Personalized power amplifier designed based on PAM8407

1. Introduction to project functions

Personalized amplifier:

Beginner, zero basics, as long as you can make a sound.

I want to use a "personalized power amplifier" to amplify the audio output from the Bluetooth headset and play it out through a speaker. At the same time, you can also plug in ordinary wired headphones for playback.

2. Project attributes

This project is made public for the first time and is my own original work. It's purely a personal hobby, just for fun.

3. Open source agreement

GPL3.0 open source license, no commercial use without permission.

4. Hardware part

4.1 Chip selection:

The preamplifier uses TDA1308. I have this chip myself, but it was not in stock in the mall when I placed the order.

The power stage uses PAM8407, which is the chip used when I watched Lichuang's Mo Gong explain it. It is simple, has high power, and suits me.

4.2 Circuit composition:

4.2.1 Power circuit

I chose the LM317, which I picked from the mall. I want to use a 12V adapter and use LM317 to transform it to 5V to power the chip.

Because the power polarity of some adapters is that some pins are positive and some are negative, in order to prevent reverse connection, I used an NMOS tube here, and the effect is pretty good.

The output voltage of this circuit is about 5.6V, which is higher than I expected. It is within the working voltage range of the chip, so that's it. I built the circuit according to the LM317 data sheet, why is it higher than 0.6V? Is there something wrong with my choice of R9 (5K) and R10 (15K)? Does anyone know? Please give me some advice. Thank you.

4.2. 2 Headphone amplifier

I chose TDA1308, and the circuit is built exactly according to the circuit in the data sheet, and the sound is okay. Since I don’t have headphones with high impedance, I don’t know if the thrust force has been improved. PS: Can the two resistors R5 and R6 adjust the amplification factor?

This circuit worked the first time without modification.

4.2. 3 Speaker amplifier

PAM8407 has relatively few pins and simple wiring, making it more suitable for beginners like me who are drawing PCBs for the first time.

This picture was modified by me. The prototype PCB is different from this one. Pins 14 and 11 were directly grounded by me. As a result, there was no sound from the speaker.

The line was drawn wrong, so I had no choice but to fly the line. After flying the line, the sound came out.

PS: I also drew the wires of my audio input socket backwards, and the left and right channels were connected backwards. Because I drew it from memory, I didn’t think it would be wrong, so I didn’t check it.

If you have obsessive-compulsive disorder, you must fly to solve it. I forgot to take a photo of the finished product. This photo is a semi-finished product.

Also, originally I wanted to connect an audio socket to output to headphones, but it failed at first. Because as long as the headphones are plugged in, the output current of the power supply increases sharply, approaching a short circuit. How could this happen? Plugging in the headphones only disconnects the signal from the TDA1308. I don't understand why the current increases sharply.

Later, I found that when the speaker was first connected to the power supply, the diaphragm bulged. At this time, the ammeter pointer jumped abnormally and the current was high. I guess it's a problem with the speaker wiring. My circuit is based on the circuit taught by Mo Gong during the live broadcast. Logically speaking, it should not be wrong, but the current is indeed abnormal. Since it was a problem with the speaker and the diaphragm was bulging, it was probably because of DC power. There was no capacitor connected to the speaker in Mo Gong’s circuit, so I soldered a 100uF capacitor on the speaker’s binding post (see the picture below), and a miracle happened. The current is not large anymore, and the current does not increase sharply when the headphones are plugged in. At this point, all the functions I wanted to implement have been achieved, and I am excited.

(PS: The second and seventh pins of PAM8407 have been connected in series with a 0.1uF capacitor, and the circuit diagram has been corrected)

4.2. 4 Bluetooth input

In this input area, I have reserved the pad for the Bluetooth headset input, so that there is no need to design a separate Bluetooth module. I can directly remove the circuit board from the old Bluetooth headset and connect the wires to achieve Bluetooth wireless playback.

4.3 Final product display:

The flying line is a bit ugly and the drawing has been changed .

5. Demonstration video

Please watch the video