line level audio out?

dennis · ‎2016-06-05

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.

dennis · ‎2016-06-05

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 · ‎2016-06-06

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 · ‎2017-04-16

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