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permanent live

Discussion in 'A4/A4 cabriolet/S4 forum(B6 chassis)' started by TwedA4, Jun 13, 2014.

  1. TwedA4
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    TwedA4 Member

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    [Jun 13, 2014]
    hi guys,

    would it be ok to take a permanent live feed from the big distributor block under the dash for a 300W max active sub?

    be so much easier than poking a piddley wee cable through the bulk head... lol
    #1
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    [Sep 22, 2014]

  3. dualmono21
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    dualmono21 Well-Known Member

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    [Jun 14, 2014]
    which active sub is this ???
    you ideally need to know what kind of current draw you will be consuming and through what size of cable it is being put down

    from this you can work out if its suitable to be powered off the rear charging point
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  4. TwedA4
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    TwedA4 Member

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    [Jun 14, 2014]
    well using the max power at 300W and cars charching voltage (14 ish?) theres a draw of around 20 amps. there's alread a wiring kit with it and it has what looks like 2 30amp cable which is more than enough as we had the earlier model in one of our vans.

    it's a blaupunkt xlf 200a.

    edit: should have read all your post. there is a separate power option available which does use a cigarette lighter... lol

    the unit has a 10A fuse in it too if that helps
    Last edited: Jun 14, 2014
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  5. TwedA4
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    TwedA4 Member

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    [Jun 14, 2014]
    one of these. :)
    25498_0.jpg
    #4
  6. Stevieb2905
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    Stevieb2905 Active Member

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    [Jun 14, 2014]
    image.jpg
    You could use one of these and plug unto a spare socket, maybe towbar assembly, or piggy back off something else. Bit confused though, if it's drawing 20a why is it fitted with a 10a fuse?
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  7. TwedA4
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    TwedA4 Member

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    [Jun 14, 2014]
    well, thats using my maths... not sure how power is calculated for powering amps and stuff... might just be the speaker that's 300W max.

    but that's all that was fitted to our last one and it was fine. until we blew the speaker. lol

    edit: ok, i was doing the math wrong. it's only gonna draw 10a using RMS power which is 150. :)
    Last edited: Jun 14, 2014
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  8. dualmono21
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    dualmono21 Well-Known Member

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    [Jun 15, 2014]
    its measure like this

    maximum power output of the amplifier x 2 (this is because all car audio amplifiers are only generally 50% efficient )

    300 x 2 = 600 600 divided by 12 is 50

    so an amplifier producing 300 watt has a current draw of 50 amps
    there is also an equation for cable size vs cable length

    which is why i ask which unit is it ?
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  9. TwedA4
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    TwedA4 Member

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    [Jun 16, 2014]
    it's a blaupunkt xlf 200a.

    it's a D class amp so think it's pretty efficient
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  10. TwedA4
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    TwedA4 Member

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    [Jun 16, 2014]
    hang on... surely if it's 50% efficient, then that's only 150W. it's not going to draw 50 amps, it'll cook its on board fuse in seconds?
    #9
  11. S4_dan
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    S4_dan Fire up the Quattro!

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    [Jun 16, 2014]
    The safest bet with any amp install is to fit direct to the battery and fuse it shortly (within 20cm) of battery terminal.
    Pulling your power from the block under the dash could cause an overload on the thinner, older wires. It's not worth the risk. There are a few ways of going through the bulk head using some of the original grommets already there with wiring in them.

    Dan
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  12. dualmono21
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    dualmono21 Well-Known Member

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    [Jun 16, 2014]
    i have read the specs of the active sub

    it is only 150 wrms not 300 as quoted rms is the proper way of measuring an amplifier r.m.s roots mean squared
    to make 150 and maximum output the amp is only 50% efficient
    so you have to put twice as much power into it to make 150wrms out of it

    2 x 150 = 300 ... 300 divided by 12 is 25
    so you need a cable which is capable of holding 25 amps over whatever length the cable is
    but the longer the cable is the less it is capable of holding , you have to bear this in mind when installing
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  13. easy going Si
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    easy going Si Member

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    [Jun 22, 2014]
    TwedA4 for a 25amp load you also have to take into account the fuse you are using if you are using glass slow blow

    the equation becomes fusing factor over load which is a 1.7 multiplier for any cable I normally double the cable for the load
    which gives you a 2 times safety factor i.e. 25 amp load 50amp cable

    hope this helps
    #12

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