12-02-2019, 08:52 AM
So I just took some time to mess around with the audio circuitry on my PBP. I desoldered L11 (0 Ohm resistor) and connected some wires so that I can easily try different components. Long wires aren't ideal, especially when troubleshooting EMI issues, but as you'll see from my findings below, my setup shouldn't be a problem here.
Okay, so here's what I tried:
Replace L11 with a resistor.
75 Ohms: Noise goes down noticeably (but not gone), but the audio amp cuts out (audio gone for half a second, then back for half a second) already at low volumes.
23 Ohms: Noise goes down only a bit, and still cuts out at high volumes
Replace L11 with an inductor:
Random one I had lying around: No effect whatsoever
Add a 100 nF in parallel with the PA supply:
No effect.
Power the PA from my external lab power supply:
No noise, full volume achievable. (That also sorta validates my test setup.)
Just because it might be interesting, here's the power draw of both power amps:
disabled: 0 mA
idle: 20 mA
quiet listening: ~30 mA
full volume: up to 300 mA
That's a lot more than I expected, and it explains why the RC filter above caused the amp to cut out, but it's what we have to work with. Also, I was hoping I could maybe run the PA off of 3.3 V, which would've allowed to maybe add a linear regulator, but I tried it with my lab supply and it started to cut out at higher volumes already at 4.5 V.
So, in summary, I don't have a solution yet. At least we know for certain now where the noise is coming from (the 5V supply).
Any ideas on what else I could try? I guess taking a look at the source (the 5V regulator) might help (and that would be the proper way to do it), but it's also the one that's a lot harder to fix now that the boards are finished. Not to mention it's way easier to fry the board by playing around with these components.
Thanks,
Markus
PS: Notebooks with random wires coming out of the side sure look fun^^
Okay, so here's what I tried:
Replace L11 with a resistor.
75 Ohms: Noise goes down noticeably (but not gone), but the audio amp cuts out (audio gone for half a second, then back for half a second) already at low volumes.
23 Ohms: Noise goes down only a bit, and still cuts out at high volumes
Replace L11 with an inductor:
Random one I had lying around: No effect whatsoever
Add a 100 nF in parallel with the PA supply:
No effect.
Power the PA from my external lab power supply:
No noise, full volume achievable. (That also sorta validates my test setup.)
Just because it might be interesting, here's the power draw of both power amps:
disabled: 0 mA
idle: 20 mA
quiet listening: ~30 mA
full volume: up to 300 mA
That's a lot more than I expected, and it explains why the RC filter above caused the amp to cut out, but it's what we have to work with. Also, I was hoping I could maybe run the PA off of 3.3 V, which would've allowed to maybe add a linear regulator, but I tried it with my lab supply and it started to cut out at higher volumes already at 4.5 V.
So, in summary, I don't have a solution yet. At least we know for certain now where the noise is coming from (the 5V supply).
Any ideas on what else I could try? I guess taking a look at the source (the 5V regulator) might help (and that would be the proper way to do it), but it's also the one that's a lot harder to fix now that the boards are finished. Not to mention it's way easier to fry the board by playing around with these components.
Thanks,
Markus
PS: Notebooks with random wires coming out of the side sure look fun^^