Repair subwoofer plate amp?

Discussion in 'DIY Corner - Tips & Techniques' started by jbond04, Feb 8, 2011.

  1. jbond04 Audiophyte

    jbond04
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    Hey everyone, I've got a subwoofer that doesn't function properly, and I suspect it may be the plate amp. Here's a rundown of the symptoms and the tests that I've performed:

    1. Subwoofer is inaudible during normal operation, but the power light is on.
    2. Cranking the volume on the receiver to the maximum will produce some bass output, but still very low levels.
    3. The problem occurs regardless of whether I'm hooked up with the high level input or the line input
    4. Crossover is set to the highest value (150Hz) and subwoofer volume is set at the maximum
    5. Power is set to "On", not "Auto-On"
    6. I have tested the sub with another receiver, but the result was the same: low output even with volumes cranked

    First, let me state that my definition of "low volume" is "I can barely hear it when my ear is right next to the driver". I can confirm that sound is playing through the subwoofer, not just power noise, so the inputs are definitely working. These facts have led me to the conclusion that my amp has gone bad. I pulled the amp out of the subwoofer, and I can't see any damaged or fried components, bad solder joints, etc. It looks fine.

    Now I know that I could just order a plate amp for $100 and move on, but I'd rather not spend the money when there is likely just one component in the amp that has failed. I'm looking for advice from people who have encountered a similar issue, and what component they suspected was the cause. I'm an electrical engineer, so I have access to plenty of tools, measurement devices, and soldering equipment. Unfortunately, amplifiers were never my strong suit in school, so I can't speculate what might be causing the volume problem that I'm having. Maybe a bad op-amp? Broken feedback loop?

    Please chime in with any experience or advice that you may have, even if it's directing me to another forum! I've been an occasional lurker around here, but this problem has finally brought me out into the open. Thanks for any help that you can offer!
    Last edited: Feb 8, 2011
  2. TLS Guy Audioholic Overlord

    TLS Guy
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    You need a circuit for the amp at least. These days unlike the days of old they seem hard to come by. Without at least a circuit diagram you are stuck.

    Then you need test gear, at least a FET VOM, signal generator and scope at the minimum.

    A service manual if you can get one is very helpful.

    If it is a class D amp, and most plate amps are, they are very difficult to service. If the problem is in the switching power supply of these units, and it usually is, they are virtually impossible to service, and not worth the time.

    Basically those types of units are disposable devices and not worth the trouble to service.
  3. Alex2507 Audioholic Slumlord

    Alex2507
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    We have been waiting for you Mr. Bond. There is a trap door under your computer chair that has you go down an impossibly long slide on the way to some hungry alligators. This time you're in big trouble.
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  4. lsiberian Audioholic Overlord

    lsiberian
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    amps aren't impossible to fix, but they are a pain. You could see if you have a short in something, but I suspect you plugged something in wrong.

    yeah because if it produces sound that normally means it works. I suspect your didn't hook it up correctly or you have a messed up setting.
  5. TLS Guy Audioholic Overlord

    TLS Guy
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    He seems to have checked it out carefully. My money is on a low voltage coming out of the power supply. That can cause just this sort of fault.

    The switching power supplies in those things are a pain, just like computer power supplies, and they both have the same modus operandi.
  6. 90gstman Audioholic Intern

    90gstman
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    Drive by shooting

    Check the driver also before buying anything. Lightly push on the driver to see if it moves freely with no rubbing (if it has a hard dustcap/voicecoil cap push with lightly from the center, if its a soft cap push evenly from right and left side of the cone). Also, hook the raw driver up (by itself) to the left or right side output of your receiver and make sure it works.

    You would probably want to shoot yourself if you bought an amp and the driver was bad.
  7. jbond04 Audiophyte

    jbond04
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    I just got done checking the driver with my receiver; it's nice and loud, so I think we can rule that out. (Thanks for the suggestion, though!)

    I also discovered another interesting tidbit during testing. Even if I turn the amp off (i.e. put it in "standby" rather than "on" or "auto-on"), I can still barely hear sound coming from the driver. Essentially, turning my amp on or off has no effect on the subwoofer performance.

    I haven't been able to find a schematic/circuit diagram of the amp, but I was hoping that the design would be similar enough to other plate amps that it wouldn't be necessary. I would imagine that all Class D amps have some design features in common, right?

    So if my amp severely attenuates the input signal to the point where I need to turn the volume up to the max on both my receiver and the sub in order to barely hear the output, and turning the amp on or off makes no difference, what does that tell me? TLS Guy, your theory of low voltage out of the power supply sounds plausible. Is there a way that I can test this? I have a multimeter, but I'm unsure where I should measure or how much voltage I should be seeing.
  8. TLS Guy Audioholic Overlord

    TLS Guy
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    It sounds as if the amp is dead. There is just a small signal getting through the amp from the input, but it is not amplifying.

    All manufacturers will have different circuits. Circuits all differ quite markedly.
    There are lot of means to similar ends.

    As to voltage, only a circuit diagram with crucial voltages marked will allow you to service the amp.

    To service it, you need: -

    An understanding of how the unit is configured from the circuit diagram.

    Appropriate equipment.

    A logical plan to diagnose the fault, based on the above, in the absence of a service manual.

    Without any of the above, you will have no success.

    It is disgraceful, that in modern gear there is no circuit and parts list in the user manual, just pages of BS about not getting it wet, taking the top off and giving yourself a shock etc. They are more often than not in pigeon English to boot.

    There was time when these matters were taken for granted, and people knew you could get a shock digging around inside if you did not know what you were doing. Circuit diagrams and parts lists were expected to be in the manuals or readily available.

    So the overwhelming probability is that your unit is a recycle job.

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