12-01-2019, 04:57 AM
(This post was last modified: 12-01-2019, 04:58 AM by m42uko.
Edit Reason: spelling
)
On my PBP, I can also hear a fairly loud noise (whine) through the speakers under some conditions.
The noise in the circuit seems to be coming from the 5V regulator for USB + Periphery (VCC5V0_USB). Probably, it's operating under non-ideal conditions (too low load), which causes some ripple at "high" frequency, which then couples into the output power amps (U19/U20), and thus out the speakers. I haven't gotten around to do any real measurements yet, but here's my initial analysis.
The symptoms are:
* PBP idle, with nothing connected: No noise
* PBP idle, with low-power USB device connected (like a mouse): quiet noise
* PBP playing audio (but muted), with nothing connected: fairly loud noise
* PBP playing audio (but muted), with higher power USB device connected (USB drive): no noise
* Backlight brightness has some effect, but compared to the USB load changes, it's negligible.
* Headphone jack has no noise (it's directly driven from the DAC, and not through the power amp ICs)
Now about possible solutions. There is a filter in the power supply of the audio power amp (L11, C307, C308), which, however, is pretty much ineffective as the series inductor (L11) has been replaced with a 0-Ohm resistor (at least on my board). It might help replacing this part with either a properly dimensioned inductor or a low-value resistor (maybe around 10 Ohms?). Adding a 100 nF-ish capacitor in parallel with C307 might also help.
If we're unlucky, the noise doesn't couple through the power supply lines, but through data lines or even through the air. In that case, the solution would be a lot more tricky.
I don't have the components to test this right now, so I can't say for sure whether this will help or not.
Has anybody else tried anything like this already? Also, does anybody have the datasheet to the power amp (XA9108). This would help to estimate the proper values for the filter.
The noise in the circuit seems to be coming from the 5V regulator for USB + Periphery (VCC5V0_USB). Probably, it's operating under non-ideal conditions (too low load), which causes some ripple at "high" frequency, which then couples into the output power amps (U19/U20), and thus out the speakers. I haven't gotten around to do any real measurements yet, but here's my initial analysis.
The symptoms are:
* PBP idle, with nothing connected: No noise
* PBP idle, with low-power USB device connected (like a mouse): quiet noise
* PBP playing audio (but muted), with nothing connected: fairly loud noise
* PBP playing audio (but muted), with higher power USB device connected (USB drive): no noise
* Backlight brightness has some effect, but compared to the USB load changes, it's negligible.
* Headphone jack has no noise (it's directly driven from the DAC, and not through the power amp ICs)
Now about possible solutions. There is a filter in the power supply of the audio power amp (L11, C307, C308), which, however, is pretty much ineffective as the series inductor (L11) has been replaced with a 0-Ohm resistor (at least on my board). It might help replacing this part with either a properly dimensioned inductor or a low-value resistor (maybe around 10 Ohms?). Adding a 100 nF-ish capacitor in parallel with C307 might also help.
If we're unlucky, the noise doesn't couple through the power supply lines, but through data lines or even through the air. In that case, the solution would be a lot more tricky.
I don't have the components to test this right now, so I can't say for sure whether this will help or not.
Has anybody else tried anything like this already? Also, does anybody have the datasheet to the power amp (XA9108). This would help to estimate the proper values for the filter.