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Thread: Noise!

  1. #1
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    Noise!

    Mornin!

    I finally got rid of my head unit and connected the carputer straight to a preamp but I've come to a problem I havent got to before. NOISE! I didnt have this much noise when I was hooking up to my head unit.

    From what I've read of previous posts ( )..

    a) Grounding need to go to the same point (Please explain why, isnt ground supposed to be ... ground?)

    b) Need to ground the carputer case. Hrmm, my case is made of glass.. how do you ground glass?

    c) Unclear on this one, exactly what needs to be grounded to the same point.. Preamp? Amp? Inverter? PSU? glass carputer case? external HD?

    d) Someone mentioned in another post that if I ground on multiple points on the car body, there will be an ohmic value if I attach a multimeter between those points. Meaning there will be current flow between those 2 ground points if my high school physics memory serves me correctly. I checked with my consumer-grade multimeter, everypart of the body gives an ohmic value of 0. So should I still ground on the same point?

    e) Do I need a big wire for this or can I use the small ones like those used in making small PCBs in small electronic projects. If I do need a big wire, why? Isnt a wire just a wire?

    f) What's the purpose of the ground loop isolator? I connected one and it still gives me the same amount of noise.

    g) I read somewhere also that noise cannot be rid off completely. It can only be reduced. Could it be that I've done all that I can?

    h) What is this filter circuit inside the PSU someone here mentioned? Even if I know how to open my PSU up, how do I know which one is the filter circuit, what should I be looking for? And how do I remove it? Keep in mind I have no idea what is the difference physically between a capacitor, resistor and transformer on a pcb.. they all look the same to me.

    I think that's all for now. Heh, I should have paid more attention to electronics in high school.

    Thanks in advance.

  2. #2
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    Ouch problems..

    Well I tried to ground everything like was suggested in some other threads. But everytime I tried to turn it on, it blew the fuse that I connected between the inverter and battery.

    I narrowed it down to the inverter's ground.

    Right now, every ground is connected to the inverter's ground except the amp. The amp's ground is connected to the vehicle body.

    Preamp's ground is connected to the inverter ground
    PSU's body is connected to the inverter ground.
    Carputer's body is connected to the inverter ground.
    A 3-pin switch with DC LED's ground is connected to the inverter ground.

    But once I try to connect the inverter ground to the vehicle body, the fuse blows.


    And then there's another problem. Its regarding the noise. I played around and noticed that there is no noise if I dont connect the RCA's coming from the preamp to the RCA's coming from the carputer (duh).

    But if I connect just the RCA ground of the preamp to the RCA ground of the carputer, I start to hear whining.

    I then took out a multimeter and measured between the preamp RCA ground and found:
    Resistance : 28 mOhms with power off
    Current : (dammit I forgot but it was around 2mA or microAmps)

    Are these readings right? In my opinion it should be around 0 because its ground, but I really have no idea.

    If this isnt right, do you think something is wrong with my setup?

  3. #3
    FLAC DodgeCummins's Avatar
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    The inverter output probably shouldn't be grounded...

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    No no it isnt the inverter output, at the inverter in I see a red connector and a black connector, to which I assume to be +ve and -ve. At the bottom there is a screw like connector labelled ground. At the output there is no ground connector except at the plug where you're supposed to put in the PSU power cable.

    I noticed something strange yesterday. WHen I put one pin of the multimeter on the ground of the inverter (or body of the PSU) and one more at the car body inside one of the big screws that hold the car seat in place, I notice there is a difference of 12V!

    I'm thinking, how can that be? The car body is supposed to be ground no?

    Either something is connected to the car body that is 12V or my PSU body is charged at 12V. Right now I dont know where to look.

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    I think I solved the problem. Apparently the preamp has 3 wires, one to be connected to the + battery, one to ground and one to power on.

    I had connected the preamp one to +12, one to -12 and one to ground, and I guess that's why the fuses kept blowing if I touch the body of the inverter/psu or anything else to the car body.

    The noise is gone, but if you really try to listen you can hear a slight whining noise that seems to follow the engine i.e. higher rev, higher frequency. That one I dont know how to solve.

    Since I've fixed the wiring on the preamp, i've wired the following to car body:
    1. 240/12V PSU
    2. Inverter
    3. Preamp
    4. Amp
    5. Power on Switch (LED 3 pin - 1 source, 1 dest, 1 ground)

    The only thing I think that might help is wiring the carputer itself to ground (like hard drive screw mounts and mobo screws) but that's gonna take a lot of wires.

    One other possible solution is to wire the amp to the same ground as the rest of the equipment.. because right now the amp is wired to the passenger seat of the car and all the rest are wired to the driver's seat.

    Any input would be mucho appreciated.

  6. #6
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    Quote Originally Posted by masch
    The noise is gone, but if you really try to listen you can hear a slight whining noise that seems to follow the engine i.e. higher rev, higher frequency. That one I dont know how to solve.
    I heard this noise even with my regular head unit. I think it's caused by slight fluctuations in voltage coming from your car battery to the amp created by the alternator. Did you not hear this with your old head unit?

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    Now that you mention it, I think I did. You may be right, perhaps using a voltage regulator could get rid of this noise?

  8. #8
    FLAC MP3DUB's Avatar
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    The car as a whole is a very electrically nosiy environment, as you have discovered. The same ground is recomened because, while the entire body is "ground", the current doesn't flow uniformly through it, creating potential differences in ground, causing voltages to go ways they shouldnt. Another cause can be poorly sheilded cables picking up emi/rfi from the cars systems. And I don't know how you have your wires run and where they run to, but the general rule of thumb if they're in the back is to run power down one side of the car and the audio down the other, to limit the noise. The ground loop isolator, if memory serves, creates a floating ground, so that if you do have differences in ground voltages, you wont have it flowing from the trunk to the front of your car via your audio cable. The downside of the isolators is that they can much reduce the overall sound quality, muting the tones to an extent. Inverters also dont create the cleanest voltage for the computer to run on, creating more noise in the computer itself, and, as mentioned, an alternator is quite electrically noisy. Without going to insane lengths, you can only really try to minimize the noise as much as possible. If i dont have any music playing and I turn my stereo up allll the way, I can hear a bit of emi/rfi noise, but otherwise, its unnoticible.
    -Nick

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