High frequency sound (noise) at audio output

giuliomoro

that really depends on the design of either ... power banks use LiPo + voltage booster themselves internally.

Mind you, I am not saying the problem is not the wiring of the grounds. However, we can't look into it until we figure out the source of the noise! Did you verify via the recording method whether the signal before being sent the DAC is clean?

I see from the schematics that you are using a PAM8403 breakout board. Did you try listening to the output straight from the headphone output of Bela ? Is that noisy already or does the noise only occur in the outputs of the PAM8403?

luis

Yes, you are right. I didn't test it yet because I can't change anything now before the concert. I made a PCB with the Bela on it, so it is not easy to change anything, but it certainly is worth trying the Audio Outs instead of the Line Outs. I'll work on it and report here. Thank you again for your precious comments.

luis

Hi Giulio, here's what I have done until now:

recorded the signal from the audio output directly from the software. Result: no noise.
listened to the sound through the phones output: no noise
visualized the output of the voltage booster with an oscilloscope. Result: there is a tiny but perceptible audio frequency about 1400 Hz. I could amplify it and listen to the signal.

After that I tried to use the amplifier with the 3.3 V output of the Bela. Result: the amp works (although the spec. is for 5 V power supply) and there is no noise.

So now it works has I would like it to and there is no noise at the audio output.

My question is: is there any issue when using the 3.3. V from the Bela to power the power amp? What would be the max current usable?

Thank you very much for your support!

giuliomoro

luis the amp works (although the spec. is for 5 V power supply) and there is no noise.

What amp? There's only about 100mA on the 3.3V line and part of that is used by the cape itself.

luis

The amp is the PAM8403.

giuliomoro

Right, that can be used down to 2.5V, so you wouldn't be operating it out of specs if using 3.3V. However, the current limitations on that rail are probably going to be a severe limitation. What speaker impedance are you using?
As you see from this table it can drive up to 1.3W at 3.2V (i.e.: 406mA) on 4 ohm, or 0.9W at 3.2W (i.e.: 281mA) on 8 ohm, both far above the current capability of the 3.3V line.

You could see if you can add a filter to the 5V power rail to the PAM8403 to make things quieter. Consider something like a zener or a LC low-pass filter. Or an adjustable LDO (low dropout) linear regulator or step-down DC-DC converter targeting 4.5V?

luis

Ah, right. Thanks for the research. That is really very useful.
I tried some condensers at the output of the power booster but with no effect. So a more elaborated filter would be the next step.

luis

One thing I didn't try yet: would it work if I use directly the LiPo to power the amp? This should have no noise.

giuliomoro

Yes that would work.

What booster are you using? The noise may well be coming from poor filtering there. Figuring out what IC is in use and then whether they are following the manufacturer's recommendations for that chip would be beneficial. It may be that you could "fix" it.

luis

OK great, that could be the simplest solution. I'm using the Sparkfun 5V/1A LiPo Charger/Booster.

giuliomoro

Their schematic is identical to the switching regulator's datasheet so there's probably not much that can be done there ... It's weird you'd get 1400Hz: allegedly this device operates at 1MHz.

Could it be a wiring issue? How are you wiring the booster to the headphone amp and the headphone amp to the Bela?