# Sixth Lichuang Electric Competition# 2.1-channel Bluetooth 5.0 desktop audio

progress:

        -8.2 - The left and right channels are currently normal. There is a problem with the original designed op amp part of the bass channel. The board has been re-printed and the shell is being drawn.

        -8.8 - The hardware part is completed. After the speaker casing was designed, it was found that the processing cost was far higher than expected, so it was now replaced by Chevron board to make it manually.

        -8.11- The physical objects are all completed.

 1. Introduction to project functions

        This project is a Class D digital power amplifier based on TPA3118. The audio source can be Bluetooth and external AUX input. The output is 2.1 channels. The maximum power of the power amplifier board supports 2×30W+60W@24V, 8ΩBTL. The power amplifier uses an external 2-serial 2-parallel lithium battery to boost 12V power supply, and supports separate adjustment of bass, mid-range and high-pitched volumes. This project uses two 8Ω 12W full-range speakers and one 8Ω 15W woofer.

 2. Project attributes and open source agreement

        This project is an original project that has been made public for the first time, participated in for the first time, and has no defense. The open source license is GPL 3.0.

 3. Hardware part

(1) Power strip

        This audio is designed to be powered by an internal battery. Since the minimum voltage of the power amplifier requires 12V, it uses a lithium battery structure of two series and two parallels. Each cell is a 2600mAh 18650 battery. The boost circuit uses the MIC2253 DC-DC chip, which can increase the voltage of two strings of lithium batteries to 12V, with a maximum output current of 3.5A. The charging circuit adopts the SY6982E solution, which can charge two strings of lithium batteries with a 5V input, and the maximum charging current is 2A. Charge and discharge protection and charge balancing adopt the classic solutions of HY2120 and HY2213 respectively.

(2) Power amplifier board

        The power amplifier board is designed for 2.1 channels and is driven by two TPA3118 chips. The two chips are connected in asynchronous mode. Refer to the official schematic diagram for the peripheral circuit. The anti-pop circuit of the power amplifier is the classic power-on mute method.

As shown in the picture above, C4 is charged at the moment of power-on, and the mute pin is pulled to high level to silence, thus eliminating the popping sound.

        The mixing circuit of the bass channel is composed of two inverting proportional amplifiers composed of NE5532. The left and right channel post-stages are attenuated by 10K resistors and then superimposed.

The low-pass circuit uses a typical Sallen-Key active low-pass filter, and the bass dive depth is about 60Hz.

Physical pictures of the power board and amplifier board:

(3) Knob plate

        In order to facilitate the adjustment of the volume, the knobs are designed on a separate board, and a single 50K potentiometer and a double 50K potentiometer are used to adjust the volume of the bass and mid-treble respectively. At the same time, the Bluetooth module is placed on this board, and Bluetooth and AUX are controlled through a double switch. The knob board and the amplifier board are connected through a flat cable.

Knob plate physical picture and assembly diagram:

(4) Shell design

        At the beginning, I planned to use black walnut wood to process the shell. However, when I designed the shell model and prepared to contact the seller for processing, one of them said that the model was too complex and could not be processed. The other two sent the model file for inquiry and then heard nothing from them. Later, the delivery time was not enough and I planned to switch to 3D printing. However, the total processing fee for 12 parts was about 500 yuan, so I was directly persuaded to quit. I had no choice but to use Chevron boards to manually process the shell...

Designed speaker enclosure:

Actual handmade…

Rear and bottom:

        Compared with milling and 3D printing, the span is much greater. The circle is not round or square, but after all, we just listen to the sound and make do with it first.

        In terms of sound quality, I am not a professional and cannot comment. Anyway, the bass is so powerful that the whole table is vibrating, but the bass volume cannot be turned up too high, otherwise the speakers will explode. There is basically no noise when connected to AUX, but there is a very obvious noise floor when connected to Bluetooth. I have tried several solutions but to no avail.

 4. Software part

        This project does not involve the software part.

 5. BOM list

Power Board:

Amplifier board:

Knob plate:

 6. Contest LOGO verification

 7. Demonstrate your project and record it as a video for uploading

see attached