Do it yourself and satisfy your audiophile addiction in one step-EEWORLD

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Selection of main circuit:
　　Here, we take the circuit designed by Comrade Guo Yuhua of Liuzhou, Guangxi, published on the 14th page of the 20th issue of "Electronics News" in 2001 as an example to introduce its assembly process in detail. This circuit is designed with reference to the first prize of the fourth Japanese amplifier competition. It seems simple, but it is not easy to assemble it well.
　　I made a small change to its circuit, see Figure 1. In order to improve the sound quality, the original design designed the negative feedback into a DC form (R7 and R8 in the figure). After my actual assembly, the stability of the midpoint is slightly inferior to that of AC feedback, so I still use the commonly used circuit.
　　Selection of printed boards and components:
　　I ordered my printed boards from Guangxi Liuzhou Aijiu Electronics Store by mail. Except for special requirements in the figure, the resistors are all 1/2W American DALE or CGW resistors. VR1, VR2, VR3 use precision multi-turn fine-tuning resistors. The source (or emitter) resistor of the final power tube uses a 0.22Ω5W Japanese KOA non-inductive ceramic resistor. It is necessary to use a bridge test to select so that the resistance values of each resistor are as consistent as possible. Otherwise, when adjusting the static current, the voltage drop on each resistor will be very different, causing the current of each power tube to be asymmetric.
　　Electrolytic capacitors C7, C8, C9, and C10 are all selected from Japan ELNA special audio capacitors, C1, C3, and C6 are selected from Germany Weimar capacitors, and C2, C4, and C5 are selected from monolithic capacitors (or genuine ceramic capacitors). The selection of transistors and field effect tubes can refer to the introduction on the 14th page of the 20th issue of "Electronics News" in 2001. I chose K389/J109 (Q1, Q2), A1191/C2856 (Q5, Q6), K214/J77 (Q7, Q8), and K1529/J200 (Q9, Q10, Q11, Q12). To prevent accidents, all resistors need to be tested with a digital multimeter; all capacitors need to be tested with a digital capacitance meter; transistors need to be paired with a graphic display. It should be pointed out that audiophiles can replace the final amplifier tube according to their own taste, but try not to use low-end tubes such as K413/J118, K399/J113, A1941/C5198, otherwise the sound quality will be greatly reduced. In short, the quality of the components of the left and right channels is guaranteed, which also ensures that the working points of the left and right channels and the size of the signals passing through are consistent.
　　Power supply selection:
　　It is better to use a high-power bridge-type ordinary rectifier power supply. I use two 500VA ring cows here, with output voltages of 2×36V and 2×42V, one of which is also a 2×18V for speaker protection. The front and rear bridge stacks use 35A American gold seals, and the capacitors are 8 71V6800μF and 8 63V6800μF Japanese ruby electrolytics, and a self-made high-current power supply printed board.
　　Speaker protection circuit:
　　Common speaker protection circuits include μPC1237 and simple speaker protectors that detect the DC positive potential at the output end. Here, we recommend a dual-channel sampling professional audio protector assembled with discrete components that can detect the DC positive and negative potential at the output end. The circuit is shown in Figure 2. In addition to the functions of conventional protectors that delay the start-up of the speaker and automatically protect when a fault occurs, this circuit can immediately cut off the speaker line when the DC potential at the dual-channel output end increases or decreases, and has a delay indication and relay self-test alarm function. Input voltage ±24~50V.
　　Assembly and debugging:
　　Select solder containing more than 3% silver, first weld all resistors and capacitors, and then weld the triode. Q13 is fixed to the power amplifier tube radiator and connected to the printed board with a wire. Finally, weld the field effect tube. When welding the field effect tube, the soldering iron power plug must be unplugged intermittently to prevent static electricity from damaging the expensive field tube. The internal wiring and signal lines are all fever-resistant famous wires. It should be noted that the "ground" of each level of the circuit (including the speaker protector) adopts the "one-point grounding" technology.
　　Before debugging, connect a 60~100W incandescent lamp in series at the 220V input end to prevent the power amplifier tube from being burned out. The specific debugging method is: first use a dry battery to test the sensitivity of the speaker protector. After passing the test, connect the circuit back. Short the signal input end, turn VR1 and VR2 to the middle position, and turn VR3 to the maximum resistance. Connect the ±60V front-stage power supply, use a digital multimeter to monitor the voltage across R14, and use a screwdriver to rotate VR2 to make its voltage 2V; then use a digital meter to monitor the voltage at points A and B in Figure 1, and adjust VR3 to make its potential 3V; then connect the ±50V power supply of the rear stage, use the digital meter millivolt range to monitor the potential of point C to the ground, and rotate VR1 to make it 0V; finally, use the digital multimeter millivolt range to monitor the two ends of the 0.22Ω5W resistor, and adjust VR3 to make its potential 80 millivolts, that is, the static current of each tube is about 360mA, work in Class A state, burn in the machine for one hour, and then readjust it again. If your amplifier heat sink is large enough, the quiescent current can be adjusted higher, but do not let the heat sink temperature exceed 70℃. Note: During the adjustment of the quiescent current, if "avalanche" occurs (when adjusting to a certain point, the current suddenly increases or decreases), it means that the circuit is self-excited. You can adjust the capacity of C4 and C5 and increase the capacity of the capacitor appropriately, but it is best not to exceed 50pF to avoid affecting the sound quality.
　　After all the assemblies are completed, use a 3kW dedicated audio AC voltage stabilizer to stabilize the whole machine and all your audio equipment. Remove the input signal short-circuit and connect the audio source.
　　Everything is ready, sit in the emperor's seat and enjoy it. The beautiful and pleasant music will fascinate you and make you truly experience the endless fun of satisfying your audio fever addiction by yourself.

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