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DRV135: How to remove Hissing Noise

Aman Shah71
Prodigy 210 points
Part Number: DRV135
Other Parts Discussed in Thread: PCM1681,

Hello,

We are working on a design where we used PCM1681 and DRV135 to get balanced output. The design is completed but we have an issue in audio quality. When we plug additional audio mixer (such as iRig Pro) on XLR output then it gives Hissing sound in speakers. But when we plug direct speaker from XLR, we are not getting any noise in it.

Test Setup flow: Audio from PC -> PCB -> XLR Output -> Audio Mixer -> Speaker

I am attaching partial schematic of Board. Please review. We have placed 10uF Aluminum Capacitor in series of Sense+ and Sense- signal and XLR input. When we place film capacitor in between the output signals, we are able to reduce the hissing noise (In the case of additional mixer).

Please let us know how to remove Hissing noise at the output side?

Thank you in Advance.

Regards,

Aman ShahPCM1681&DRV135.pdf

  • 0
    TI__Genius 12890 points
    Hi Aman,
    This sounds like it might be a ground loop issue being created by the additional device, especially if a direct connection between the device and the speaker does not produce the noise. It might be worth checking if breaking the ground signal (pin 1 on your xlr) with a capacitor helps fix the issue.
  • 0 in reply to Alex Davis
    Prodigy 210 points
    Hello Alex,

    Thank you for your quick response. We will check this thing and see if it can help to fix it.
    But, I would like to add one point. The chassis ground (Pin 1 of XLR) is not connected to Analog or digital ground on the board.
    So is it creating any issue?

    Regards,
    Aman Shah
  • 0 in reply to Aman Shah71
    TI__Genius 12890 points
    Hi Aman,
    Not having a path for CGND to AGND could cause an issue if the common mode level of the outputs is different on two different pieces of equipment.
  • 0 in reply to Aman Shah71
    Guru 140200 points
    Hi Aman,

    as Alexander already mentioned, the cable shield should be connected to AGND as shown in figure 29 of datasheet. The easiest way to provide this is to connect AGND to the chassis.

    Sometimes it's a bit tricky to find a good place for doing this, but it should be near the place where the cables enter or leave the chassis. It helps when all cables enter or leave the chassis at the same place. Connect AGND to the chassis only at one point to avoid ground loops. If you want to connect signal ground to chassis at several points to improve the shielding you can make the connection by the help of hybrid grounding. This technique uses a 1...10n cap for the connection and prevents low frequency hum loops.

    If digital ground return currents contaminate the AGND, you could additionally shield the digital setion and reference the shield to the common AGND/DGND point near the PCM1681.

    Kai
  • 0 in reply to kai klaas69
    TI__Genius 11135 points
    Aman

    We haven't heard back from you so we assume this resolved your issue. If not, post another reply below.

    Thanks
    Dennis
  • 0 in reply to Dennis Goeke
    Prodigy 210 points
    Hello Dennis,

    Sorry for the delay, but at this point I would like to close this issue.

    Thanks all for the help.

    Regards,
    Aman Shah