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RCA-in-to-differental-out interface anyone?

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  • RCA-in-to-differental-out interface anyone?

    Hi there,

    I have a BMW e36, 3 series and I am eliminating the head-unit and replacing it with PC RCA line-level outputs. However I wish to retain the factory amp which takes differential speaker inputs only from the head-unit (+/- 5V peak-to-peak max). I need a way to convert RCA inputs to differential outputs to drive my stock amp. Is there any such device out there which will do this job?

    I have no space for the stock stereo head unit.

    Thanks

    Drdee
    Drdee

  • #2
    If I read this right, you just need to get more voltage (speaker level type) out of your sound card. If that's the case, you just need a preamp / linedriver. I used a Crossfire Car Audio piece CQ20.2L, that is also a 20 band EQ. It will give you up to 7.5V RMS output, and is adjustable.

    The problem I've had with trying to interface with BMW factory amps is that they use more than the typical 2 or 4 inputs.

    Toby J

    Comment


    • #3
      If you need to go from RCA's to XLR type differentials you need a thing called a DI-box, they're used in production audio all the time (think bands and DJ's at big shows plugging into 64-rail sound desks)

      Problem is, most of them are big-ish, need phantom power (+48V on the XLR lines) and i can't remember what they output as differential but it's probably not something simple like +/-5V

      But, if you NEED to go differential that's the box you need, making the signal levels proper and powering the thing becomes trivial after the signal is conditioned properly.

      Question: You said differential SPEAKER inputs, are the inputs to your amp designed for preamped outputs? I'm sure i'm reading that wrong because it doesn't make sense but hey, maybe maybe, german engineering...

      PS. You can't just use a preamp if you need a differential signal, your **** will get all confused and might reject all the signal as if it were noise... What I would do is work from your stock amps backwards,

      step 1) Identify the current inputs, are they indeed differential (+ - gnd signal lines) and what is the peak-peak voltage from ground to +, ground to - and + to -

      step 2) Figure out what my DI-box output signal levels are, and what kind of power they require

      step 3) make some neat little boxes to amplify/attenuate the signal into your amps and power the DI's, hook them up and go nuts.

      Comment


      • #4
        Unbalanced to Balanced conversion with pre-amp

        Hi there,

        I see what you mean about the DI boxes. In fact there is a device http://www.autosound2000.com/linedrivers.htm N645 that has isolated supplies, unbalanced inputs, pre-amp and balanced outputs. As you rightly metioned, DI takes split supplies and the N645 has split supplies that are generated internally using switching PSU.

        The enginner at autosound told me that it would be better to have a sound-card that could generate higher voltage and then to simply go through a transformer, hence you get unbalanced to balanced output.

        Having read many articles on BMW amps, they mention that levels into amp are differential (balanced). As you said, I need to double-check this with a scope and measure levels WRT ground and WRT to each-other. However I have a feeling that what I'm reading on the net is correct.

        When I said SPEAKER outputs, what I mean is balanced outputs from head-unit (absolute levels need to be verified with a scope so I'll get back to you on that one), but its either going to be 10v p-to-p (+ to -) with ground being halfway point (half VDD?), or 5V p-to-p (+ to -).

        Originally posted by greenwire
        If you need to go from RCA's to XLR type differentials you need a thing called a DI-box, they're used in production audio all the time (think bands and DJ's at big shows plugging into 64-rail sound desks)

        Problem is, most of them are big-ish, need phantom power (+48V on the XLR lines) and i can't remember what they output as differential but it's probably not something simple like +/-5V

        But, if you NEED to go differential that's the box you need, making the signal levels proper and powering the thing becomes trivial after the signal is conditioned properly.

        Question: You said differential SPEAKER inputs, are the inputs to your amp designed for preamped outputs? I'm sure i'm reading that wrong because it doesn't make sense but hey, maybe maybe, german engineering...

        PS. You can't just use a preamp if you need a differential signal, your **** will get all confused and might reject all the signal as if it were noise... What I would do is work from your stock amps backwards,

        step 1) Identify the current inputs, are they indeed differential (+ - gnd signal lines) and what is the peak-peak voltage from ground to +, ground to - and + to -

        step 2) Figure out what my DI-box output signal levels are, and what kind of power they require

        step 3) make some neat little boxes to amplify/attenuate the signal into your amps and power the DI's, hook them up and go nuts.
        Drdee

        Comment


        • #5
          Toby,

          will thie device do unbalanced to balanced conversion with amplification? Guess when I mention speaker-level, what I really mean is unbalanced to balanced conversion with floating ground output. Also this is an expensive device, and I need 4 channels. There must be something cheaper out there to do the same job. See my other reply, I think I might have found something (N645) x 2 = $140.

          Thanks

          Originally posted by Because
          If I read this right, you just need to get more voltage (speaker level type) out of your sound card. If that's the case, you just need a preamp / linedriver. I used a Crossfire Car Audio piece CQ20.2L, that is also a 20 band EQ. It will give you up to 7.5V RMS output, and is adjustable.

          The problem I've had with trying to interface with BMW factory amps is that they use more than the typical 2 or 4 inputs.

          Toby J
          Drdee

          Comment

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