Keywords: simple field effect tube amplifier
Simple field effect tube amplifier
Author: Fang Junkun
The field should have a rich harmonics, and the sense of the tube is very good. Moreover, its output power is much larger than the tube, and the cost is not high, so the daring front-level matching effect tube amplifier is very popular in the fever circle.
This circuit input part is more simple. The audio low noise tube C1815 is used in a pair of performance indicators to form differential amplifying circuits. The 33kΩ resistor and 1kΩ resistor in the load output feed back feedback to the base of the BG2, the working voltage of the output tube is automatically balanced, and the trouble of adjustment of the midpoint voltage is saved. 1881.50mV.
BG3 is a constant current source to improve the common mode inhibition ratio of differential amplification circuits. The base potential is provided by the light emitting diode, the performance, and signal-to-noise ratio are superior to the regulator diode, and the base potential of the BG3 is controlled around 1.8V. When working, the tube can also function as an indicator. The BG5 is another constant current source, making the voltage gain of the BG4 have a great improvement.
BG6, BG7, BG8 are medium power voltage amplification push tubes. Where BG6 functions as a static current of the whole machine, the adjustment element has only one VR with locking bolts. Before debugging, the adjustable power is placed in the intermediate position, and the D pole of the positive power supply and the two power tubes is connected in series with 1A DC. The trim value is large, the value is large, generally adjusted to 200mA, and lock can be Turn resistor. Since the negative temperature characteristics of the field effect tube, the static current will automatically reduce. But it will not affect the normal work of the whole machine. 8 field sensitivity should be the most matching, which prevents the greatest conditions of a fever. K85L, K854 and other field effect tube disassembled the price is very low, the performance is stable, and the sound quality is good.
The native coupling capacitor has an input per channel, and must use the CBB-free capacitance. The author has been experimenting. Any electrolytic capacitance (including the best) due to a certain leakage rate. Wound inductance and transition rate is low, etc. The sound is not good.
The power supply uses a transformer of 100W. The secondary AC voltage is 2x26V or 2x28V. The rectifier bridge current is greater than L0A. Filtering capacitors with 4 6800μF / 50Velna high speed electrolytic capacitance. And in parallel with a LμF CBB capacitor for high frequency decoupling. The principle of circuit is shown in the figure.
The speaker protection circuit is installed after installation.
It can be tested is normal, the method is to turn on the protector power supply separately, and the secondary relay J is inherent. At the upper end of the 10KΩ resistor and the land 1.5V dry battery. The relay is turned off immediately. It indicates that work is normal.
Enter the central point of the terminal, composed of R, C and K, which is ultra-low tone boost circuit. After the small sound is in the first time, the bass is significantly improved, and it feels good to feel the sound wave. The native sound is not much different from the amplifier, but less charm, but the output power is greater. High and low frequency extension is better.
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