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PCM3070 60Hz Loud Hum

Max Makhnyk
Prodigy 155 points
Other Parts Discussed in Thread: PCM3070

I have followed the reference design as closely as I possibly could for the PCM3070. After getting my board back and connecting it together, I get a VERY loud hum at line out. I have it connected to an amplifier to test audio and after measuring with an oscilloscope, it exactly 60Hz which I can assume is like a ground loop, even though shorting GND and CHASSIS does not help. I am relatively new to audio, including the PCM3070 chip and would love to know what is going on. 

As of right now, I cannot get the Hum to turn off nor can I get any audio to pass through.

Also; I cant use PurePath since its only for people that use the exact same setup as the eval boards, so if anyone has additional scripts (for example: Analog pass through) I would be very thankful.

Thank you! 

  • 0
    TI__Mastermind 27850 points
    Hi Max,

    I would start by probing the power rails to make sure they are the correct voltage. Next, to find the source of the hum, probe the signal path starting from the PCM3070, do you see the 60 Hz hum, if not probe until you do find it. How are you connecting the PCM3070 to the amplifier?

    Justin
  • 0 in reply to Justin Bohr
    Prodigy 155 points

    Hello Justin,

    Thank you for the quick response. I have probed all the rails, and I am getting the correct values. 

    LDOIN and LDO_SELECT: 3.29V

    IOVDD: 3.29V

    AVDD: 1.69V

    DVDD: 1.69V 

    REF: 0.75V

    Probing for the 60Hz hum is a little more difficult at the moment. For some reason my oscilliscope is reading 0Hz everywhere, even on MCLK. Ill get back to you if I figure out the problem on this.

    After messing with the PCM3070 all day yesterday and today, I manage to get audio to pass through after soldering a headphone jack to the HPL/HPR lines. This was also accomplished by study each register and writing the ones related to HPL/HPR, LOL/LOR and IN1L/IN1R etc. As of right now, I am able to bypass the Line in signal directly to the Headphone pins, without any disturbances. The gain adjustments work well and audio is crisp and clear.

    I got a a small bit of audio to go through the Line level out pins, but the 60Hz hum is still present and loud. I cranked up the signal gain up to 20dB on Line Out, and the hum was identical in amplitude, but the music was quite a bit louder. So the 60Hz hum was not amplified. It could be a possibility that my external amplifier is not working well (ST Micro part), but I just tested it on a DVD player and there is no 60Hz hum there.

    Thank you!

    Regards,

    Max

  • 0
    Prodigy 155 points

    60Hz Hum has been resolved!

    For anyone having a similar issues, remember you are sending out an UNBALANCED signal, so if the amplifier you are going into is looking for a BALANCED signal, you will have a whole lot of noise. 

    This was solved by grounding LEFT(-) and RIGHT(-).

  • 0 in reply to Max Makhnyk
    TI__Mastermind 27850 points
    Hi Max,

    Thank you for posting your solution, I am glad you were able to solve it.

    Justin