Rpi4 + USb Sound Card + PAM8403 Amplifier Whine/Noise

[bangity]

Hi All,

I am making a Game handheld system with the Rpi4. Some of the problems I encountered and what I have tried so far:

1. Tried using the 3 ports of the TRS jack of the RPi4 but I could not eliminate the buzzing noise when connected to the Amplifier PAM8403 (5V to 2x3W)

2. I then bought a USB soundcard and run the wires from the TRS jack there. The sound is crisp without the Amplifier. But when amplifier is connected, the noise returns but less than if connected to Rpi4

3. Then I made a 600:600ohms audio transformer. This has greatly reduced the noise. BUT whenever it is close to the PSU or power wires, the whines get worse. The closer it is the worse, about 2 inches away it's barely audible. However, when you put your ears to the speaker, it's there.

4. Then I connected a 10R resistor to each of the + output to speaker, and this has further reduced the noise but volume is a little less.

5. What I have found is that, when I connect any external speakers to the switching TRS jack, the audio is very very low, bare hear at maximum. If I connect the Common Ground of the audio transformer, to any power ground, the audio becomes loud again but the noise returns.

I have tried grounding to avoid common ground but this wasn't the main issue as I have tried external power to the amplifier but the problem remains.

Here is a schematic.