Building my first audio amp: I started with a simulation in LTspice

I've been interested in DIY audio, and after reading a blog post about someone building a small power amplifier with transistors, I wanted to try it myself. My reference was Chapter 4, "Designing a Mini Power Amplifier," of "Teihon Transistor Kairo no Sekkei" (The Definitive Transistor Circuit Design). It walks through a 0.5 W amplifier very carefully, from choosing component values all the way to evaluation, and I really recommend it to anyone about to build one.

The design specs are a voltage gain of 10x (20 dB), at least 0.5 W into an 8 Ω load, a frequency response of 20 Hz to 20 kHz, and distortion under 1%. The circuit consists of a common-emitter voltage amplification stage and a class AB push-pull current amplification stage. I learned a lot from the parts about adding diodes to eliminate crossover distortion in class AB, and thermally coupling the transistors to the heatsink to prevent thermal runaway.

I don't own an audio analyzer, so I filled the gap with LTspice, a free circuit simulator. Measuring the gain from the input and output waveforms in a transient analysis gave about 10x (20 dB), almost exactly as designed. I then checked the harmonic content with an FFT and calculated THD (distortion) with the .four command, getting about 2%, which closely matched the figure in the book.

I designed a PCB for the first time in EasyEDA and ordered it from JLCPCB. With a coupon it came to about 500 yen, surprisingly cheap, and shipping from China took about a week. I was nervous about whether it would actually work, but it produced sound properly. Distortion is low, and it plays as pleasantly as any ordinary speaker setup. Next, I'd like to take on power supply circuits and digital circuits too.

Source: よもつひらさか