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dennis
dennis
Associate II
June 5, 2016
Question

line level audio out?

  • June 5, 2016
  • 3 replies
  • 926 views
Posted on June 05, 2016 at 15:42

I'm using the STM32F746G to play with some audio DSP stuff and it's working fine with headphones but when I connect the output to external amps I get whine and low signal level.  I'm using the

OUTPUT_DEVICE_HEADPHONE

from the demo.  I would expect it to be a fairly strong line level signal.

Any tips?

Thanks.

P.S. I would spend more time searching this forum if it were not so painfully slow.  They must be hosting on a 286.

 
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    3 replies

    dennis
    dennisAuthor
    Associate II
    June 5, 2016
    Posted on June 05, 2016 at 22:18

    Still wading thru the datasheet for the WM8994 DAC but it's fairly complex.  Is there any chance the headphone output is class D and the lack of an apparent signal there as a ''line output'' is due to lack of inductive/capacitive loading?  What would I put in place, just a single pole LPF?  470 resistor 0.01 cap maybe and tap signal off the cap?

    dennis
    dennisAuthor
    Associate II
    June 6, 2016
    Posted on June 06, 2016 at 19:40

    At this point I'm just presuming the audio path on this board is not super pristine.  I seem to see a lot of crosstalk between the line inputs and the headphone outputs and that in turn limits my levels.  So I just do a balancing act between levels and noise floor.

    I'm open to suggestions/workarounds though obviously.

    dennis
    dennisAuthor
    Associate II
    April 17, 2017
    Posted on April 17, 2017 at 04:42

    I posted this question almost a year ago.  I still have low level whine in the audio output.  Turning the volume for 0 for the DACs doesn't seem to affect it so it seems like it's in the hardware.

    Any ideas?  Anyone ever added some more uF across the power supply?  Tantalum?

    Dennis

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