The PAM8403 amplifier is very noisy

Decided to quickly try that amp board, my results won't necessarily help at all since my setup is completely different and I don't have LEDs/microcontroller attached. I'm not near 'scope so I couldn't grab any traces.

I'm using a very high quality audio cable (double shield), but there is about 3 cm bare at the board end. Speaker is 8 ohm (I only wired to the right channel).

For the power supply, I plugged into an Anker mains-to-USB adapter.

It's quite a noisy amp, there's reasonably noticeable hiss. With the 3.5 mm plug unplugged, I hear mains buzz; it needs to be shorted for that to go away, leaving just the hiss.

Then I plugged the audio cable into a battery-powered MP3 player. It sounded fine.

However, then I attached the audio cable to my laptop instead, which was powered from another port on the same Anker adapter. It was intolerable, lots of electronic noise sounds. If I unplugged the laptop and let it run on batteries, then the sound became acceptable again.  

I tried a few things; additional decoupling (220uF electrolytic cap on the power rails, no difference. Then added a choke preceding that (a 1 mH common-mode choke with the windings wired together to be a normal single choke), still no difference. Then removed the electrolytic and re-soldered the choke as common-mode, then tried two of them : ) and also tried adding it to the audio connections too. Still no difference.

Then I remembered I was not using the official laptop charger - I attached the laptop to the correct charger, and the amplifier remained on the Anker one. Noises gone. The official laptop one has 1k resistance from the USB shield to the earth pin. The Anker one doesn't have an earth connection (i.e. SELV).

I'm guessing perhaps the laptop puts enough noise into the charger, that it egresses on all the other ports (common-mode or differential or both - don't know, guessing both!), and the filtering I was doing wasn't sufficient - I should have tried both differential and common-mode simultaneously but I didn't get a chance.

Decided to quickly try that amp board, my results won't necessarily help at all since my setup is completely different and I don't have LEDs/microcontroller attached. I'm not near 'scope so I couldn't grab any traces.

I'm using a very high quality audio cable (double shield), but there is about 3 cm bare at the board end. Speaker is 8 ohm (I only wired to the right channel).

For the power supply, I plugged into an Anker mains-to-USB adapter.

It's quite a noisy amp, there's reasonably noticeable hiss. With the 3.5 mm plug unplugged, I hear mains buzz; it needs to be shorted for that to go away, leaving just the hiss.

Then I plugged the audio cable into a battery-powered MP3 player. It sounded fine.

However, then I attached the audio cable to my laptop instead, which was powered from another port on the same Anker adapter. It was intolerable, lots of electronic noise sounds. If I unplugged the laptop and let it run on batteries, then the sound became acceptable again.  

I tried a few things; additional decoupling (220uF electrolytic cap on the power rails, no difference. Then added a choke preceding that (a 1 mH common-mode choke with the windings wired together to be a normal single choke), still no difference. Then removed the electrolytic and re-soldered the choke as common-mode, then tried two of them : ) and also tried adding it to the audio connections too. Still no difference.

Then I remembered I was not using the official laptop charger - I attached the laptop to the correct charger, and the amplifier remained on the Anker one. Noises gone. The official laptop one has 1k resistance from the USB shield to the earth pin. The Anker one doesn't have an earth connection (i.e. SELV).

I'm guessing perhaps the laptop puts enough noise into the charger, that it egresses on all the other ports (common-mode or differential or both - don't know, guessing both!), and the filtering I was doing wasn't sufficient - I should have tried both differential and common-mode simultaneously but I didn't get a chance.

Hi  shabaz 

Here are the datasheet and an app-note. The PAM8403 is obsolete, the PAM8406 is recommended.

As I mentioned you will need filtering on the output to the speakers. Also, the PCB needs improved routing.

In the PAM8406 schematics BOM list you have the values of the components.

 A huge problem is a GND loop, -ringing and -bouncing.

-> Poweranalog is now part of Diodes:

https://www.diodes.com/search?q=pam&action_results=Go&start=20

Regards Gerald

Hi Gerald,

Thanks for the info, that part looks much better than the 8403. No reason to use that old part anymore. I can't recall the reason I had the 8403 board, I think I got it several years ago.

You're welcome.

Oh, yes me too - I also have toys I can't remember why I have it. :)

You mentioned the Vref _pin and -capacitor:

It is important to know that this pin is a bi-directional pin.

In the datasheet is descripted you need a X7R cer. cap. between 470n to 1u.

>>> Analog Reference Bypass Capacitor (CBYP)
The Analog Reference Bypass Capacitor (CBYP) is the most critical capacitor and serves several important functions. During start-up or recovery from shutdown mode, CBYP determines the rate at which the amplifier starts up. The second function is to reduce noise caused by the power supply coupling into the output drive signal. This noise is from the internal analog reference to the amplifier, which appears as degraded PSRR and THD+N.
A ceramic bypass capacitor (CBYP) with values of 0.47μF to 1.0μF is recommended for the best THD and noise performance. Increasing the bypass capacitor reduces clicking and popping noise from power on/off and entering and leaving shutdown.

>>>

But on the EMB is mounted a 100n X7R cap. This value is too small.

This pin, Vref can modulate the signal through the signal path if that Vref value is not stabile. 
Noise and popping noise signals are going to the amplifier.
This cap. must be mounted very close to the pin and the GND must be a polygon field. - The polygon should be around the IC as well.

I'm waiting for the PAM8406 version which should not be so noisy.
I didn't achieve much using the ferrite bead + capacitors filter.

Can you make photos of your assembled amp., that show us details about your built? 

Of course, you can see in the post.