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Original question:

PCM2704C: Asking for the recommendation on PCM2704C design

PCM2704C: some questions about the network and capacitor of the output

Alex Lee
Prodigy 190 points
Part Number: PCM2704C


Hello

There are two questions about PCM2704C I'd like to figure out.

1. is C10+R9 the Zobel network for stability? is it safe the remove it?

2. is R19+C14 the High pass filter? I saw the datasheet use an 32ohm headphone for reference. I wonder if this calculation is correct 1/2πRC = 1/2π(32||10k)(100) = 49.9Hz. The result seems not good enough for the bass?

Thank you!

  • 0
    TI__Mastermind 31561 points

    Hello Alex.

    Yes, C10 along with R9 is Zobel network to provide a path for the speaker current when it is flowing in the other direction. Although it is recommended to leave it, if you have to , it  should be okay to remove it.

    Also the R19 is not a part of HP filter, it is there mostly as a discharge path for that node.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 190 points

    Hi Arash

    Thanks for the quick reply!

    Would you please explain more about how the R19 works as datatsheet metions "depending on tradeoff between required THD performance and pop-noise level for suspend" ? I saw the value is 330ohm on datasheet, but 10k in EVM board. quick confuse how to choose the value.

    And for the HPF, does it completely depend on the C14 and impedance of load? if the headphone is 
    32ohm, the cut off frequency is 1/2π(32)(100) = 50Hz
    16ohm, the cut off frequency is 1/2π(32)(100) = 100Hz
    it seems still not so well in bass region?

  • 0 in reply to Alex Lee
    TI__Mastermind 31561 points

    Hi Alex, 

    The R19 provides a discharge path, when the cap connected to that node is charged and suddenly audio is stopped , you might hear a pop .The value of the resistor effects the  distortion at the output vs the loudness of the pop noise. So depending on your need for pop free vs. less distortion you can select the value for that resistor in the range that is suggested in datasheet. I believe  the EVM value is for better THD, but I don't have the board to confirm it.

    Besides the Zobel network and discharge resistor,  there might be  an  AC coupling cap which is connected b/w the output node  and load .

    Before that you might  have a LP or HP filter-depending on what freq range you want to let it pass.

    Please refer to Figure 36. in datasheet for the HP filter connection along with the Headphone.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 190 points

    Thanks Arash

    I just check the Figure 36

    R7 is the discharge resistor, C9 and R10 should be the HPF? here come to my question at the beginning.

    if the impedance of headphone is
    32ohm, the cut off frequency is 1/2π(32||3.3k)(100) =~ 50Hz
    16ohm, the cut off frequency is 1/2π(16||3.3k)(100) =~ 100Hz

    Not sure if this is the correct calculation...

  • 0 in reply to Alex Lee
    TI__Mastermind 31561 points

    Yes, you marked the HP correctly and the calculation is correct. If you need a different cut off freq simply change the  cap value.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 190 points

    o....got it. So the R10 is not necessary because the cut off frequency mainly depends on the imdepance of the headphone. That's why the EVM board remove it.

    Thank you so much Arash.