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Damaged split supply amplifier

  1. Jul 22, 2014 #1
    I have a boss subwoofer amplifier which I recently discovered was causeing the distortion I have been hearing in my car.

    this amplifier consists of a PWM powersupply turning the 13.5v from my on car to a +45 -45 power supply.

    Then there are 8 mosfet transistors, 4 in parralel powering one phase of audio, then another 4 in parrelel powering the other phase of audio.

    I am reading on my DMM that the amp when put to the max I can get a 40 ish volt output which is plenty more than I need and to be expected.

    But when the amplifier is hooked to my 2 subwoofers
    at 2 ohms (which the amp is rated for) I can get an absolute max of only 14-ish volts and a lot of distortion.

    This sounds exactly like the power supply pukse width modulator is not working correctly. So I checked all of the power supply switching mosfets and they all work.
    so then I replaced the modulator chip itself expecting that to fix the problem and yet it did not.

    any ideas?
  2. jcsd
  3. Jul 22, 2014 #2


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    Have you measured the power supply rails to see if they are sagging? Also, what do they do at various loads? Can you see a change in the output to the switcher on a scope as the load changes? Try these things with your subs hooked in series or just one at a time to change the load. Also, is the 12 volt supply holding up feeding to the amp? If you figure the amp at 100% efficient (which it obviously is not) a 2 ohm load at 80 volts peak to peak will pull about 30 amps from the 12 volt source. Do you have a way to measure this current? Also, do you know if both phases are doing the same thing? If so, it indicates a power supply problem.
  4. Jul 22, 2014 #3

    Why do I read 100V AC between each rail and ground?

    I have no scope,
    also the power supply is a running car useing 10 guage coper wire, this is a little thinner than it should be but not nearely thin enough to make such a difference in output power.
    At 8 ohms it does the exact same thing and starts dieing at 14v.

    right now I am trying to figure of there is any transistor or anything damagable leading to the PWM chips current sensor.
    Last edited: Jul 22, 2014
  5. Jul 22, 2014 #4


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    I haven't worked without a scope on stuff like this probably ever. I was fortunate enough to own a scope before even having any formal training in electronics. I would not like doing this without a scope. You seem to do a lot of amplifier tinkering. I think you could justify the investment. Can you temporarily disable the current sense to determine if this is causing the problem?
  6. Jul 22, 2014 #5
    Ya, i really need to get one.
    but i dont have any money, that is why i need t fix this lol.

    I am working on it but the datasheet is not a big help...
    and it looks like the feedback pin is just grounded with a few caps and resistors.
    it's real weird.

    another thing is that i have a homemade work bench power supply that can put out a few amps at 12v,
    the entire amplifiers idle current is just on 800ma, i put a 4ohm dumby load on the amplifiers output and for some reason i was reading that the amp was drawing 2 amps while i only had about 1.5v output on a 50Hz sine wave!





    The math shows that i should only be reading 0.845A, not 2.0A
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