Fixing a hum in audio amplifier

Discussion in 'General Electronics Chat' started by mbuiter, Jul 6, 2015.

  1. mbuiter

    Thread Starter New Member

    Jul 4, 2015
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    Dear forum users,

    Please excuse me if I have come to the wrong location with my question (and excuse my terminology, I am no expert). I have spend many hours trying to detect and fix the problem I'll describe below, but I got stuck.

    I have got a power amplifier (NAD T761) which has a loud hum on its output, the hum is about 50 Hz judging by its sound. After having opened it I found and replaced a burst capacitor (C104 in the schema, next to the U125 7805 IC), but this didn't fix the problem. After the problem persisted, I bought a second-hand scope to investigate further. The scope shows repetitive power fluctuations on the outputs of both 7805 ICs. Going by this knowledge, I replaced both 7805s (U125 and U123) but still the problem persists. I have included a picture of the scope, showing the power fluctuations on the output of the U123 7805 IC (scope settings AC coupled, 0.1V/div, 5ms/div). Similar fluctuations (somewhat different shape, same frequency) are found on the output of the U125 7805 IC. The output of the U121 IC is free of any such fluctuations. Apart from the fluctuations, the 7805s output a decent 5V.

    The attached schematic is part of a (much) larger schematic describing the digital processing board of the amp. I expect this board to be the source (or partly the source) of the trouble, because if I switch the amp's input to external 5.1 input (bypassing this board), the sound is crisp and clear. The board contains both digital and analogue circuitry.

    One coincidental (accidental) find that I did is that when I very briefly short-circuit the U123 output, the power fluctuations disappear completely (and the amp continues to work, without hum :)). The power fluctuations return when I switch input channels, as that apparently resets some systems, and the hum returns.

    This amp is much more complicated than I had expected, and my electronics knowledge is far less than I had hoped for, so I am in way over my head. I have already accepted that I will probably not be able to fix this problem, but I will go, and have already gone, to some length to try and learn. The diagnostics stuff was already almost as expensive as the amp itself.

    I hope somebody can help me with some directions into which I could proceed further. Does anyone have experiences with such power fluctuations on DSP boards? Is there anything I can do to get the problem domain smaller (it's a complicated PCB)?

    Kind regards,

    Maarten
     
  2. #12

    Expert

    Nov 30, 2010
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    Try C107.

    The regulator chips must have a few volts more supply than their output voltage. When the primary filter goes bad, the main voltage supply wobbles too much between power pulses from the power transformer. When your primary voltage supply sags down to less than about 7 volts, the 5 volt regulator can't compensate.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Either C107 has much less than its rated capacitance, or excessive load on either +5Va or +5Vd due to a downstream fault. If overloaded, one of the 78M05s should be much hotter than the other. The scope would also show which of the two 78M05s is overloaded.
     
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  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That power module is just for the logic chips in the AC3 (Dolby Digial/DTS decoding) part of the AV receiver. You have to find the main power capacitors (Two big ones) on the main amplifier board. Those are likely the issue. e.g. C561, C562 .

    http://elektrotanya.com/nad_t761.pdf/download.html

    upload_2015-7-6_18-4-53.png
     
  5. #12

    Expert

    Nov 30, 2010
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    Some people are just better at dealing with Nads than I am. :rolleyes:
     
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  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That's hitting blow the belt!
     
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  7. MikeML

    AAC Fanatic!

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    How come the if the main filters are bad, it effects the output of the 78M05s, which are fed from an isolated winding on the main power transformer?
     
  8. #12

    Expert

    Nov 30, 2010
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    I believe you are correct, but I was busy having a bit of fun at the moment. :oops:
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I just wouldn't expect the little power supply for the AC3 decoder to be the source of the hum. Also, if there is no capacitance on the big cap for the main power supply, ripple could fall below the drop out voltage of the 7805 if the load on the main cap is significant.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    According to the partial schematic posted by the op, the 5V supplies in question have their own transformer winding, their own rectifier and filter capacitor, independent of the main supply.

    If the OP is seeing dropout of those 5V regulators, then that should not be effected by the big caps in the main power supply. Maybe the transformer is overloaded somehow, or has a high source impedance, or the wrong input voltage???
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The partial drawing shows that the AC3 has its own bridge rectifier, but the AC feeding the bridge comes from the 8vac secondary on the main transformer.

    image.jpg

    The whole page with AC3 circuit is attached. The 7805 circuits are in bottom left. Enlarged in second (last) image.

    image.jpg
    image.jpg
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    I don't believe that the problem is on the amplifier board. If it was, the hum would persist no matter which input is selected.

    The amplifier board's audio input comes in on J603B. This connects to the "Multi Board", which appears to be a signal switching board. The audio components run on +-12V.

    Once you get the logic power supply functioning well, then work through the signal path, starting at J603B on the amplifier board. I suspect you have more than one power supply problem, (check +-12 volts on the Multi board.)
     
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    Imagine if NAD sponsored a football team and the crowd started cheering.....GO....
     
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  14. mbuiter

    Thread Starter New Member

    Jul 4, 2015
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    Hello Guys,

    Thanks for your very quick replies! I will try to check your suggestions tonight, and see if I can come up with new information.

    Greets, Maarten
     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Check that the main transformer primary has the correct AC voltage across it.
     
  16. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Does the HUM disappear when the volume is at 0
    Are the regulators hot to the touch
     
  17. mbuiter

    Thread Starter New Member

    Jul 4, 2015
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    Hello R!f@@,

    The hum cannot be heard with the volume at zero. I can control the volume of the hum just as good as the volume of the music; I believe it is inserted through the connection between the AC3/DSP board and the multi-channel board which people referred to above. I will post a scope picture of the hum on this connection later today, the hum appears as tiny spikes on these signal lines.

    You are right, the regulators are hot to the touch. One (I believe U123) is equipped with a heatsink, but my feeling is that they get hotter than they should.

    Regards, Maarten
     
  18. Lestraveled

    Well-Known Member

    May 19, 2014
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    Can you hold your finger on the regulators?
     
  19. mbuiter

    Thread Starter New Member

    Jul 4, 2015
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    I'll try tonight but I guess: yes. You are right, they are not that hot.
     
  20. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    So it's not the main PSU, Amp or caps.
    So what is the exact voltage measured at the outputs of the regulators
     
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