Noise form PAM8403 amplifier board?

[sanchaz12]

Hello everyone,

Recently I got a mini amplifier board based around the PAM8403 2x3W stereo amplifier chip connected to a pair of 4 Ohm speakers for maximum power output.
Everything seems fine until I connect one specific device, my LG G2 smartphone. When I start to play music there is a huge amount of noise present, which seems to blend in with the music when increasing the volume level. When the music stops and the phone's DAC enters presumably a 'sleep' mode, the noise is gone.
Maybe I am doing something wrong in my wiring. I tried to keep the wires as short as possible and used shielded cables where possible.
Another thing to note is that the power supply used does not affect the level of noise. When connected to a 5V AC adapter or a mobile power bank, there is an equal level of noise.
Connected to other devices (MacBook Pro -with or without being connected to charger-, despktop PC or a Sony Walkman MP4 player) there is no audible noise present.

Attached are some pictures of the board and wiring and a recorded MP3 of the noise when connected to the LG G2 and lowering/increasing the volume.

As reference, here are the datasheets of the PAM8403 amplifier, and the Wolfson WM5110 DAC used in the LG G2:
https://www.openhacks.com/uploadsproductos/pam8403.pdf
https://www.alpha-audio.nl/wordpress/wp-content/uploads/2013/05/Wolfson-wm5110.pdf

[fgomes]

Hi, sorry for such a late reply, but at least as a reference to other users with similar problems, it seems to me that the problem is that the audio out from your mobile probably is also a Class D amplifier, and the noise you are hearing is caused by the sampling done by the PAM8403 of the frequency used by the Class D out of the mobile phone amplifier. Try to use a low pass filter between the mobile phone audio output and the PAM8403 input and see if it solves the issue. This is probably the reason why you don't hear this problem with the MAC audio out (I don't know if it is Class D also, but if it is it could have a good output filter).

Fernando

[djococaud]

Hi, I had the same problem with a tablet connected to a small amplifier. Once the tablet was connected to the mains charger, the audio was full of switching noises...

Your problem is that you have to isolate the grounds in the audio path.

I placed a small audio transformer (600 ohms, 1:1 ratio) between the tablet audio output and the amp input, and now, all the unwanted noise is gone !!!

[dteck]

I had a lot of trouble with these as well. I bought a 5 pack of them off ebay and was using them to try out some bluetooth audio ideas I had. I had the noise problem just like you did. I also had the problem of it resetting my BT module when the amplifier would start up. That didnt happen all the time but there were huge voltage dips that required adding some bulk capacitance after a regulator just for the BT modules.
Really what made me give these up was the horrible clipping I ran into on the audio out. That not only sounded horrible but again caused a bunch of noise on my power lines.

I moved over to the MAX98306. Its about the same power and it has differential inputs (helped cut the noise out). It also handles clipping better and it doesn't suck up all the juice when it powers on. Its was only a few bucks more to spin my own boards so for me it was worth it YMMV

[eddythebreaker]

I had this problem when playing with a library TMRpcm for arduino. I solved this using that low pass filter that Fernando pointed (https://en.wikipedia.org/wiki/Low-pass_filter) and it worked well!

I only had to play with values of the capacitor and resistor to get a best result. In my case resistor=150ohm and capacitor(I used a filter)=3.3uf worked well.