functional. Need to trace it back (I believe it's a straight run between HU and speaks?). I don't have a wiring harness that goes to the driver's side rear trunk next to the power antenna (which is where I believe the stock amp lives).

    I want to direct the sound to the LC7i (basically an LC6i but with a 3rd channel + accubass) and then to the Alpine amp and to the speakers. I have a small powered sub that I will plug into the 3rd channel via RCA.

    I believe that I need to take only the + from the speaker outs on the CM5908 and run those to the LC7i.

    I have found posts from bmwman91 on building custom RCA cables with proper shielding and I plan to have the LC7i very close to the HU (not sure where yet).

    Can anyone confirm that this is the approach? I believe the Alpine handles high-level inputs (differential) but can't find reference as to if the LC7i does that. I want to clean up the signal and pull out bass too, which is why I want to keep the LC7i (and I have it already too).

    CM5908 --> LC7i --> Alpine MRV-F300 --> 2 channels speaks (front/rear)
    |
    |-----> Powered Sub


    All thoughts appreciated! I'll probably test it all out of the car first but if you have insight I'd welcome it.

    ~Jason

    #2
    If you use the speaker outs, I HIGHLY recommend using the pair, not just the "+" line. The outputs are called bridge tied loads, and while they are not perfectly balanced, using them as pairs will allow a good degree of common mode noise rejection at the LC7i. If you do use only one leg of the outputs, then find some quality shielded RCA cables. If using the pair, find some STRANDED cat5/6 cable. It has 4 tightly twisted pairs and is small which makes it easy to run.

    Honestly, I don't think that the LC7i is needed. The Alpine amp you list has speaker level inputs, and those should work just fine. You can also simply tee-off of a left/right pair where they will tie into the Alpine amp and run those to the powered sub since it looks like that also takes speaker level inputs.

    From what I have read, Audiocontrol's accu-bass thing sounds neat, but most people end up turning it off because it does weird stuff. I am re-doing my sound system next week, tossing a 5 channel MB Quart amp and a 6XS crossover out, all in favor of a LC6.1200 amp which will have Accubass disabled.

    Comment


      #3
      Originally posted by bmwman91 View Post
      If you use the speaker outs, I HIGHLY recommend using the pair, not just the "+" line. The outputs are called bridge tied loads, and while they are not perfectly balanced, using them as pairs will allow a good degree of common mode noise rejection at the LC7i. If you do use only one leg of the outputs, then find some quality shielded RCA cables. If using the pair, find some STRANDED cat5/6 cable. It has 4 tightly twisted pairs and is small which makes it easy to run.

      Honestly, I don't think that the LC7i is needed. The Alpine amp you list has speaker level inputs, and those should work just fine. You can also simply tee-off of a left/right pair where they will tie into the Alpine amp and run those to the powered sub since it looks like that also takes speaker level inputs.

      From what I have read, Audiocontrol's accu-bass thing sounds neat, but most people end up turning it off because it does weird stuff. I am re-doing my sound system next week, tossing a 5 channel MB Quart amp and a 6XS crossover out, all in favor of a LC6.1200 amp which will have Accubass disabled.

      The LC6.1200 amp looks nice. I may go there at some point - I have the LC7i + Alpine already and want to use them but I do know there are better options.

      I like the theory of the LC7i but if it's not needed it's just adding complexity.

      So, the + and - of the CM5908 will go directly into the + and - of the Alpine, for each channel. If I need to extend the wiring, I'll use stranded cat 5/6. I have a box of CAT 5 in the garage, will check if stranded.

      Any thoughts on AMP placement? I know most put in the trunk, up against the back of the rear seat. I'd prefer to keep the amp up front so that I can leverage the existing wiring easier. I don't like putting in the glove box. Maybe under the passenger seat.

      Jason
      Last edited by jsnppp; 11-28-2021, 06:30 AM.

      Comment


        #4
        I'm going to mount the Alpine Amp in the trunk and route the stock speaker wires to and from using CAT6 cable.

        I won't cut the original plugs off of the existing stock wires and so need an adapter to allow me to connect..

        I've looked and cannot find a female adapter that will allow me to go from male speaker wire plugs to CAT6... I can solder the CAT6 wires, just need the female adapter ends and some bare wires.

        Anyone point me in the right direction? I'll keep looking and post here when find. Worst case is I solder female spade connectors to a pigtail... would love something nicer looking.

        Jason

        Comment


          #5
          Ok, I finally found a search term that worked. "2 pin DIN plug female"

          I think these are the ones: https://www.amazon.com/10pcs-Female-.../dp/B07SQVJFFX

          They look the same but can't tell if the dimensions are what I need..
          Last edited by jsnppp; 11-29-2021, 10:44 AM.

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