Using the TDA2822 integrated circuit, you can create a compact hearing aid amplifier that can also be used as a preamplifier. This audio amplifier design uses the popular TDA2822 chip, commonly found in traditional table USB-to-PC speakers. With a maximum output power of 600mW per channel, totaling 1.2W, this integrated circuit is ideal for a range of audio applications.
Applications
This headphone amplifier offers versatility beyond its standard use. It can be connected to the output of the signal summer to monitor audio signals from the console. Additionally, it can function as a preamp to boost weak computer signals. Another benefit is its ability to strengthen signals in computers that have amplifiers built into the speakers. This helps prevent noise interference caused by long signal paths. We successfully utilized this amplifier in our project, which had an external amplifier setup
The TDA2822M integrated circuit is a compact, 8-pin MINI-DIP stereo amplifier. Common applications include radio amplifiers, handheld game consoles, and computer speakers. This versatile chip can operate with DC voltages ranging from 3V to 12V. However, for use as a hearing aid amplifier, a lower voltage of 5V is sufficient. Alternatively, it can be powered directly from a computer’s USB port, which provides a convenient 5V supply. When paired with speakers and powered by the USB port, the TDA2822M delivers acceptable and pleasant sound quality.
Constructing this circuit is a straightforward and cost-effective process. The integrated TDA2822 is accompanied by only a few resistors and capacitors. To begin, create the printed circuit board (PCB) using the provided drawing in the downloadable PDF file. Next, carefully place the components in their designated positions, referring to the component mask in the PDF file as a guide. Be mindful that electrolytic capacitors, the diode bridge, and the integrated circuit all have specific polarity requirements, so ensure they are positioned correctly before soldering. Double-check the orientations to avoid errors.
The quality of the assembly determines a project’s success.
- Solder joints should be flawless, smooth, and shiny.
- Ensure there are no shorts or bridges between tracks or solder joints, as these can compromise the circuit’s performance and reliability
Clean the circuit board thoroughly using thinner and a toothbrush. Gently scrub the board repeatedly to remove grease, excess solder particles, and any other residue. Once clean, dry the board completely. For added protection, consider applying a layer of enamel to the copper traces to prevent oxidation and corrosion.
The circuit board incorporates a compact power supply, consisting of a 1A diode bridge and a 1000μF capacitor. Connection to the transformer is made via a 4-pin MOLEX connector (2.54mm pitch), with the center pins omitted. The diode bridge rectifies the input voltage, creating separate positive and negative supply rails, while the capacitor smooths the output to deliver a stable DC voltage.
To power the circuit using a 9-volt battery, simply omit the diode bridge and connect the battery directly to the 1,000 uF capacitor, ensuring correct polarity. Don’t forget to maintain the correct orientation of the 1,000 uF capacitor.
The connectors shown in the photograph are for signal input and signal output. Notably, the output connector features a reverse current protection circuit, known as a Zobel Network. This network comprises a 4.7Ω resistor and a 0.1μF capacitor (104), connected in parallel across each output and in series between the two outputs. Additionally, a 10kΩ resistor is included on the right-hand side to safely discharge the voltage when the amplifier is powered off, preventing prolonged noise or oscillations when power is removed.
The signal input features a pair of 200KΩ resistors that serve as a volume attenuator, helping to prevent excessively loud sound levels when using headphones. This is particularly important for protecting hearing. The value of these resistors can be adjusted based on the output volume of the player being used. For example, if the player produces very high volume levels, resulting in overly loud sound through the headphones, you can replace the 200KΩ resistors with higher-value ones, such as 330KΩ. Conversely, if the volume is too low, you can use lower-value resistors instead.
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Amplifier Audio Inputs and Outputs
For headphone output, we utilized a stereo jack, which requires a stereo shielded cable to maintain sound quality. If you plan to use this circuit as a preamplifier, ensure that the output connection is compatible with the input coupling of your main amplifier, to guarantee seamless integration and optimal performance.
Typically, this amplifier is used with computers (PCs) or cell phones, which often feature 1/4-inch stereo jack connectors for audio output. To accommodate this, we’ve created a custom cable with a stereo plug on one end and a 3-pin GP connector on the other. Additionally, the double-pressure terminal visible on the left side of the photo allows for easy connection of a loudspeaker, providing the option to use speakers instead of headphones.
Now, let’s examine the fully assembled amplifier, ready for use as a hearing aid amplifier. To power it, you’ll need a transformer with an AC output of 6-9 volts and a minimum current rating of 300 milliamps. Note that when AC voltage is rectified, it increases by a factor of 1.4141 (the square root of 2). Therefore, if you use a 9V AC transformer, the integrated circuit will receive approximately 12V DC from the power source.
Simple DIY speaker enclosures
A creative and straightforward approach to building decent-sounding speakers is to repurpose a pair of plastic jars.
The ideal speaker pairing for this amplifier would be 3-10W speakers with an 8-ohm impedance. In this case, we repurposed two 3W speakers from a broken MP3 player. We found two plastic containers with mouths that matched the speaker size, and attached the speakers to the containers using tape, ensuring a sealed fit. The amplifier’s output connector is a three-pin configuration, with the left and right channels on the outer pins and the ground/common on the center pin. To connect the speakers, we linked the ground (black wire) to the negative terminal of one speaker, and then connected another wire from that speaker’s negative terminal to the negative terminal of the second speaker.
When using speakers with the TDA2822M chip, it’s common to experience moderate heat buildup. To address this, you can either use a 6V transformer, which will reduce power output, or attach a small aluminum heat sink to the chip using epoxy glue. Additionally, you can enhance speaker performance by adding ventilation holes to the baffles. This increases overall loudness and emphasizes mid-frequencies. However, for a louder sound with attenuated mids and boosted highs and lows, you can choose to omit the ventilation holes. Ultimately, the optimal configuration depends on experimentation, as the sound quality varies with container size and speaker specifications.