Heyun T99 is the predecessor of the fifth anniversary of Ober Audio. It has an elegant appearance, simple circuit and pure sound quality without sound staining.
The T99 pre-amplifier circuit is shown in the figure. It can be seen from the figure that it has almost no components except for the two electronic transistors, so before introducing it, let's talk about the electronic tube and its position in the application of audio equipment. The physical characteristics of the electron tube are still better than the transistor in some aspects, such as the modern 6N15, 6N3 electron tube, the distance between the electrodes is on the order of 10-3m, and the electron speed in vacuum reaches 107m / s under a few hundred volts of screen pressure. The overtime is in the order of 10-10s, and the frequency period for 10MHz is 10-8s. During this transit time, the voltage phase of each electrode is basically unchanged, so the tube can work without difficulty to 300 ~ 500MHz, that is to say, in the audio amplification, it is not necessary to consider the frequency characteristics of the tube, any kind of tube All can meet the audio amplification requirements of at least 10kHz. In addition, in the high-frequency high-power amplifiers above 100kW, the electronic tube is still unrivaled, and the transistor is unmatched. Therefore, in the military field and high-tech field, the electronic tube is still partly used. As for the general view that the high-frequency characteristics of the electron tube are not as good as the transistor, it is not a problem of the tube itself, but the low-pass circuit formed by the internal resistance and distributed capacitance of the electron tube during voltage amplification and the leakage inductance of the output transformer during power amplification. Caused by parasitic parameters. In short, the tube is still an excellent audio amplifier device, but the circuit design and transformer manufacturing cannot be sloppy.
From the perspective of hearing and appreciation, transistors and tubes should have their own merits and cannot be generalized. The sound of the tube is warm, sweet, and good to listen to. The air and space information are well-integrated. This has become a consensus in the audio industry. The transistor has the advantages of fast transient response, high analytical power, and deeper engraving of audio and video details.
The electron tube (triode) is composed of cathode K, screen electrode (anode) A, and grid G. The cathode is the source of the electron flow of the electron tube. When the cathode is heated to a certain degree by the filament, it will continuously emit electrons into the space. When a DC voltage is applied between the screen and the cathode so that the screen potential is higher than the cathode potential, under the action of the screen electric field, the electrons emitted from the cathode will continuously run toward the screen, the so-called vacuum tube forward guide through. According to the theorem that the direction of the current is opposite to the direction of the electron flow, the current flows from the screen electrode to the cathode. This is called the screen current Ia. The grid is the electrode that determines the amplification effect of the electron tube and is located between the cathode and the screen near the cathode. The role of the grid is to suppress the emission of electrons from the cathode to the screen. When the grid is applied with a negative voltage relative to the cathode, that is, the grid negative pressure, two electric fields are formed between the screen and the cathode in the tube: one is the positive electric field generated by the positive voltage of the screen, which attracts the electrons in the space charge region The second is the negative electric field generated by the negative pressure of the gate, which repels the electrons in the space charge region. The more negative the gate voltage, the stronger the repulsive effect, and the smaller the screen current. The gate current can be changed by changing the gate negative pressure. The grid is closer to the cathode than the screen, and the suppression of the screen current is much greater than the screen voltage, about 4 to 100 times larger. A small change in the grid voltage can cause a large change in the screen current, thereby realizing the current amplification of the electron tube.
After understanding the amplification principle of the electronic tube, let's look at the T99 pre-amplifier circuit. The T99 pre-stage circuit is a typical shunt adjustment push-pull amplifier circuit (SRPP), as shown in the figure. The two electronic transistors (6922) V1 and V2 play a regulated voltage amplification role, R5 and R6 are self-sufficient bias resistors, C3 is the output coupling capacitor, R7 is the AC load resistance, and W1 is the volume control potentiometer. SRPP is a very brilliant circuit design. The principle of the circuit is different. From the name of the circuit, it should be a push-pull circuit. Radio publications at home and abroad also call it a single-ended push-pull circuit.
