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    ugmoe2000's Avatar
    ugmoe2000 is offline Senior VIP Member

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    Humming Noise Responds to RPM's

    I just installed a new PA Plasma Series amp in place of my weaker rockford fosgate amp and the sound is MUCH clearer and louder now, HOWEVER... there is a faint humming noise that my laymen passengers wouldn't notice but us conisseurs of car audio would go nuts over.

    The noise responds to the RPM level of my car... the higher the RPM's go the higher pitched the noise is and the lower the RPM's the lower pitched the noise is... it sounds like one of those noisemakers you blow with the little fans in them that whistle when they spin.

    Now that I've explained the problem, how do I fix it? Where is this interference coming from and why was it not present at all in my old Rockford Amp (which no one will argue is of higher quality)? The same exact wiring was used, I just switched out the amps.

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    analogkid's Avatar
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    Re: Humming Noise Responds to RPM's

    check all your grounds first

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    Re: Humming Noise Responds to RPM's

    definatly sounds like ground loop


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    Re: Humming Noise Responds to RPM's

    how do I correct a ground loop and what am I checking my grounds for? Could this be related to how I hooked up my signal ground? I had no idea what it was for other than for a reference point to 0 volts... so I ran it to the ground terminal on my amp and tapped it off of there. Should I have ran it elsewhere... perhaps to a baremetal ground?

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    Re: Humming Noise Responds to RPM's

    Heres the step by step guide-

    2.1 My speakers make this high-pitched whine which matches the engine's
    RPMs. What is it, and how can I get rid of it? [IDB]
    ================================================== ========================

    The answer to this section was generously provided by David Navone of
    Autosound 2000. The material in these instructions was adapted from
    the Autosound 2000 Troubleshooting Flow Chart by Ian Bjorhovde with the
    permission of Autosound 2000. For more information about Autosound
    2000, (see Section 7).

    This is a set of instructions to debug a stereo installation if there
    is any noise present after it is completed. Follow each step
    carefully! If you have more than one amplifier, repeat level one for
    each amp to be sure that none of them are responsible for the noise.

    2.1.1 Level 1: Check out the Amplifier(s)

    After you have determined that there is noise in the system, determine
    if the amplifier is causing the noise. To do this, mute the signal at
    the inputs to the amp by using shorting plugs. If there is no noise,
    then the amp is fine, and you can proceed to level 2. However, if
    there is noise, then use a test speaker at the amp's output. If this
    stops the noise, then the problem is originating in the speaker wiring,
    or the passive crossovers. Check to make sure that none of these are
    shorting with the body of the car, and start again at level 1. If
    noise is still present when using the test speaker, then there may be a
    problem with the power supply on the amp. Try connecting an isolated
    power supply - if this does not get rid of the noise, then there is
    something seriously wrong with the amp, and it should be replaced. If
    the noise goes away, then there may be a problem with power supply
    filtering or isolation. This can be fixed by changing the amp's ground
    point or b adding external supply filtering.

    2.1.2 Level 2: Reduce the System

    The amps have been determined to be noise free. If you have any
    processors between the head unit and the amps, disconnect them and
    connect the head unit directly to the amp. If this gets rid of the
    noise, then one (or more) of the processors must be at fault, so
    proceed to level 5. Otherwise, try running the signal cables over a
    number of different routes. If you are able to find one that does not
    produce any noise, permanently route the cables in the same manner, and
    proceed to level 5. If not, then you must isolate the head unit from
    the car's chassis (except for its ground!) - don't forget to disconnect
    the antenna, since it is also grounded to the car. If isolating
    the head unit does not solve the problem, the move the grounding point
    of the head unit. Hopefully the noise will be gone, and you can
    install the head unit with a quiet ground and proceed to level 5,
    otherwise go on to level 3.

    2.1.3 Level 3: Move the Head Unit

    The amplifiers are fine, but moving both the ground for the head unit
    and the signal cables does not solve the noise problem. Take the unit
    completely out of the dash, and put it on either the seat or carpet,
    and run new signal cables to the input of the amp. If this solves the
    problem, re-install the head unit, one step at a time and skip to level
    5. But if the noise persists, then move the head unit as close to the
    amp as possible and use the shortest possible signal cables. This will
    verify that the original signal cables are not causing the problem -
    assuming the noise is gone, reinstall the head unit one step at a time
    and go to level 5. Otherwise, there may be a problem with the power
    filtering for the head unit. As with the amps, power the head unit
    with an isolated power supply (again making sure that the head unit
    isn't touching the car's chassis at all). If the noise goes away, you
    can add power supply filtering or an isolated power supply; go to
    level 2. But if the isolated power supply does not solve the problem,
    then you can either replace the head unit and go to level 2, or check
    the car's electrical system in level 4.

    2.1.4 Level 4: Testing the Car

    There does not seem to be a problem with either the head unit or the
    amplifier, and the car's charging system is suspect. To see if this is
    the case, we can use a system in a car that is already known to be
    "quiet." Bring both cars together as if you were going to jump one,
    and use jumper cables to connect the two batteries. Start the engine
    of the car with the noise problem, and listen to the "quiet" car's
    system. If the noise does not go away, there is a SERIOUS problem with
    your car's electrical system (possibly a bad alternator). Have a
    qualified mechanic check the charging system out. If there is no noise
    in the "quiet" car, then the "noisy" car's charging system is
    definitely quiet, so continue with level 5.

    2.1.5 Level 5: Adding Signal Processors

    We have proven that the amplifiers are good, the head unit is good, and
    the car's electrical system is good. Now we need to reconnect each
    signal processor. Repeat this level for each signal processor used in
    your system; if you have added all of your signal processors, and
    there is no longer any noise, CONGRATULATIONS! You've removed the
    noise from your system! Connect the signal processor. If there
    isn't any noise, then go on to the next signal processor. Otherwise,
    try re-routing the signal cables. If this cures the problem, the route
    them permanently over the quiet path, and install the next processor.
    If not, then isolate the processor from the car's chassis except for a
    single grounding point. If this works, then permanently isolate the
    processor, and move on to the next processor. If isolation does not
    help, then advance to level 6.

    2.1.6 Level 6: Processor Isolation Tests

    Now, noise enters the system when one particular processor is
    installed, but regrounding it does not help. Move the processor very
    close to the amp, and check for noise again. If there isn't any, then
    re-install the processor, carefully routing the cables to ensure no
    noise, and continue at level 5 with the next processor. Otherwise, use
    an isolated power supply to power the processor, making sure that no
    part of the processor is touching the car's chassis. If this solves
    the problem, the consider permanently installing an isolated power
    supply or possibly a 1:1 transformer, and go to level 5 with the next
    processor. Otherwise, separate the processor and isolated power supply
    from the car by many feet and re- test. If there is still noise, then
    there is a serious problem with the processor's design. Get a
    different processor, and continue at level 5 with it. If separating
    the power supply and processor from the car does solve the noise
    problem, then either the processor is damaged, or your tests were
    inaccurate. Repeat level 5.

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