Humming sound in stereo amplifier

Discussion in 'General Electronics Chat' started by lubnaan90, Jul 7, 2011.

  1. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Greetings to all Readers,
    First of all thank you for reading this post.

    I have recently built a Stereo Power amplifier, Both L & R channels uses independent amp boards, independent power supplies (transformers) are used for each channel amp boards, best quality and shielded cables are used, But unfortunately there is a humming sound in the speakers connected to this amp and i hope there is a simple solution for the same.

    Description of the problem:-

    - No Audio (RCA) inputs connected = No humming at all (FINE)

    - If LEFT Audio Input (RCA) connected but RIGHT (RCA) Not connected= NO HUMMING and crystal clear (music) sound from the left channel speaker. (FINE)

    - If RIGHT Audio Input (RCA) connected but LEFT (RCA) Not connected= NO HUMMING and crystal clear (music) sound from the right channel speaker. (FINE)

    - Now this is when the problem occurs, when Both (R & L RCA) inputs are connected = Humming sound from both the channel speakers. (PROBLEM)
    Note: During humming if either one of the RCA L or R input is disconnected, then the humming sound simply stops in both the speakers.

    I tried using a different preamp, Mp3 and CD players, but the problem is the same as mentioned above.

    I'll deeply appreciate any help, comments and advises, Thank you.

    Best Regards.
     
    Last edited: Jul 10, 2011
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    These are the symptoms of a "ground loop" problem. The answer is usually simple, but WHICH answer can be difficult. A drawing of your setup will be very helpful. For now, we are hoping you have made a simple error, but we can't tell from reading words. We need a drawing.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You really would need to post a diagram (and a photograph of the setup might also be useful).

    My first guess would be that stray magnetic fields from the mains transformers might be inducing different voltages in the wiring to the two channels, which are only causing a problem when both channel inputs share a common connection.

    There are many other possibilities though.
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi lubnaan90,

    Do the two channels have a common ground at least? If the grounds are separate, but have a single connection point, then that connection point should be at the input jacks. With no input, there should be no current in these connections.

    Do you have a "Balance" control?

    Regards,
    Ifixit
     
  5. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello Ifixit,

    Yes, both the channels have a common ground, the chassis is also grounded well.
    No Balance control, I have built this amp without a balance and tone controls.

    Thank you for writing.
    Best regards.
     
    Last edited: Jul 8, 2011
  6. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello,
    Thank you for your Interest,
    I have attached some pictures which should give a better idea of my setup.
    Yes, I really hope that the humming is due to some minor Error and also hope that this can be rectified.
     
    Last edited: Jul 8, 2011
  7. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hi, Thank you for writing, I have attached some pictures of my Amp setup, please feel free to check it.
     
    Last edited: Jul 8, 2011
  8. Kingsparks

    Member

    May 17, 2011
    118
    5
    Hi.
    That is a very neat looking job. The block digram is useful but not really informative as to exactly what is happening.

    Do you have a full schematic? Are there more then one common ground, grounded at more then one place I mean?

    You are describing a classic ground loop as observe before and being that it is a "design" problem even a full schematic may not help.

    What is the input? Does it share a ground, other then the RCA plug with the amplifier?

    High marks for an excellent looking project, and try including more grounds from each amplifier and power supply. Check your design for the preamp if it is separate from the main amplifier.

    Most of all post a schematic please.
     
  9. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello,
    Thank you for your comments, really happy to understand that you like my project.
    I have attached a Schematic, which should be understandable.
    The RCA inputs does share a ground with every other ground points (PSupply Gnd, Chassis, speaker out gnd, etc) in the amp, I have double checked it using My Multimeter, the reading shows Zero.

    Thanks Again.
    Best Regards.
     
    Last edited: Jul 8, 2011
  10. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi lubnaan90,

    Measure between the two RCA jack ground pins, with the hum present, to see if a AC voltage is present. Use a scope, or DMM on low AC Volts.

    Can you high-lite in a photo how all the grounds were run?

    Regards,
    Ifixit
     
  11. #12

    Expert

    Nov 30, 2010
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    Studied this for at least 10 minutes. I say route the input wires against the case instead of right over the power transformers (for a starter). Then tell about the red wires that are coming out of the power transformers and not properly connected. Then tell about the 4 jacks on the rear that form a single line. I think they are the speaker jacks.
     
  12. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Thank you for writing,
    If you suspect that the input wires going right over the Transformers could be the culprit, then how is it that there is zero humm when no inputs are connected (with the input wires still right over the transformer), or zero humm when either of one input is only connected, As i mentioned, the humm only begins when both the L & R inputs are connected. Anyways, i will go ahead and try as recommended by you.
    The Red Wire: Input 220VAC cable to power the transformers.
    four Jacks: yes, your guess is absolutely correct.
     
    Last edited: Jul 8, 2011
  13. #12

    Expert

    Nov 30, 2010
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    Sorry about the wire routing tip. I just got carried away with using good layout methods.

    Perhaps I could help by defining your question. How could 2 conductors, laid in a magnetic field, have a voltage induced in them such that current only flows when their ends are connected together?

    I will leave this to other people to answer.
     
    Last edited: Jul 8, 2011
    lubnaan90 likes this.
  14. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hi, So what would you suppose can be done to avoid interruption of magnetic fields with the Input cables? Keeping the cables away from thee transformer is one thing, but is there any way to shield the cable further more?

    Best Regards,
     
  15. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Try moving the wires away from the transformers first to see if this is the cause of your problem. You should also keep the signal wires away from the power wiring.

    That said, your block schematic shows a lot of connections labelled "GND". If these are all linked together (via the metal case?) there may be lots of crosstalk going on.
     
  16. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello,
    Thank your for your reply,
    To begin with, I just disconnected the GND between the chassis and the Left channel Amp Board, Strangely the Humming sound seems to have completely stopped in both the channels, even when both L & R inputs are connected. However, The connection between the Right channel Amp board And the chassis is still maintained. I am just getting a little confused here, i mean it is a good thing that the Humming has stopped but how is it that When the Right amp Board GND and the Chassis GND is still connected and there is no humming?
     
  17. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello To All,
    It seems i have found the problem, The humming only starts when i combine the ground of both the transformers together. Any comments?
     
  18. Adjuster

    Well-Known Member

    Dec 26, 2010
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    There is nothing particularly strange about the humming stopping in both channels. The situation you have sounds more and more like being a "hum loop" aka "ground loop".

    Here, emfs induced around a closed path of wiring or metalwork cause unwanted currents to flow, but breaking the path at any one point stops the current. In your case, it seems likely that hum current is circulating around the inputs to the two amplifiers when they are linked to a common point. When you remove the ground connection to one of them, the hum stops for both.
     
    lubnaan90 likes this.
  19. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Deeply Appreciate your Explanation, well understood, Thank you very much.
     
  20. bertus

    Administrator

    Apr 5, 2008
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