Beginners to make vmos field effect tube small amplifier

VMOs Field effect tube has the advantages of both the electron tube and the advantages of transistors, the power amplifier made with it sound mellow, sweet, dynamic range, frequency response is good, so in recent years in the audio equipment has been widely used.
　　
High-power field-effect tube power amplifier, electric. The road is more complex, the production and commissioning is also more difficult, and. The cost is also very high, so it is not suitable for beginners. The following introduction of this VMOs FET small amplifier, the circuit is very simple, making debugging is also very easy, very suitable for beginners enthusiasts. As long as the components used well, the circuit and welding correct, you can succeed in one fell swoop, and the sound quality is quite good. The disadvantage of the machine is that the output power slightly smaller (about 3w~4w), but used as Mp4/target=_blank Class=infotextkey>mp3, mobile phones and other digital music playback is very applicable. The circuit circle only draws one channel, the other one is identical, and the power portion is shared.
　　
A Circuit Introduction
　　
The circuit is shown in the drawings, the output stage uses a typical single-ended class A amplification circuit, because in the entire range of the transmission signal, a class amplifier is in the linear region, while the single-ended amplifiers do not have a symmetric problem, so the nonlinear distortion is very small. Because the FET is a voltage control device, the output stage requires a smaller push power, so the push stage is also very simple, by a junction-type field effect tube.
　　
The power output is output by the output transformer coupler, and the music flavor is similar to the tube amp.
　　
Second, field-effect tube detection
　　
In amateur conditions, enthusiasts can use the pointer-type universal meter of the electrical barrier to the tube for a simple test, a rough judgement of the quality of the pipe.
　　
BG1 Junction-type FET uses 3DJ6, 3DJ7, etc., test with the rxlk or rx100ω to measure the gate and the source, gate and drain of the positive and negative resistance, positive resistance should be in the skω-lokω around, the reverse resistance should be infinite. Measurement of the source and drain resistance, positive and negative resistance response, when the hands near the gate, there is induction phenomenon, the needle swing, the greater the induction, the greater the cross-conduction. The source and drain poles can be used interchangeably.
　　
Bg2vmos Field effect Tube uses v40at, VN66AF, IRF132, 2sk134, etc., when measuring the gate and the source or the gate and the drain between the positive and negative resistance, the resistance should be infinite, so that the gate can be found first.
　　
The source and drain poles are equivalent to one PN Junction, and for N-channel reinforced pipes, the resistance value of the tube is very unstable if the black-form pen is used to connect the drain-Red watch pen to the source and the gate is open. When the contact gate with the hand, the resistance has a significant change, the greater the change, indicating that the higher the cross-conduction value of the pipe (at this time the application of Rxlok block, the table voltage is high, the test changes significantly).

Three Winding of output transformers
　　
As the output impedance of the FET is lower than that of the electron tube, it becomes smaller and the primary impedance is lower, so it does not need a large inductance to get a good frequency response, and a simple winding method can be used to meet the requirements. Iron Core Area s=20mmx20mm. Primary coil with φ0.5mm enameled wire around 220 laps, secondary with φ0.8mm enameled wire around 110 laps 80 speakers, around 78 laps connected to the 4ω horn, first around the secondary and around the primary. The size of the core and enameled wire can be flexibly selected, as long as the core window around, the relative can be substituted.
　　
Iv. Selection of power transformers
　　
The use of cassette recorders, black and white TV and other demolition transformers, the secondary voltage 18v~22v around can be used. If the voltage is not suitable, it can be solved by the redundant winding series or threading and so on, so that the DC voltage 24v-30v around the rectifier is obtained. The output power will vary depending on the supply voltage. If the secondary voltage is too low, such as about 10V, can also use the voltage Doubler rectifier to obtain the required DC voltages, but the corresponding increase in filter capacitance. Because it is a class amplification state, the current is relatively constant, so there is no need to use a regulated power supply. Filter inductor L can be replaced with daylight lamp ballast.
　　
Five, assembly and welding
　　
VMOs field effect tube Eight impedance is very high, in order to prevent accidental damage, should be at the end of the assembly to re-weld the VMOs tube, and at the time of welding the GS bipolar (or three poles) temporarily shorted. Soldering iron should be reliable grounding the shell, or when welding unplug the power cord with waste heat welding.
　　
Because the circuit is simple, the printed circuit board can be made by means of a knife engraving method, even if the use of scaffolding welding, but also to achieve success. Adjusting the resistor R3 makes the BG1 D, S-pole voltage and resistance R2 both ends of the voltage basically equal, adjust the resistance R6 change the power amplifier level quiescent current. The resistor R4 is a negative feedback resistor. The 10V regulator is an external protection diode for the VMOs tube. Because of the high calorific value of the Class A amplifier, the heat sink of the vmos tube should be large enough to ensure that the temperature rises at work.

From Shanyang Electrical Maintenance

Beginners to make vmos field effect tube small amplifier