PAM8403 Class-D Amplifier Module with Micro-USB

Joshua R. T. Purba
Electronics

I just bought a PAM8403 amplifier module. PAM8403 IC is a “filterless 3W Class-D stereo audio amplifier.” The module is small: 18mm × 17mm. The variant that I bought has a micro-USB connector (USB micro-B) for the power connection which are paralleled to through-holes (J4 on the PCB) with non-standard pitch. For the audio input connections there are through-holes (J3 on the PCB) with standard pitch (0.1"). For the audio output connections there are through-holes (J1 for left channel and J2 for right channel) with non-standard pitch. Quite an inconsistent PCB design.

Top view of the module showing the PAM8403 IC and the other components.

PAM8403 module from top.

Bottom view of the module showing only silkscreened connection labels. There are no components on the bottom side.

PAM8403 module from bottom.

Apparently this module was meant to be used in tandem with a bluetooth daughterboard which I didn’t bought. This would explain weird things on the PCB that doesn’t make sense on their own:

  1. LED indicator connection that is not connected anywhere.
  2. 5V output connection that is passed-through a diode.

It is possible that these connections make sense when connected to the bluetooth daughterboard. Here’s some picture I found that explained the weird connections and the possible bluetooth daughterboard.

Explanation from an eBay vendor about the connections of the PAM8403 module. This is not my picture. Credit goes to the vendor on https://www.ebay.com/itm/323411528426

Vendor explanation of the connections.

Example of the horizontal installation of the bluetooth daughterboard from an eBay vendor. The daughterboard is below the PAM8403 module. This is not my picture. Credit goes to the vendor on https://www.ebay.com/itm/323411528426

Horizontal installation of bluetooth daughterboard below the PAM8403 module.

Example of the vertical installation of the bluetooth daughterboard from an eBay vendor. The daughterboard is standing vertically on top of the PAM8403 module. This is not my picture. Credit goes to the vendor on https://www.ebay.com/itm/323411528426

Vertical installation of bluetooth daughterboard on top of the PAM8403 module.

The 3 pictures above wasn’t mine. I got them from https://www.ebay.com/itm/323411528426.

I reverse engineered the schematic. This was fun because of the small size and low component count.

PAM8403 module electronic schematic showing the components and wirings.

The reverse engineered schematic of the PAM8403 amplifier module. All capacitance values are guesswork.

It appears that the left and right audio input on the PCB are wrong, they should be the other way around. On the PCB the through hole marked as “R” is connected to pin-7 of the PAM8403, “L” is connected to pin-10. But in the datasheet pin-7 is INL (left channel input) and pin-10 is INR (right channel input). I have corrected this in my schematic.

I tested the PAM8403 module using a pair of 8Ω 10W speaker drivers. These speaker drivers were salvaged/used and I bought them in Pasar Loak Jembatan Item in Jatinegara.

These are the speaker drivers I used to test the PAM8403 module. On the left is the 5V USB charger that I will use to power the PAM8403 module. On the right is the PAM8403 module.

Top view of the salvaged speaker drivers.

I flipped the drivers so the bottom markings can be seen.

Bottom view of the salvaged speaker drivers.

I used regular unshielded wires for the test. The insulation is so thick the shell for the 3.5mm AUX plug cannot be installed.

Everything soldered and ready for the test.

For the test I connected the 3.5mm AUX plug the the jack on my laptop and played a song at 40% volume. The module has no volume control so I controlled the volume from my computer.

The music I played is Goodbye To Spring by Josh Woodward (CC BY 4.0). The little skip at the end of the recording is due to my phone that I used to record the video.

Not depicted in the video: after powered from the USB charger but before the 3.5mm AUX plug is connected to the computer, there is a buzzing sound from the speaker drivers. The buzzing got bigger as I touched the input cable. After connected to the computer the buzzing disappeared. Other than that everything works acceptably well 👌

The PAM8403 module is cheap and useful. The salvaged speaker drivers are bought used cheaply. After assembled they can play music from computer or phone with power being supplied using a regular 5V USB charger.

See ya folks!

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