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I'm using a PCM5122 to output audio to a line level output as well as a PAM8406 amplifier.

If I connect the PCM5122 to the PAM8406 AFTER the output load RC filter, then there is noise introduced into the PAM8406 (exterior noise that can't be filtered).

What does work is connecting the output of the PCM5122 directly to the PAM8406 through a ferrite bead, into a 1uF DC blocking capacitor and into the 18k input resistor on the PAM8406.

My question is: Are there any potential issues that would arise from not using a load resistor?

Here is the output side of the PCM5122: enter image description here

Here is the input side of the PAM8406: enter image description here

Notes:

  1. The PCM5122 output filter differs from the TI recommendations because the extra output load thwarts the noise present on the output (it's inherent to the system that the unit is being plugged into).

  2. Output gain on the PCM5122 is 0dB right now. I may actually tune that further down to prevent clipping via the device gain registers.

  3. The amplifier is set to 24dB gain as the system it's expected to drive an in-car telephone system (and must therefore be quite audible). The PAM works well in its current configuration, FWIW.

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    \$\begingroup\$ A bit more info would be useful. Why the PCM5122 output filter is different from datasheet recommendations? It now presents 4x more load on output than the original filter. And what is the output gain (volume) setting, and what is the expected digital range used? I mean, the DAC can push out 2VRMS/5.6VPP ad 0dBFS signal, and I see that the amplifier is set to 24dB gain and it uses 5V supply, so I am just trying to double check if what you are doing makes any sense. \$\endgroup\$ – Justme Oct 31 at 20:29
  • \$\begingroup\$ @Justme - Updated my OP with answers to your questions. \$\endgroup\$ – t3ddftw Oct 31 at 21:00
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The PCM5122 datasheet says it supports both DC and AC coupled loads, if the load is within specification. Minimum load resistance is 1k and typical is 10k, but there is no maximum resistance so 18k AC coupled into inverting PAM8406 input should be fine. But when the audio output to PAM8406 is taken directly from the chip instead of taking it after the output filter, there will be more high frequency modulator noise present, and in general this should not be fed into the next stage. But if this fixes your issue it might be fine. My only advice is that carefully design the best balance between digital signal amplitude, digital gain, analog output amplitude and analog gain, so that digital domain and analog domain do not clip, and so that least analog gain is used not to amplify noise. And as I've used that chip before, it was hard to make pop-free (they even removed that from datasheet because it wasn't pop free), everytime it gets stable clocks after a pause, there is a pop on analog output which you don't want to amplify.

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  • \$\begingroup\$ Perfect -- Thanks @Justme. I actually ran into the same pop issue on the PCM5122 as well. My fix was to feed a volume-less C# note to the system on boot and then disable the SCK/BCK watchdog (register 0x25) to prevent the PCM5122 from entering power down mode moving forward :) \$\endgroup\$ – t3ddftw Oct 31 at 22:22

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