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TPA6120A2: Buzz without earth ground connection

James VDC
Prodigy 240 points
Part Number: TPA6120A2
Other Parts Discussed in Thread: ALLIGATORHello. When utilizing TPA6120A2 with various SMPS devices I am encountering quite loud buzzing if a solid earth ground connection is not made. Two Examples: 1. VDC is supplied via a two prong wallwart. 2. VDC is supplied via a enclosed SMPS that has an internal capacitor between the incoming "earth ground" and the DC output "gnd" In either instance, if I establish a connection to the safety ground pin at the wall outlet itself, or via another piece of equipment (ex. alligator clip between headphone circuit ground and the chassis of another piece of equipment that is plugged into the wall outlet and shows continuity from its case to the safety ground at the outlet) the buzz is completely eliminated and the headphone circuit runs extremely quietly. Is there a method of eliminating this buzz without providing a direct earth ground connection? Thank you
  • 0
    TI__Guru* 95385 points

    Hi James,

    That is called a ground loop.
    It is most likely that the audio source of TPA6120A2 has a different ground reference.

    You can take a look at this App Note about Ground Loop Break circuits, it mentions TPA613x and DRV60x but apply to this device as well: http://www.ti.com/lit/an/sloa143/sloa143.pdf

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators

  • 0 in reply to Ivan Salazar
    Prodigy 240 points
    Hello Ivan and thank you for your response. I believe that you are misunderstanding. The issue at hand is not a ground loop. The TPA6120A2 is the last stage in my circuit. It is being fed directly from a preamplifier circuit. They share the same ground on the same PCB. The solution to the "buzz" I am encountering at the output of the TPA6120A2 is not established by breaking any ground loops. It is established by making a solid earth ground connection to the circuit ground at the wall outlet. In the two scenarios I described in my initial post, the power supply for the audio circuit was either a SMPS Wallwart with 2 prong connection on the AC side or an enclosed SMPS with 3 prong connection on the AC side, but an internal capacitor between the AC earth ground and the DC output ground. In either scenario, there is a buzz at the output of the TPA6120A2 and the only manner I have found to cancel the buzz is to make a direct connection between the circuit ground and AC earth ground. For instance, with the enclosed supply, jumping the internal capacitor directly connects the AC earth ground to the DC output ground and thus eliminates the issue. There is no ground loop being broken; it is the introduction of a solid earth ground connection that eliminates the issue. I am looking to see if there is another way around this in order to use the SMPS wallwart and also eliminate the buzz. Thank you for your help with this .
  • 0 in reply to James VDC
    TI__Guru* 95385 points

    James,

    That still sounds like a ground loop, it is not at the amp, but at your supply.
    Your short from AC to DC side on the SMPS would be the Ground Loop Break.

    One thing I can think of is about your input signal to the amp. Are you using single-ended or differential input? It would be best to use differential, perhaps it can reduce the impact of this noise, if it's coming along with the input signal.
    Are you able to inspect the signals along the path? You could be able to identify where is the noise source.
    Other way to try this is to disconnect the amp from the input and check if the noise is still present; if the noise is present even when the input is not connected then the noise would be coming from the supply/GND, in that case you can try to identify the frequency and apply filtering at the supply input to the amp.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators

  • 0 in reply to Ivan Salazar
    Prodigy 240 points
    Ivan, I'm going to try the troubleshooting you suggested. When you wrote "disconnect the amp from the input" are you indicating to disconnect the headphone amplifier from its input? Or disconnect the input of the preamplifier circuit itself? In either respect, wouldn't leaving the input floating create an antenna and induce more noise? Thank you very much for helping James
  • 0 in reply to James VDC
    TI__Guru* 95385 points

    James,

    I mean disconnect the headphone amp from its input.
    What I try to know is if the noise is coming from previous stage, or its due to supply/GND coupling noise.

    You could try floating or grounded inputs as well.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators

  • 0 in reply to Ivan Salazar
    Prodigy 240 points
    Ivan, I went back and tried some further troubleshooting. I was able to find that if I shorted the input to my preamplifier the headphone output no longer had any buzz at all. Further, if I then injected signal into the circuit at various other point,s downstream from the first preamp stage, the headphone output was robust and free from any buzz. Fortunately, it seems there isn't an issue with the headphone amplifier or its connection to the previous stage. Unfortunately, it appears there is some issue with the front end of the unit. Thank you for all your help James
  • 0 in reply to James VDC
    TI__Guru* 95385 points

    James,

    It is good to know that you've narrowed down the source of the noise in your system.
    Please let us know if you have any further questions about our devices.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators