The following diagram is the circuit of Stereo OTL Audio Amplifier. It based on IC circuit, so the circuit looks simple. You can use IC LM379,LM377,LM378 for this amplifier project. Refer to the table, maximum voltage supply for IC LM379 is 28V, it will will deliver 4W/channel. For LM377, maximum Vcc voltage is 18V single supply and delivers about 2W/channel. While for LM378, use maximum 24V regulated DC power supply and you will get 3W hifi audio power per channel. Use heatsink on the IC to prevent overheating.

This is a one chip sound level meter that can be use for displaying sound level of an amplifier or simply the sound level from a microphone.The heart of the circuit is IC LM 3915 Audio level IC.Even though it is a stand alone IC , a peak detector based on Transistor BC 558 and diode 1N4001 is also included for better performance. Supply voltage can be from 3V to 20V.The input is set for audio line voltage (1V peak to peak) and has a max input voltage of 1.3V. To make the circuit use a moving dot display instead of bar graph display,Pin 9 can be should be disconnected from +V. Part List C1 2.2uF 25V Electrolytic Capacitor C2, C3 0.1uF Ceramic Disc Capacitor R1, R3 1K 1/4W Resistor R2 10K 1/4W Resistor R4 100K 1/4W Resistor R5 1M 1/4W Resistor D1 1N4001 Silicon Diode Q1 BC 558 PNP Transist...

This is the schematic diagram of mini microphone amplifier circuit, which all of the active components are based on transistor. This circuit project is called "mini" since its size is small and the output is small too. It uses surface mount technology. Circuit works The output is push-pull and consumes less than 3mA (with no signal) but drives the earpiece to a very loud level when audio is detected. The whole circuit is DC coupled and this makes it extremely difficult to set up. Basically you don't know where to start with the biasing. The two most critical components are 8k2 between the emitter of the first transistor and 0v rail and the 470R resistor. The 8K2 resistor across the 47u capacitor sets the emitter voltage on the BC547 and this turns it on. The collector is directly connected to the base of a BC557 transistor, called the driver transistor. Both of these transistors are now activated and the output of the BC557 causes current to flow through the 1k and 470R...