Making Single-Ended Class A Amplifier with Five-pole Power Amplifier FD422

1. FD422 dual channel power amplifier circuit principle

See the attached circuit for the circuit. The audio signal is sent to the input terminal by the input terminal, and then sent to the gate of the 6NI electron tube of the input stage through the lOOkΩ volume potentiometer. Rl is the gate leakage resistance. 6N1 is a bile duct with high resolution and soft tone, suitable for SRPP circuit.

The characteristics of SRPP circuit are better linearity of high-frequency amplification, high input impedance. Low output impedance, low distortion, wide frequency response, large dynamic range, good high-frequency transient response, clear and soft sound quality. The SRPP circuit has the advantages of common cathode amplification and cathode follower, which can make the input stage and the power amplifier stage achieve the best impedance matching. The power amplifier stage is composed of FD422 direct heating pentode to form triode, forming a single-ended Class A power amplifier circuit. The load impedance of the screen is 3.5kΩ, the voltage of the screen is 416V, the voltage of the cathode is 27V, the current of the screen is 77mA, and the self-sufficient grid negative pressure method is adopted. The power consumption of the power amplifier stage is P = UI = (416-27) x0.077 = 30W. Calculated according to the maximum output efficiency of Class A power amplifier of 35%, the maximum output power of this machine is P = 30x0.35 = 10.5W.

2. Power supply

The power supply is the key to ensuring the performance of this machine. Heavy materials must be applied to provide sufficient power to the circuit. The power of the power transformer of this machine needs to be more than 250W. In order to reduce the internal resistance of the power supply, crystal diode rectification and large-capacity electrolytic capacitor filtering can be used. This machine uses an Italian "Cardinal" 1000μF / 450V electrolytic capacitor. If there is no large-capacity electrolytic capacitor at hand, it can also be used in parallel.

3. Selection of components

Components are important factors that affect the sound quality of the amplifier. Output transformers, volume potentiometers, resistors, coupling capacitor cathode bypass capacitors, and power filter capacitors will all affect the sound quality, so they should be carefully selected. Don't think that the bile duct is cheap, just be careless about other components.

The output transformer is the key to the whole machine. The author uses 50W finished product. Inter-stage coupling capacitors and cathode bypass capacitors are important components that affect the sound quality and timbre. No matter what brand or second-hand disassembly parts are selected, their withstand voltage must be sufficient and there must be no leakage, and the capacity should be close to each other. Paired, conditionally preferred oil-immersed capacitors. In addition to the resistance power, other than IW can be used, the cathode is 30W glazed wire wound resistor.

The chassis can be designed according to its own component volume and size.

4. Production and debugging

The design of this circuit is based on the principle of "conciseness first", which can reduce installation errors. Install the power supply part, then the output part, and finally the input part. After installing each part carefully, install the next part after carefully checking, testing, and adjusting, so that the success rate is high. The components should be connected as close as possible, the star point should be grounded, the solder joint should be smooth, and the welding should be firm to avoid missing soldering. The working voltage at each point is marked in the figure for reference. After preliminary adjustments, the working point of each tube is basically in a normal state. If you play a familiar disc. The playback effect will be good, but it is not very beautiful, because the bile duct, transformer, resistance-capacitance component, and wire are new You also need patience to cook the machine, you can listen to it while cooking, and the sound is different after a period of time. If you feel that the low frequency extension is not enough, you can increase the bypass capacitance of the output stage by an appropriate amount. In addition, the value of the gate-drain resistance of the output stage can be adjusted within a certain range according to the effect.

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