Hey, friend, if you’ve come across this 100W audio amplifier schematic from Easy Electronics, you’re probably thinking about cranking up the volume for your next audio project. I’ve seen designs like this plenty of times in my workshop days, and this one looks like a solid Class AB amplifier using bipolar transistors. It’s perfect for powering speakers in a home stereo, a DIY PA system, or even a car audio setup. The image shows the full circuit with labeled components, a speaker icon, and a ±50V power supply, so let’s dive in. I’ll walk you through how it works, what each part does, and how you can build it yourself. Think of this as a casual chat over a soldering bench—practical and straightforward.
Why Build a 100W Audio Amplifier?
First off, why bother with a 100W amplifier? If you’re driving a decent speaker, especially one needing more than a few watts, this setup delivers. It’s enough power for medium-sized rooms or outdoor gatherings without distortion, assuming your speaker can handle it. Class AB amplifiers, like this one, strike a balance between efficiency and sound quality—better than Class A but not as complex as Class D. They use less power than Class A while avoiding the crossover distortion of early Class B designs.
This design uses common transistors like TIP41, TIP42, and 2SC5200, which are affordable and widely available. It’s a great learning project if you’re into audio electronics, and it can be a cost-effective alternative to buying a prebuilt amp. Just a note: handling ±50V and high currents means safety is key—use insulated tools and a proper enclosure. If you’re new to this, start with a lower power supply voltage to test.
Overview of the Circuit
Looking at the schematic, this is a classic push-pull Class AB amplifier. The input signal enters at the left, gets preamplified, and then drives a pair of output transistors that work together to boost the signal to 100W. The circuit splits into two halves: one for the positive signal (handled by NPN transistors) and one for the negative (PNP transistors). Power comes from a dual ±50V supply, and the output feeds a speaker through a filter.
The design uses a differential input stage with Q2 and Q3, followed by a voltage amplifier stage with Q5 and Q6. The output stage features heavy-duty transistors Q9 and Q10 (2SC5200 and its complement) to handle the current. There’s also a bias network to set the idle current, ensuring smooth transitions between positive and negative halves. The speaker is protected with an inductor and a capacitor, and the layout suggests a single-sided PCB is possible.
Key Components and Their Roles
Let’s break down the main parts of the schematic. I’ve pulled these values directly, so you can match them when sourcing.
- Transistors:
- Q2, Q3 (2SA1015, 2SC1815): Small-signal NPN/PNP pair for the differential input stage.
- Q5 (TIP41): NPN driver for the positive half.
- Q6 (TIP42): PNP driver for the negative half.
- Q9 (2SC5200): High-power NPN output transistor.
- Q10 (2SC5200: High-power NPN output transistor.
- Resistors:
- R1, R13 (22k): Input and feedback resistors.
- R2, R14 (4.7k): Bias resistors for the input stage.
- R5 (68 ohm): Emitter resistor for stability.
- R8 (10k): Part of the voltage divider for Q6.
- R7, R12 (150 ohm): Base resistors for driver transistors.
- R9, R10 (100 ohm, 1W): Bias spreader resistors.
- R15 (0.47 ohm, 5W): Emitter resistor for output stage current sharing.
- R18 (0.47 ohm, 5W): Another emitter resistor for balance.
- Capacitors:
- C1, C2 (2.2uF): Input coupling caps.
- C3, C4 (10pF): Compensation caps to prevent oscillation.
- C6, C7 (1nF): Feedback and decoupling.
- C8 (100nF): Output coupling cap.
- Diodes:
- D1, D4 (1N4007): Protection diodes.
- D2, D3 (not labeled, but implied): Bias diodes for thermal stability.
- Inductor and Speaker:
- L1 (0.47uH, 5W): Output inductor to filter high frequencies.
- Speaker: 8 ohm, 100W rated.
The power supply is ±50V, delivering around 100W into an 8-ohm load at full swing, assuming 60-70% efficiency. Parts cost could be $10-15 if you shop for surplus or bulk transistors.
How the Circuit Works: Step by Step
Let’s trace the signal path. Imagine your audio input—maybe from a phone or mixer—coming in at the left.
Input and Differential Stage
The signal hits C1 and goes to Q2 (2SA1015), with Q3 (2SC1815) as its partner in a differential pair. This stage amplifies the difference between the input and feedback signal, rejecting noise. R1 (22k) and R2 (4.7k) set the gain, while C3 (10pF) stabilizes it. The output is a small voltage swing across R5 (68 ohms).
Voltage Amplifier Stage
Next, Q5 (TIP41) and Q6 (TIP42) take over. Q5 boosts the positive signal, driven by R7 (150 ohm), while Q6 handles the negative side via R8 (10k). These drivers push current into the output stage. The bias network—D2 and D3 (implied)—sets a small idle current to minimize crossover distortion. R9 and R10 (100 ohms) adjust this bias.
Output Stage
Here’s where the power happens. Q9 (2SC5200) and Q10 form a push-pull pair. When the input is positive, Q9 conducts, pulling current through the speaker. When negative, Q10 takes over. R15 and R18 (0.47 ohm) balance the current between them. The output swings up to ±40V peak, giving around 100W into 8 ohms (P = V^2/R, or 40^2/8 = 200W peak, derated for efficiency).
Output Filtering and Protection
C8 (100nF) and L1 (0.47uH) form a low-pass filter, blocking high-frequency noise from reaching the speaker. D1 and D4 (1N4007) protect against reverse voltage spikes. The 5W ratings on R15, R18, and L1 show they’re built to handle heat.
The circuit’s gain is set by the feedback loop (R13, C6), likely around 20-30dB. Total harmonic distortion should be low—under 0.1% with good biasing.
Building and Testing Tips
Ready to build? Start with a breadboard for the input stage, then move to a PCB. Use a heatsink for Q9 and Q10—they’ll get hot at 100W. Here’s a step-by-step:
- Assembly: Solder transistors first, ensuring correct orientation (check pinouts). Use thick traces for power lines.
- Power Supply: Build a ±50V supply with a center-tapped transformer, bridge rectifier, and 4700uF caps per rail. Start at ±20V for testing.
- Bias Adjustment: Set R9/R10 for 10-20mA idle current through Q9/Q10. Measure voltage across R15—should be ~5-10mV.
- Testing: Apply a 1kHz sine wave. Check the output with an oscilloscope for clipping. Ramp up volume slowly.
- Safety: Enclose it, add fuses (1A on each rail), and ground the case.
Common issues: Oscillation (add C3/C4 if needed), overheating (improve heatsink), or no sound (check bias). Efficiency is 50-60%, so expect heat.
Pros and Cons of This Design
Pros: Powerful output, simple design, uses common parts, good for learning. Cons: Requires a large heatsink, moderate efficiency, and needs careful biasing. Not hi-fi quality, but solid for general use.
Wrapping It Up
This 100W audio amplifier is a great project for anyone into audio electronics. It’s a practical build that delivers solid power with accessible components. If you put it together, start safe and tweak as you go. Let me know how it sounds in your setup!
Frequently Asked Questions
What transistors can I use instead?
TIP35/TIP36 or MJL21193/MJL21194 work, but match ratings.
How much power supply do I need?
±50V at 2-3A per rail, so a 200VA transformer minimum.
Can it drive 4-ohm speakers?
Yes, but output drops to ~70W and heat rises—use a bigger heatsink.
Why is it getting hot?
Class AB wastes power; ensure proper cooling and bias.
What’s the frequency response?
Typically 20Hz-20kHz, check with a sweep test.
Is it safe for beginners?
Not really—high voltage and current need experience.