From the figure, the negative voltage of the V2 gate is taken from the voltage drop of R6. The screen current that changes with the change of the signal flows through R6 and produces a correspondingly changed voltage drop. The equivalent internal resistance of V2 changes accordingly, and the change amplitude is opposite to V1. , V1 constitutes a common negative amplifier, V2 is the load of V1.
When the input signal is positive half cycle, the V1 screen current increases, the voltage drop on R6 increases, the V2 grid voltage moves in the negative direction, the V2 equivalent impedance increases, and the V1 screen voltage decreases; when the input signal is negative half cycle, the V1 screen current decreases Smaller, the voltage drop on R6 also decreases, the gate voltage of V2 moves in the positive direction, the equivalent impedance of V2 decreases, and the screen voltage of V1 increases.
In other words, when a positive signal (positive half cycle) is input from the V1 gate voltage, the stronger the signal, the more positive the gate voltage, the greater the screen current, the more negative the V2 gate voltage, the higher the equivalent internal resistance, and the greater the tube voltage drop. , The lower the ground voltage of the signal taken from the cathode of V2; when the negative signal (negative half cycle) is input from V1, the stronger the signal, the more negative the gate voltage, the smaller the screen current, the more positive the V2 gate voltage, the more the equivalent internal resistance The lower the tube voltage drop, the higher the voltage to ground from which the signal is taken from the V2 cathode. Therefore, the output signal has an increased amplitude relative to the input signal and a phase difference of 180 °. The voltage amplification characteristics of this common cathode amplifier circuit and the voltage amplification characteristics of the transistor common emitter amplifier circuit are identical in principle. The SRPP circuit has a variable load, which expands the dynamic range of the output voltage.
Compared with the fixed load common cathode amplifier circuit, the output voltage distortion of the SRPP circuit is small, the input impedance is high, and the output impedance is low. Its biggest feature is that the distortion decreases with increasing frequency and has good high-frequency characteristics. Therefore, the SRPP circuit is also a popular circuit in recent years. In the application of this basic circuit, designers have made many improvements and compensations. The design feature of T99 is that the cathode self-sufficient bias resistor R5 has a small value, only 100Ω, and the bypass capacitor is eliminated. R5 hours, the grid bias voltage is also small, under the same screen pressure, the static operating current is relatively high, the Q point is high, and the linear region is more balanced (of course, it cannot be higher than the middle position of the linear region) to reduce the static work. The point is too low and cut-off distortion occurs at large dynamics, thereby expanding the dynamic range of the amplifier. Due to the small resistance of R5, the AC signal feedback is negligible, and the elimination of the bypass capacitor will reduce the frequency distortion generated by the capacitor, and the high and low frequency response is better. Even if the bypass capacitor is removed, it will have a certain effect on the signal gain, and because the 6922 has a higher transconductance, it can still guarantee a higher signal gain. The sound of 6922 is neutral, transparent and hearty. Therefore, this design just makes the overall tone more plump and mellow.
T99 is a very simple and classic design. It does not have a cathode follower circuit, nor a high-level filter circuit such as a choke coil, but entirely lies in the reasonable design of the circuit, careful selection of components, optimal layout and careful adjustment of the structure and circuit, which makes the T99 production cost lower , The price is low and the sound quality is pure. T99 adopts one-point grounding, the internal structure is compact, the main line is welded to the ceiling, and the line loss is small, which is also one of the reasons for the pure sound of T99.
After audition, T99 has the characteristics of good balance, high analytical power, transparent sound, moderate dynamics, strong driving ability and so on. It also has good collocation characteristics, that is, no matter what level of collocation, it shows a true and natural easy-going. It is like a passive front stage with gain, except that it provides enough driving voltage to the rear stage to overcome the thin and dry sound when there is no front stage, and there is nothing else.
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