Loud high pitch tone over speakers

mngyuan

I'm getting this high pitch tone over my speakers from the audio out of my bela mini anytime bela is outputting audio. I'm using an Adafruit MAX98306 amp connected to two 8Ω 1W speakers. I don't have an oscilloscope but based on my ear its between E7 and F7 at about 2700Hz. It doesn't change in loudness and sounds like a triangle (or square kind of) wave. The weird thing is I don't get the tone when using my laptop's headphone out directly into the amp, and I don't get the tone on my headphones when I connect them into the same pins on the breadboard as what the amp's getting. It also only happens when bela is outputting audio, and it happens regardless of whether I use the audio out or the line level out.

@yannseznec recommended I buy a bela instead of a bela mini to not deal with speaker amp headaches and I completely ignored his advice for some reason 🙃 and I guess this is my karma. Is there a quick fix (seems like it'd be a common noob problem) or ... should I just get a regular Bela?

I snooped a bit in the forums and attempted a DC coupling circuit fix but it didn't seem like the right thing to do / didn't work.

giuliomoro

How is it wired up to Bela? Where does the power for the MAX98306 come from in each of the scenarios you describe?

mngyuan

Bela is powered thru USB from my laptop, and the speaker amp is getting power from the 5V on the mini. I swapped it over to the 3.3V and it cleaned it up immediately. Thanks, I hadn't thought of that.

giuliomoro

that's not what I was aiming for really ... 3v3 has a smaller current capacity than 5V, so your amp will distort at low power and/or the board will become unstable when it reaches the maximum current on 3v3.

3v3 is indeed cleaner than 5V on Bela, which could explain the improvement. However, the power-supply rejection rate (PSRR, i.e.: how much the IC can attenuate noise at the power supply) is 83dB for the MAX98306, which means that any noise on the power supply rail should be pretty quiet, so it's still a bit of a mystery.

What 5V were you using on BelaMini? If I remember correctly, P1.05 is the voltage straight as it comes through the USB port, so it's probably the best one to use, as it will be limited only by the USB bus's current limit and not by the on-board current limiter (which I believe is below 500mA).

Another thing I wanted to know is : when the amp is used like this:

when using my laptop's headphone out directly into the amp,

where is the Adadfruit board powered from?

Also, where do you get the ground pin that goes from Bela to the Adafruit board?

mngyuan

ah ok. Actually fiddled with it some more and realized I forgot to if you don't have differential outputs, simply tie the R- and L- to ground from the store page. That's eliminated the tone even on 5V.

I was using P1.24 but I've swaped over to P1.05 for now. Seems about as clean as P1.24, the tone is much, much quieter now.

when the amp is used like this: ... where is the Adadfruit board powered from?

It was from the same P1.24. Everything I was doing earlier was from there, I hadn't thought about changing where the power came form.

where do you get the ground pin that goes from Bela to the Adafruit board?

The bela ground is the same ground as the Adafruit, so I suppose it's the USB ground? And now I've got the R- and L- properly connected to it as well.

I'm gonna try connecting it to the wall later through a 12V adapter + DC step down converter to ~5V or straight from a USB wall charger, depending on whichever sounds less noisy.