This is the circuit diagram of a 35W audio amplifier based on LM391. The LM391 is a comprehensive driver to the power transistors. It needs only few external components to work. With the TR1 we can adjust the bias current to 45mA. Inductor L1 consists of 20 turns of wire thickness of 0.9mm, around the resistor R16. The transistors Q3-4 must be placed onto a heatsinks. If you use the Q3-4 with type of TIPxxxx, you can be placed directly onto the board. But if you use packaged transistors TO-3, then it should be connected with short cables to the respective positions of the board.
Parts List
| R1,3,5,8,10 = 1Kohm | C1 = 2.2uF 63V | TR1 = 10K Trimmer |
| R2,7,11 = 100Kohm | C2 = 4.7uF 25V | D1,2 = IN4002 |
| R4,6 = 4.7Kohm | C3,17 = 47uF 63V | IC1 = LM391 |
| R9,12 = 100ohm | C4,8,9 = 1nF 100V MKT | Q1 = BD139 |
| R13 = 47Kohm | C5 = 47nF 100V MKT | Q2 = BD140 |
| R14,15 = 0.15ohm 5W | C6 = 22pF ceramic | Q3 = TIP2955 |
| R16 = 1ohm 2W carbon 5% | C7,11,12,13 = 100nF 100V MKT | Q4 = TIP3055 |
| R17 = 10ohm 1W carbon 5% | C10 = 22pF ceramic | L1 = See text |
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