OTL stereo mesh (20WX2) made, 20W OTL Power Amplifier
Keywords: OTL power amplifier, 20W amplifier, power amplifier circuit, amplifier production, 2N3055
In the transistor, the push-pull power amplifying circuit is widely used in the transistor. The general push-pull circuit generally needs to output transformers and input transformers. There are some disadvantages coupled by such transformers, such as: Since the magnetization curve of the transformer core is non-linear, it will cause nonlinear distortion to the amplifying circuit, due to the transformer For circuit input loops, the parasitic coupling of the intermediate frequency loop will make the machine work unstable, especially due to the existence of the transformer, and severely affect the frequency characteristics of the circuit, because the inductance of the transformer winding cannot be done too much, so At low frequencies, the sensing resistance is smaller (XL = WL), so that the signal is divided into part, so that the low frequency end gain is lowered. This makes it high, and the bass is not full enough. In order to overcome these disadvantages, another type of circuit is called "no output transformer circuit". Such a non-transformer power amplifier circuit has offset the input of stage coupling, output transformers, and use direct coupling. In this way, the circuit structure is complicated, but it is convenient for the negative feedback circuit, which makes the frequency amplitude, small distortion, and satisfies high-power and miniaturization requirements. There are many kinds of non-output transformers circuits, and the OTL circuit (capacitive coupling), OCL circuit (direct coupling), BTL (bridge) is connected to the speaker. This generator uses a typical OCL circuit, which has the advantages of high stability, wide range of range, good fidelity, and this circuit is often used in high-altitude sound. This OCL stereo megaphone is suitable for the use of electronic enthusiasts and audio enthusiasts. First, the working principle of the circuit Figure 1 is a principle circuit diagram of the present OCL stereo power.
As can be seen from Figure 1, the output of the speaker and the amplifier is directly coupled, the output capacitor for the discharge flow is saved, in order to cause the DC potential of the circuit output to zero volts, the positive and negative symmetric power supply, differential amplifier input Wait for measures. In Fig. 1, VT1, VT2 is the differential amplification input stage, and VT3 is an excitation stage, and VT4 to VT7 is a composite complementary output stage. The audio signal is sent to the base of the VT1 through the coupling capacitance C1 and R1. After amplification, the collector of VT1 is output, and the VT3 is further large, the excitation signal output from the VT3 collector is pushed to push the power output stage VT4 ~ VT7 work, Such a power amplified audio signal can drive the speaker work. In order to facilitate further analysis, Figure 1 can be simplified into the form of Figure 2. The VT4 and VT6 were equivalent to a NPN transistor, while VT5 and VT7 composite equivalent were a PNP transistor. From the VT4, 6, and VT5, 7 and the power filter capacitance C9, C10 of the circuit of Figure 3, which are equivalent to a bridge. When VT4, 6, VT5, 7 is exactly the same, C9, C10 is also identical, bridge arm balance, and the speaker has no DC. If the two sets of power supplies are completely symmetrical, the output terminal potential can be guaranteed to zero volts. Since all circuits are all direct coupling, any variations of ambient temperature and component parameters affect the potential of the output (A point, in Figure 2). To this end, VT1, VT2 constitutes a differential amplifier to overcome zero drift, and DC negative feedback is applied in the circuit, that is, the output terminal is added to the VT2 base, which ensures the potential of the output (A point) to zero volts. . The feedback process is: a point potential ↑ -ube2 ↑ -ie2 ↑ -ur4 ↑ -ube l ↓ a IC L ↓ a uc 1 ↑ -ube3 ↓ First IE2 ↓ -ur7 ↓ a UBE4, 6 ↓ (Ube5, 7 ↑) A VT4, 6 internal resistance ↑ (VT5, 7 internal resistance ↓) an A point potential ↓. Conversely, if the A point potential ↓ will make a point potential ↑ through the opposite change process.
Second, the selection output stage of the component uses the imported high-quality high-power triode; 2N3055, β value as much as possible, the remaining transistors use South Korea imported triode 90 1 4 and 0 1 2, VD3 ~ VD6 selected bridge 1 N400 1, VD1, VD2 is selected 1 N4 1 48. The electrolytic capacitors of the power filter capacitor C7 to C10 are selected from 1000 μ / 35V, and the remaining elements are given in the component list.
Third, the printed circuit diagram of the installation of this circuit is shown in Figure 3. It includes two OCL power amplifier circuits and DC power supply circuits, and 4 high power tubes 2N305 5 is required to be loaded with a heat dissipation plate between printed circuit boards, and secure it with a screw. The resistor is horizontally installed, capacitors and triodes are installed in a vertical installation, and close to the circuit board, the welding requirements are firm and reliable, and there are two jumpers on the board, which can be soldered with iron wire. 4. Debugging the power transformer used in this circuit must be prepared by self-suction, using a center tip dual output transformer (AC: 1 5 V × 2), the power is not less than 40 watts, connected to the AC ~ and top of the printed circuit board, after power-on The DC voltage of ± 1 8V is generated at both ends of C9 and C 10, and the voltage across the speaker is zero volts. First adjust the current of the differential amplifier VTL, VT2, in order to avoid the power tube, the power tube has a large current flow, first use the wire to short of VT4, VT5, so that the VT4 ~ VT7 is closed. Then weld one end of the resistor R6 to the ground. The emitter total current of the differential magnification is determined by R4, and the adjustment R4 causes the total current of the VT1 and Vt2 to 1 mA, and after recovering the resistor R6, the voltage of the speaker should be 0. If there is an offset, the R3 can be adjusted.
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