A quality complete 2N3055 amplifier project with bass, treble tone control at 2x10W (4-Ohm) or 2x20W (4-Ohm) power that can be used with USB mp3 module. 2N3055 transistors are used in the output stage. There is also an additional 12V DC power supply so that you can operate the USB radio-player.
If we want to obtain 2x10W at the amplifier outputs, it can be operated with two supply voltages: 24V DC, if we want to obtain 2x20W, it can be operated with 35V DC. The amplifier input sensitivity is approximately: 400mV rms/27Kohm.
After the amplifier direct transformer voltage is rectified, it is fed with regulated voltage via 2 LM317 (parallel connected). In addition, a slow start circuit has been added to the LM317 regulator.
2N3055 Amplifier Transformer voltage for 2X10W RMS power should be 24V AC power 75VA 2X20W RMS voltage should be 30V AC power 105VA
Although both LM317s have built-in short circuit protection on their outputs, the two amplifier channels are powered via fuses; these fuses can be removed to check the correct setting of the idle current (bias) on the ammeter display.
The bias current can be set to a range of 20…50mA, depending on how well we match the output transistors. 40mA is ideal.
If you are using a Bluetooth USB mp3 player module, you may need to use different values for the R13 and R40 resistors, depending on the output voltage of the module.
The value is not critical, but the value 5.1kOhm was used in the sound tests with the mp3 player. So if the sound is low, set the value of these resistors to 5.1kOhm.
