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Thread: Car PC Audio / Amp setup

  1. #1
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    Car PC Audio / Amp setup

    Hey guys,

    Looking into amps now I have my car pc kind of sorted hardware/software wise (I.e. I know what I want it to do, and have most of the components up and running and wired in). Bear in mind I am not in any way knowledgeable in audio so please be nice!

    I have a 1000w Shark V9 amp (L, R and Sub) that I got for free, so I wired that up to a battery charger to provide 12V and simply test it with my car speakers. My car speakers are 4ohm each, and I simply joined them in series (8ohm left and 8ohm right), leaving the sub connection disconnected. That works fine, sounds SO much better than my HU - tape adapter - phone setup (apart from slight buzzing which stopped when I touched the case, grounding issue?). The problem is this amp is quite big, and I was looking to get something small that I can mount behind the dash.

    Shark V9 1000w amp spec:

    Average power output:
    4 ohms: 300W x 3
    2 ohms: 400W x 2 or 400W x 1
    Bridge : 1000W

    Drew 0.47A at idle, and less than 2A at nearly loud enough to be painful levels, being run from my phone (headphone socket) - no distortion). This was on my DMM's 10A setting. If it draws this little power, why has it got a 30A fuse?

    I was looking at one of these:

    http://www.ebay.co.uk/itm/Amp/230836632175
    or
    http://www.ebay.co.uk/itm/Amp/300759514850

    Should I stick with the current amp I have, or get one of these? (They are small enough to mount behind the dash, and would be easier to wire in with smaller cables).
    Last edited by sparx; 08-21-2012 at 04:59 PM.

  2. #2
    Raw Wave
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    Because you are measuring the current at the wrong place? Or only outputing about 20W RMS. (RMS (or "continuous power") being the base meaningful rating for amps & speakers - forget peak power or any other power UNLESS they state exactly how those figures were derived.)

    With a 30A fuse, that amp at 14.4V can put out a max of 30 x 14.4 = 432W RMS,
    It's probably under a 300W RMS output when amp efficiency (~80%) and normal fuse loading (70-80%) is taken into account.


    Your 8ohm speakers also means less output

    At 2A, you might be outputting about 20W RMS,
    To double that volume, you need 4x the current, hence 80W.

  3. #3
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    The DMM was placed between the positive lead and the B+ input of the amp, using the 10A mode of my DMM with the probe in the 10A socket. I didn't measure the voltage (I meant to but forgot!). I did consider an error with my DMM as it may have been out of range, but that results with a "1" in all other modes.

    The level dial on the amp was only at 10 o clock, before it started becoming too loud - as I increased this, the current draw did too (not linearly).

    So if I have measured the current draw correctly, then (at least) the red MA-150 amp in the link above should be sufficient?

    (Just fixed the links, sorry!)

    Edit: If they don't lie about the specs that is. Just read reviews for it and i'm definately not even going to try ordering one of those! May just stick with the amp I know works well, just need to get the correct cabling for it and find somewhere to stick it out of the way!
    Last edited by sparx; 08-21-2012 at 06:40 PM.

  4. #4
    Raw Wave
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    Well the red says 12V 0.5A power, hence a max of 6W output or 3W per side. It is way below your 2A draw.
    As to wtf their 500W or 75W means....


    The blue says 12V 5A input hence max 60W output.
    But the 5A is more than your 2A so it should be ok power-wise. (Distortion and "sound" in another issue.

    60W RMS is normally 120W "Music Power", but PMPO is a useless measurement (is that for 1uS or 1mS or 1 second or...?).

  5. #5
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    the others that have used that smaller amp rate it as a 'factory replacement amp' not amazing, but not horrible.

  6. #6
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    Quote Originally Posted by soundman98 View Post
    the others that have used that smaller amp rate it as a 'factory replacement amp' not amazing, but not horrible.
    I'm toying with the idea of using this shark amp. I am assuming i'm measuring the current correctly, so would it be suitable to fuse it at a lower value at both ends of the cable (3-5A), and then use smaller cable? It would make it cheaper, and easier to install cabling if this is the case (following the wire size guide posted in one of my other threads).

    Just need to see if I can install it somewhere close to the front of the car... maybe behind the glovebox - but not quite sure how much space is available. Either that, or under the passenger seat.

  7. #7
    Raw Wave
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    You only need one fuse at the power source end (battery end).
    That fuse is to protect the cable.
    Hence as long as the fuse rating is less tha or equal to the cable's current rating, you'll be fine.
    Note that a general rule is to run a fuse & cable at no more than 70%-80% of its nominal rating (eg, 5A fuse/cable => 5x 0.8 = 4A max.

  8. #8
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    Quote Originally Posted by OldSpark View Post
    You only need one fuse at the power source end (battery end).
    That fuse is to protect the cable.
    Hence as long as the fuse rating is less tha or equal to the cable's current rating, you'll be fine.
    Note that a general rule is to run a fuse & cable at no more than 70%-80% of its nominal rating (eg, 5A fuse/cable => 5x 0.8 = 4A max.
    Wired in my current amplifier - i'm not driving a sub with it which I believe may be why it's not drawing anything more than 3A. I've fused it at the battery at 20A and at the amplifier (built-in fuse socket) at 10A. I've attached it under the passenger seat, it doesn't get warm even after an hour or more of use.

    One more issue i've come across is a turn off thump. I originally used the built-in audio out of my Joggler, but that had some weird noises on boot, I believe which are down to the component spacing (everything is crammed in pretty tight!) or a ground loop or similar inside the device. Tried another cheap external USB sound card, but that had a god awful hiss at full volume and an extremely nasty thump whenever it was re-initialized by windows, or unplugged/plugged in.

    Third go with a Numark DJIO I had hanging around, near perfection! A slight click on boot up (barely audible, especially if driving), however I have a nice thump when I remove the key (Joggler is powered off a 5V 5A dc/dc converter, along with a usb hub powered off the same converter, and the sound card powered off that). The relay which turns on the 12V supply to the 5V dc/dc converter also provides the remote turn-on signal to the amplifier.

    Is there any way to stop the thump at power off? I think it's the DJIO sound card when it loses power ( a similar, but quieter thing happens if you unplug it from the USB socket), but the amp isnt turning off quick enough to stop it getting to the speakers - would a capacitor help anywhere in the circuit?

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