Portable Audio Amplifier

thehardwareguy

This was my singleton project. It involved an extensive amount of research and practical work such as designing, constructing, and testing multiple circuits.

I did an entire dissertation on this project so I won't be sharing every single detail. If you have any questions feel free to contact me.

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Overview

The aim of the project was to build a portable amplifier on a budget of £50. It proved rather challenging. The challenge was finding the right balance between performance and functionality. The cabinet is constructed entirely out of recycled wood, meaning it was not added to the budget.

The amplifier is loud enough for a small room, I still use it to this day. Looking back, I wish the budget was higher because there are additional features I would've liked to add, such as Bluetooth & remote control.

Specification

TPA3122D2 Class D chip.
5W/Channel Output Power.
3" Visaton 10W Speakers.
LED Battery Level Monitor.
3.5mm Input Jack.
5+ hours battery life.
Rechargeable 12V battery.
Portable.

Speaker Selection

Speakers are the most important component in terms of sound quality. Skimping out on speakers will result in poor sound quality, no matter how good your amplifier.

The drivers I chose were full-range Visaton FR8's (10W RMS each), a pair cost me £20. The speakers lack some low end frequencies, this was to be expected due to their small size. Overall they offer good performance for their price and size.

Rated Power: 10W.
Nominal Impedance: 8 Ohm.
Frequency Response: 130-20000Hz.
Resonance Frequency: 150Hz.

Battery Selection

The battery is an important component in any portable device, it must be reliable and efficient. I prioritised the following factors when choosing a battery:

Small.
Light.
High Capacity.
12V / 1A supply.
Affordable.

It was a weigh up between a lead acid battery and a lithium ion battery. I decided to choose the lithium ion battery due to it's small form factor, low cost, consistent voltage levels and high cycle life.

The battery is made in China, which initially raised some reliability concerns. However, I've had no issues with it so far.

Chip Selection

A lot of research went into selecting a chip. There are many different amplifier classes to choose from. Two of the most commonly used are Class AB and Class D. Class AB offers peak performance, but with low efficiency. Class D offers slightly less performance particularly at high volume, but a much higher efficiency. The graph demonstrates the efficiency of both, relative to output power.

Texas Instruments TPA3122D2

Up to 15W per channel.
10-30V Operating voltage.
Efficient Class-D Operation.
4 Adjustable Gain Settings.
Internal Oscillator.
Thermal / Short Circuit protection.

Amplifier Circuit

I designed my own circuit. I won't be including the schematic here. After designing the circuit, initial testing was undertaken on breadboard. Breadboard is extremely useful to test small circuits because it makes interchanging components a lot easier. After circuit functionality was confirmed, I transferred everything over to veroboard.

Front I/O Design

My focus was to keep the front I/O simple. Given the budget restriction, I was unable to add any fancy features. The front I/O consists of:

Battery LED Monitor.
LED Power Button.
3.5mm Input Jack.
Volume Potentiometer.

Battery Level Monitor

Given that the device was going to be portable, it was essential that the battery level be visible somewhere. In order to do this I built a simple circuit using the LM3914 chip. This chip is very popular and used in many modern devices.

I used 5 LED's, each representing a different voltage. The voltage levels are calibrated using a potentiometer in a simple voltage divider arrangement. The image above demonstrates the configured voltage levels.

Cabinet Design / Construction

The design consists of three main compartments. Two sealed speaker compartments and one larger compartment for the battery and circuits. A dummy version of the cabinet was made up to ensure everything would fit inside. The whole thing is held together using wood glue, excluding the removable back panel. Below is a very early, basic CAD model of what I planned it to look like.

The entire cabinet is constructed from recycled wood, that being two old shelves and some cut-offs. All of the images below were taken throughout the construction process.

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