How to Build a 50W RMS MOSFET Audio Amplifier – DIY Project

Simple 50W RMS Audio Amplifier with Mosfet

🌐 You can read this article in: Português | Español

🔬 Amplifier Technical Specifications

Output Power	50W RMS
Load Impedance	8 ohms
Supply Voltage	+/- 35V DC
Maximum Current	2A (mono) / 4A (stereo)
Output Configuration	Complementary push-pull with Mosfets

1️⃣ Input Stage: Differential Amplifier

The first stage of amplifier is a differential amplifier based on PNP transistors BC556, Q1 and Q2. This stage is responsible for receiving low-amplitude audio signal and preparing it for following stages. The capacitor C2 functions as an input DC decoupler, ensuring that only audio signal passes through. The resistor R1 limits input current, protecting circuit, and capacitor C1 diverts unwanted high frequencies, functioning as a simple low-pass filter.

2️⃣ Driver Stage: The Heart of Amplifier

The second stage is drive stage composed of transistors Q3, BC546, and transistor Q4. This section functions as a voltage amplifier, increasing the signal received from the input stage and preparing it for the output stage. Additionally, this stage aims to regulate the bias of the output transistors, ensuring they operate in their linear region and minimizing distortion.

3️⃣ Output Stage: Power and Efficiency with Mosfets

The output stage is a complementary push-pull stage based on MOSFETs IRF530 and IRF9530. This configuration is ideal for power amplifiers, as one MOSFET conducts the positive half of the signal while the other conducts the negative half, resulting in high efficiency and low distortion. The output is coupled to the speaker using inductor L1, which helps to isolate the speaker from any DC component that might be present in the output signal.

💡 Expert Tip

If you don’t have access to any of these specific transistors, don’t worry! We provide on our website a tool called:

Equivalent BJT Transistor by Data Cross-Reference

With this tool, you ensure the correct functioning of the circuit, even using components different from those suggested here.

📌 Additional Components: Stability and Quality

The network composed of R15 and C5 is intended for noise reduction, functioning as a Zobel filter to stabilize the load and prevent high-frequency oscillations. The capacitors C6 and C7 are power supply filters, essential for eliminating ripple and ensuring a clean power supply for the circuit. The variable resistor R6 is a crucial component intended to adjust the quiescent current of the bias transistor, allowing optimization of the MOSFETs’ operating point for lower distortion.

🔌 50 Watt Amplifier Circuit Diagram

In Figure 2 below, we have the schematic diagram of the circuit. Despite being a circuit with few components, we must pay special attention during assembly, as we are working with at least 6 transistors, diodes, and electrolytic capacitors with defined polarities.

Fig 2 – Schematic Diagram 50W RMS Audio Amplifier with Mosfet

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⚡ Adequate Power Supply

To ensure the maximum performance of your amplifier, it’s crucial to use an adequate power supply. The required power supply is of the symmetric type +/- 35V DC, with a current of at least 2 Amperes. This voltage and current are essential for the amplifier to deliver the promised 50W RMS with low distortion.

🌀 Construction of Coil L1

For coil L1, wind 12 turns of enameled copper wire 18AWG with a diameter of 3/8″ or 1cm without a physical core. This coil is crucial to isolate the speaker from any DC component and improve the frequency response of the amplifier.

🔵 Selection of Capacitors

The electrolytic capacitors C6 and C7, which act as power supply filters, should have minimum voltages of 50V to ensure safety and durability. The other electrolytics can be 25 or 35V, as long as they respect the specified capacitance values.

📏 Essential Heat Sink

An adequate Heat Sink is necessary for MOSFETs. We recommend a heatsink with medium dimensions of 22x12x12cm with sufficient dissipation area to ensure that the transistors operate at safe temperatures, even at higher volumes. Remember to use thermal paste between the MOSFETs and heatsink for better heat transfer!

🤔 Possible Problems and Solutions

If you encounter problems during the assembly or operation of the amplifier, here are some solutions to the most common problems:

Problem	Possible Cause	Solution
No sound at output	Incorrect connections or damaged components	Check all connections and test the components
Distorted sound	Incorrect bias or insufficient power supply	Adjust RP1 and check the power supply
MOSFETs overheating	Inadequate heatsink or excessive bias	Improve the heatsink or reduce the RP1 adjustment
High-frequency noise	Instability or interference	Check capacitor C5 and board layout

🛠️ Optional Improvements

If you want to take your amplifier to the next level, here are some improvements that can be implemented:

• Short-circuit protection: Adding a protection circuit can prevent damage to the MOSFETs in case of a short at the output.
• Enhanced power supply filter: Larger capacitors or addition of inductors in the power supply can further reduce noise.
• Mute circuit: Implementing a mute circuit can prevent noise when turning the amplifier on/off.
• Power indicator: Adding LEDs or a VU meter can give a more professional look to your project.