This is a bass booster circuit that use transistors as base. It is designed to increase a low frequency of audio on cheaper than ICs or normally circuit.
This is a 4 transistor audio amplifier circuit. Which is a 4-transistors complementary push-pull amplifier, that shows the basics of audio amplifier design.
This is three Simple 50w OTL audio amplifier circuit by 2N3055, can build easy and cheap, It's durable and very loud sound , Not needed speaker Protection. [...]