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LM4911Q-Q1: Audio output to headphones is extremely quiet

Erik Friedel
Expert 6105 points
Part Number: LM4911Q-Q1
Other Parts Discussed in Thread: PCM1690, LM4911

Hello Experts,

I am using the LM4911QMM/NOPB with the PCM1690 DAC to have an audio output to a 3.5mm jack. The 8 outputs of the DAC are brought together to be mix-left and mix-right as shown in the schematic below:

MIX-LEFT and MIX-RIGHT are used as the inputs to INA and INB of the LM4911QMM/NOPB. The external passive components follow what is detailed for a capacitive coupled output as shown in the schematic below:

And line-output4L/R are routed to a 3.5mm audio jack. 

Do you have any ideas as to why the output volume is so low for the stereo output to a speaker?

Thanks in advance,

Erik

  • 0
    TI__Guru** 105835 points

    Hi Erik,

    Could you please share some captures of the audio signal at different spots of the circuit?

    • Before (input side) of C117 and C119
    • After (output side) of those same caps C117 and C119
    • At the output of the amplifier VOA and VOB

    Are you testing with conventional 16-ohm or 32-ohm speaker loads? Would it be possible to connect an external audio signal source directly to LM4911 in order to isolate this part of the circuit?

    Please feel free to follow up either here or through the email thread. We can also debug in person if possible.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    TI__Expert 6105 points

    Hello Ivan,

    I attached the capture using a Saleae:

    Before C119 was 0V, After C119 was 2.34V, Before C123 was 2.43V

    I was able to confirm that if I connect an audio signal directly from the source that it outputs on the speaker clearly at a normal volume. I did this by connecting an audio source to MIC input 4 and using a jumper cable from the output side of C98 to the input side of C119. 

    .

    Do you have any insight as to why the McASP->DAC->LM4911->3.5mm jack produces such quiet volume? I have no issues with the audio output of MIX-LEFT and MIX-RIGHT as mono outputs to 3.5mm audio jacks. 

    Are there any additional debug steps that I can try to try to overcome the volume issue?

    Regards,

    Erik

  • 0 in reply to Erik Friedel
    TI__Guru** 105835 points

    Hi Erik,

    The captures you show are without any audio signal at the input, correct? I just see DC voltages; it would be meaningful to see an actual sinewave or similar test tone to check how the amplitude changes along the signal path.

    Regarding your last test, you mean using an analog input signal directly to the input of LM4911 works better? I was concerned about SD and MUTE pins, but if you have a good output with a different input signal source then that may not be a problem.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    TI__Expert 6105 points

    Ivan,

    Removing the capacitors as we discussed caused the LM4911 to work as expected.

    Regards,

    Erik