The power amplifier circuit with single triode output has small output and low efficiency, which is rare in daily electrical appliances. At present, the push-pull circuit is often used.

    Figure 1 is a schematic diagram of a push-pull circuit using a coupling transformer. Its characteristic is that the static working current of triode is close to zero, and the power consumption of amplifier is small. When there is information input, although the working current of the circuit is large, most of the power is output to the load, and the loss itself is small, so the power utilization is high. In this circuit, each triode works only in half a cycle of the signal. In order to avoid distortion, two triodes work in coordination. The secondary of input transformer B1 in the figure has a grounded center tap. When the audio signal is input, two signals of equal size and opposite polarity in B1 secondary are sent to the transmitting junction of BG1 and BG2 respectively. In the positive half cycle time of the input signal, BG1 tube is cut off because of the reverse bias voltage. Only BG2 can amplify the signal and output it from the collector; In the negative half cycle of the signal, BG1 gets a positive high bias voltage, which can amplify and output the signal of this half cycle, while BG2 turns off. Although the two triodes in the circuit amplify half of the signal synchronization respectively, their output currents pass through the output transformer B2 successively, so the induced currents obtained at the secondary of B2 can form a complete output signal.

    In this power amplifier circuit, in order to solve the problems of impedance turn matching and signal phase, input and output transformers are indispensable. However, it is difficult to make a high-quality transformer in terms of materials and technology. It always consumes some energy and reduces the efficiency of the circuit. Moreover, the frequency characteristics of the transformer are not good, which makes the circuit output of signals at different frequencies very uneven and will cause distortion. Therefore, in order to improve the quality of power amplifiers, people use more transformer free (OTL) power amplification circuits.

    When designing the power amplifier circuit, the circuit should be designed simply. If the original taste of music is pursued, the links between the signal input circuit and the power amplifier circuit, such as tone, balance or other functional circuits, should be reduced or abandoned as far as possible. If you like to modify music, in addition to the above circuits, you can also add circuits such as BBE, 3D and SRS that can greatly modify music signals. Please follow the following principles in arranging the location and wiring of components on the circuit board:

    1. Reasonably arrange the position of each component on the circuit board according to the signal transmission path from small to large, and try to shorten the distance between each component (jumper can be used when the distance is too large or obstacles are encountered) to reduce the introduction of external interference.

    2. In the power supply line, it is not allowed to place the high current printed circuit or wire in the middle or near the low current path, so as to avoid interference to the low current line.

    3. The path (printed line or wire) of high current on the circuit board shall be designed to be wider (thicker) so that the current can pass through smoothly.