MAX7219 and PAM8403 : amp pickup noise from MAX7219

Hello !
I have a project with the following elements:

  • NiMh 6V batteries pack powering MCU (arduino nano) and DC-DC busck converter ;
  • XL4015 DC-DC buck converter tuned to 5V and powering the rest of the circuit ;
  • MAX7219 drinving a 10 leds bargraph ;
  • DYPlayer storing and playing audio file on command from MCU ;
  • PAM8403 : input from DYPlayer and output to speakers ;

When turn on, humming noise synchronised with the bargraph(MAX7219) is produced by the PAM8403.

If I plug the speaker in the DYPlayer, sound is A1, but not load enough for my needs.
On the MAX7219, I have a 110uF and a 0.1uF capacitors accross VIN and ground ;
On the PAM8403 I have a 3300uF and a 0.1uF capactiors accross VIN and ground ;

Adding 0.1uF capacitor to the PAM8403 Vin didn't seem to change anything about the noise...

I did not make any particular arrangement regarding grounds, they are mostly chained... It's a DIY pcb design, so it probabaly needs some more love in the design since I'm only an hobbyiste.

So what can be made to reduce/eleminate noise picup from MAX7219 ? I would like to redesign my board with improvement. Any hints are welcome.

I could use 2 separate batteries packs and DC-DC converters for AMP and the rest of the circuit, but it's expensive and maybe overkill ?

thank you !

My old project was based on MAX7219 and was a stereo level indicator. The indicator worked very well visually according to my requirements, but when I connected it to an amplifier I was unpleasantly surprised by noise in the speakers. I used a specially designed by me board professionally made by a company for PCB orders. I thought that the problem was with the power supply, but it turned out that when I move and rotate the board in space, the noise changes. For this reason I decided that the noise was from induction. I tried to replace the chip with MAX7221, which is exactly recommended for less interference, but the result was almost the same. If you find a solution I will be glad to share it.

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Someone suggested me to use the SN74HC595 8bit shift register because it produce less noise the way it updates the output: I think it's not using PWM to update or control leds current. Did not try it yet, but migth be a way to work around the grounding and decoupling required here because I'm not sure I can acheive those.

For a 10 leds bar graph, two SN74HC595 are required in cascade. Resistors are also needed for eache bargraph led. I might use a resistors array for convenience...

It is true that the 74HC595 chip controls the LEDs statically and not dynamically like the MAX7219, and the noise would be much less or not there at all. You might consider using smart LEDs like WS2812 or PL9823 for example. So with only 1 pin you will control all the LEDs. I'm not sure if there will be any noise but it might be worth a try.

I've a lot of WS2812 on the same circuit with no noise. But I can't fit them in a small bargraph arrangement. I'll give a try with the 595 shift register for the bargraph.

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