Pure DC field effect tube power amplifier circuit-matching
[Circuit principle]
This field effect tube power amplifier is suitable for those who are obsessed with the sound of the tube, so they can't make a good sound for the post-biliary enthusiasts for various reasons. This bipolar field effect tube power amplifier is very similar to the output characteristics of the tube, with good frequency characteristics, and the timbre is similar to the post-amplifier stage. It is more ideal to match the tube pre-amplifier. SRPP electron tube pre-amplifier circuit, as shown in Figure 7-25.
The circuit is a parallel adjustment push-pull circuit, also known as a shunt adjustment push-pull circuit. It was originally used for high-frequency input stage, such as VHF / UHF tube high-frequency head circuit, but used for AF input, regardless of distortion , Linearity, amplification, dynamics and low output impedance are all better than the general class A triode amplifier, huge and contrary to many other tube circuit design, the distortion rate of the SRPP circuit decreases with increasing frequency.
This circuit is obviously two tubes connected in series, how is the parallel adjustment push-pull circuit? This is for AC work. The two tubes are connected in series, and each bears half of the power supply voltage. However, the AC signal is different. The screen of the upper tube is connected to the ground, the input is taken from the screen of the lower tube, and the cathode output (common screen circuit), so that the two tubes become parallel work. Because the grid of the electron tube is made negative when the cathode is negative, the bias circuit is also extremely simple. This principle cannot be used in transistors or op amp circuits.
The principle of the field effect tube power amplifier stage circuit, the principle of differential displacement stage is shown in the cabinet 7-26. The differential input stage uses the field effect spawning module NPD5565S, whose parameters are VDss = 55V, iDss = 6 ～ 15mA, and its input characteristics are very good (high input impedance). If there is no NPD5565S, other low-power N-channel tubes can also be used. The voltage requirement is greater than 100V, and the Gm values ​​are matched, and the error is less than 2%. VR is both the source negative feedback resistance and the zero-point potential zero-setting resistance. VT3 and VT4 are mirrored constant current sources. They are similar to DC circuits, but open for AC, so that they can meet the DC operating point of each tube and enable AC signals to be transmitted to the next level as much as possible. This is the best for the driving stage, and because the midpoint servo circuit of the traditional ordinary DC amplifier is not used, the midpoint potential zero adjustment is extremely convenient and simple. VT5 and VT6 are voltage amplification stages. Because this circuit adopts current pouring type, R4 determines the working current of VT5 and VT6. Therefore, adjust R4 to make the working current of VT5 and VT6 to 80mA. Working in the Class A state can be eliminated on the one hand The terrible crossover distortion can also increase its load capacity. If the midpoint potential is adjusted to zero but unstable (that is, the midpoint drifts), the resistance of R6 can be reduced at this time, using 6.8kohm or 4.7kohm, but the resistance should not be less than 4.7kohm, and the heat dissipation area of ​​VT3 ~ VT6 should be big enough.
The principle of the power amplifier stage is shown in Figure 7-27. The coupling between the driving stage and the subsequent stage cancels the traditional capacitive coupling method. The power amplifier tube uses a bipolar FET field effect tube. This line is 4 pairs per channel, used in parallel To reduce the output damping coefficient. The output power of this line is 50W, enough to enjoy large dynamic music. When each channel is 4 pairs, if pairing is difficult, 2 pairs can be used, and the pairing error is required to be within 3%. Each tube should be insulated with mica and coated with thermal grease. The grid resistance of each power tube adopts the metal film resistance of five-color ring 1 / 4W. The larger the resistance value, the warmer the tone, the best between 330ohm and 470ohm. The values ​​of C and R in the absorption circuit should not be too small or unnecessary.
ã€Component selection】
The power transformer adopts 600W toroidal transformer. If E-type transformer is used, it is better. The power of the driving stage and the power amplifier stage and each channel should be separated separately. The rectifier bridge is used in parallel with a high-speed bridge. The filter capacitor adopts German ROE high-speed large-capacity electrolytic capacitor. The total capacity of the filter electrolytic capacitor of each channel is greater than 33000uF, the larger the capacity, the deeper the low frequency. Large electrolytic capacitors should be paralleled with small-capacity high-speed electrolytic capacitors and polypropylene capacitors. The voltage of the driving stage is slightly higher than the voltage of the latter stage, and it is better if the driving power supply can use the servo power supply.
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