TIP41C/TIP42 COCL discrete power amplifier circuit diagram

　　In transistor receivers and amplifiers, push-pull power amplifier circuits are widely used. Traditional push-pull circuits always require input transformers and output transformers. This transformer-coupled circuit has some disadvantages, such as: since the magnetization curve of the transformer core is nonlinear, it will cause nonlinear distortion in the amplifier circuit; due to the parasitic coupling of the transformer's leakage magnetic field to the circuit input circuit and the intermediate frequency circuit, the whole machine will work unstably; in particular, the existence of the transformer seriously affects the frequency characteristics of the circuit. This is because the inductance of the transformer winding cannot be made too large. Therefore, the inductive reactance is small at low frequencies (XL=ωL), which reduces the gain at the low-frequency end. On the contrary, in the high-frequency part, due to the large inductive reactance and the large amplification factor, it is easy to produce saturation distortion, which makes the high and low frequencies not full enough.

　　This training kit uses a typical OCL circuit, which has the advantages of high stability, wide frequency response range, and good fidelity. The circuit schematic is as follows:

　　Since the two-channel circuit of the power amplifier is completely symmetrical, we only explain one of them here. VT2 and VT3 form a differential input circuit. After the input audio signal is amplified, it is output from the collector of VT3. The voltage drop on R9 and VD1-VD3 provides a DC bias voltage for VT4 and VT7 to overcome the cut-off distortion of the two tubes. After the audio signal is pre-amplified by VT4 and VT7, it has sufficient current strength and is then sent to VT5 and VT6 to complete power amplification. During the positive half cycle of the signal, the current flows from the positive current through VT5 to the load and then to the ground. During the negative half cycle, the current flows from the ground through the load and VT6 to the negative pole of the power supply. During the entire power process, VT5 and VT6 are always in a micro-conduction state. Therefore, this power amplifier is also called a Class A and B complementary symmetrical power amplifier circuit. Since this circuit adopts a direct coupling method, its frequency characteristics are very good. After the completed prototype is input with sine wave signals of different frequency bands, the waveform at the output end has extremely high fidelity. Compared with the integrated circuit power amplifier, although the circuit is more complicated, for beginners who are learning power amplifier circuits and from the perspective of school teaching, this kit has a very good teaching effect. It can help students improve their understanding of the principles of OCL circuits. After the actual object is equipped with speakers, they can enjoy high-fidelity music, thus achieving the goal of learning electronic theoretical knowledge and cultivating hands-on skills in electronic production.

　　Installation and commissioning instructions:

　　From the number of components, this kit may be considered a bit difficult for beginners. In fact, the left and right channel circuits are two completely symmetrical circuits, and the other is the power supply circuit. As long as you carefully follow the symbols on the circuit board when installing the components, you can successfully complete the production. There are two jumpers that need special attention when installing components. They are located next to VT5 and VT6, marked with "J1, J2". If there is a solder leak, the amplifier will not work properly. The amplifier circuit board after production

　　The power transformer used in the kit needs to be prepared by yourself. It needs to use a center-tapped dual power transformer with AC primary 220V and secondary 6~12V×2 sets, with a power between 10 and 30 watts (determined by the power you need). When wiring, the center tap is connected to the middle of the terminal, and the other two wires are connected to the upper and lower positions. The middle ground wire of the speaker terminal is shared, and the other two are connected to the "L-OUT" and "R-OUT" terminals respectively. It should be noted that the two output terminals must not be short-circuited, otherwise the power amplifier tube will be burned immediately. After power is turned on, positive and negative DC voltages are generated at both ends of C7 and C9, and the voltage at both ends of the speaker is zero volts. If these parameters are correct and other components are installed correctly, it can be successfully produced.