Making a high-quality electronic tube amplifier, the quality of output transformers is very important, and the high price of indicators is also very expensive, such as "West Electric" cattle. In fact, as long as it is reasonable to select the circuit, it is possible to reduce the requirements of the output transformer. The circuit made by the author is the common output transformer.
The pre-grade is a lot of clouds to use the name management 12AX7, the sound is round and delicate, and the gallbladder is strong. The power amplification stage is used as a cathode output unit with a 6P14 tube, and the output power is greater than 2.5W. Domestic 6P14 interpretation sound quality is not better than imported tubes, but the pipe parameters differ, and it needs to be paired during use. In general, the cathode DC voltage should be as consistent as possible. Since the cathode output circuit is used, its output impedance is very low, the transient response is good, the damping coefficient is improved, and the high and low frequency extension greatly reduces the performance requirements of the output transformer. The author uses a high-quality output transformer and a general output transformer to perform a pronunciation, both do not feel significant differences in sound quality.
Since the cathode power output is free, it is a 100% tandem voltage negative feedback circuit. If a conventional high-resistance output transformer is used to excite the driving, the DC power supply of the voltage push pipe 12AU7 needs to be increased by 2 times. However, because the cathode output circuit has the advantage of low internal resistance, the audio output transformer does not require a high primary impedance. After using low impedance transformers, the burden of the push-level voltage can be greatly reduced. This machine uses the impedance ratio of 1kΩ: 8Ω, a cathode output transformer, simplifies the difficulty of power production.
Output transformer finished products can be used to use "Shuguang" card, "Baore" brand, etc. Conditions can also be self-made, but should ensure a certain amount of inductance, with a low frequency end frequency, generally 10: 1 turns ratio. Although the circuit is not high in the quality requirements of the output transformer, the secondary AC impedance must be 1KΩ: 8Ω, the primary DC resistance must be guaranteed between 270Ω ~ 320Ω, otherwise the power vessel cannot establish a normal operating point. If the primary DC resistance value is small, there is too much, and the resistance should be serially connected (both ends with high quality large-capacity capacitance AC bypass), it is important to reach this. The filament is powered with a 82kΩ restriction resistance using a virtual high potential method, preventing power tube filament, cathode communication breakdown.
The power transformer uses the EI type or R-type transformer, and the transformer capacity is better. The high pressure end output voltage is 2x230V, the output current is greater than 2x120mA, and the audio is only a warm fever. The rectifier circuit uses a semiconductor diode to perform a full wave rectification, and 2 tube models are 1 n5408, with a pressure of 1kV, current 3a, and the best quality, not easy to break through.
Coupled capacitors, the author recommends using French Solen (Suleng) capacitance, which is better than other capacitors. The actual capacity should cooperate with the lower raster plate resistance, and the value should not be too large, so as not to affect the speed sensation. High-pressure filter capacitor disassemble Italy Kensone Co-cell electrolytic capacitance, cathode bypass capacitor US EC80μ small volume large-capacity film capacitance, the sense of hearing is fast. The resistance should try to use a domestic red metal film or a US Dale military resistor. The actual power has been indicated in the circuit diagram. The choke is used in a 20W high-quality silicon steel sheet for a single-way interpolation, and the frame is filled with a high-strength lacquered wire.
After the whole machine is installed, it is generally not used to make big debug, but remember that the output transformer secondary end cannot be loaded. Connect a good speaker, test 6P14 tube cathode DC voltage two channels should be consistent, so that the tube is substantially paired. If there is self-excited sound, it is caused by a large parameter deviation such as power tube sensitivity, and can eliminate self-excitement resistors at a 6P14 pipe gate to 1 kΩ ~ 5kΩ. Measuring the cathode current of the 6P14 tube should be equal to the cathode voltage to output the DC resistance of the transformer, which is suitable for no more than 45 mA, and the minimum of distortion is the best.
Push a pair of 91DB high-sensitivity high quality paper pot speaker unit speakers, listen to the sound in small area in the study, bedroom, etc., the sound quality sounds have indeed achieved impeccable levels, which can meet the more cultivated music friend.
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