Method for designing preamplifier based on 6n2
Assembling a bile preamp is really the first choice for friends of DIY tube machine.
1. The design points of the tube preamplifier The tube is an amplifier device with high input impedance, high working voltage and easy aging. It is relatively difficult to design and manufacture a tube preamplifier that can meet the Hi-Fi standard than the crystal preamplifier. Big.
Pay special attention to the following points when designing:
1. Reasonable selection of electronic tubes: There are many types and models of electronic tubes. Each type of electronic tube has different performance, structure, working conditions and use occasions, and there are certain sound differences. Because of this, different tubes cannot simply be used interchangeably. In the pre-amplifier, the triode should be generally used because the linearity of the triode is good, the noise is small, the amplification factor will not be too high, and it can be used for audio amplification to obtain better overall performance and sound quality, and the circuit is relatively simple. There are many types of transistors, which one should be decided according to the circuit structure and gain. The gain of the triode amplifier is generally about 0.6 Î¼, and the Î¼ value is mostly between 20 and 100, so the amplification factor is generally not more than 70. If the circuit only needs one level of amplification, you can use a triode with a higher Î¼ value, a multi-stage amplifier It is better to choose medium or low Î¼ tubes to obtain better stability. Some European and American brands of old-fashioned electronic tubes have been heated up by the media, and the prices have doubled. It seems that only the tubes of these brands are the most beautiful. In fact, there are many artificial factors involved in this.
China also has decades of experience in the production of electronic tubes. It is a big country in the world that produces electronic tubes. The life of the tubes has reached 10,000 or even 100,000 hours. Many brands are also regarded as treasures by foreign enthusiasts. Therefore, we should look at this issue scientifically and rationally when choosing, to avoid the cloud of people and the bias of listening. As far as the triode is concerned, domestically produced 6N1, 6N2, 6N3, 6N4, 6N6, 6N11, etc. are all very good performance tubes. As long as they are used reasonably, they can also obtain extraordinary sound quality. What's more, many foreign second-hand tubes that are popular on the Internet are almost at the end of their performance.
This production tube is locked at 6N2. 6N2 is an excellent audio amplifier tube. It has basically the same parameters as 12AX7, and the sound is similar. Only the filament pins are different. It is also very simple to replace 12AX7 in use. 6N2-PDF file
|6N2 pin||6N2 appearance (Shanghai)||6N2 appearance (Dawn)||6N2 appearance (Beijing)|
2. Correct selection of operating point: as with transistors, the operating point of the tube amplifier should also be selected appropriately to have the lowest distortion and a large dynamic range. Because the characteristic curve of each kind of electron tube is different, so have their own optimal working points. The selection should be carried out in accordance with the characteristic curves and established methods given in the manual to ensure that the tube can work in the best state within the dynamic range. However, audio equipment is used to restore music, and it is its bounden duty to be loyal to the original. The technical specifications of the finished product are the designer's only criterion, but its manufacture is different from the design of general electrical appliances. The "artistic spirit" of the product.
The voltage of the tube is amplified. When there is a slight change in the gate voltage, the screen electrode will have a large change. In other words, the influence of the gate voltage on the screen current is very large. There is no audio signal input in the gate circuit When the grid voltage is negative, the screen current is smaller. When the grid voltage is negative, the screen current will be equal to zero in the cut-off state. If the audio signal U is input to the gate circuit, the grid The potential between the electrode and the cathode will continue to change, and the screen current will also change accordingly. In addition to the DC voltage drop of the working power supply of the screen electrode circuit, an audio AC voltage drop is added. Select the value of Rl, you can get the amplified signal voltage.
In the actual production, it is found that the actual characteristics of the domestic 6N2 using different brands vary greatly (detailed below), on the other hand, it also reflects the quality differences of individual domestic brand devices.
|Characteristic curve (0)||Characteristic curve (1)||Characteristic curve (2)||Characteristic curve (3)||Characteristic curve (4)|
3. Use effective noise reduction measures: the input impedance of the tube is very high, and the cathode must be heated, and it is easy to induce or introduce various AC interference noise, so the signal-to-noise ratio (S / N) of the tube amplifier is difficult to do well. This is especially true for preamplifiers.
At present, the more effective noise reduction measures are:
(1) Supply the filament with a regulated power supply. The side-heating electronic tube generally uses an AC power supply to supply power to the filament. In this way, due to the existence of a parasitic capacitance between the filament and the grid, alternating current charges and discharges this capacitance, and a voltage drop will be generated in the grid resistance, resulting in a large hum. In addition, the filament will emit a small amount of electrons under normal operating temperature (especially at both ends of the filament). These electrons fly to the inner wall of the cathode sleeve, and then form an AC interference through the cathode resistance. If the cathode capacitance is not large or there is no cathode capacitance, this AC interference cannot be effectively suppressed. If a DC stabilized power supply is used, for example, a stabilized power supply composed of three-terminal stabilized voltage blocks is used to supply power to the filament, how can the AC noise caused by the above reasons be avoided, so that the S / N is significantly improved.
(2) Reduce the potential between the filament and the grid. If you do not want to use DC voltage regulation to supply power to the filament, you can use a traditional method to connect a (50 ~ 100) Î© wire wound potentiometer in parallel at both ends of the filament power supply, and ground the sliding arm to reduce the potential between the filament and the cathode , To reduce the impact of AC power supply on S / N. In addition, a similar effect can be obtained by grounding one of the filament pins of the input stage electron tube.
(3) The purpose of adding a certain positive voltage to the center tap of the filament winding is to make the filament have a positive potential to the cathode, pull the electrons emitted by the filament back into the filament, and suppress the alternating sound generated by the filaments. In addition, appropriately reducing the supply voltage of the filament within the allowable value also has a certain effect.
(4) Improve the filtering effect of the screen power supply. Nowadays, electrolytic capacitors with high withstand voltage and large capacity are readily available. The capacity of the filter capacitor of the screen power supply can be used a bit larger, such as those with 220 Î¼F or more. If it is a multi-stage amplifier, you can use sub-level decoupling filtering method to improve the filtering effect of the screen power supply. Those with higher requirements can also adopt the choke filter mode until they are powered by a regulated power supply.
(5) Reduce the interference of stray electromagnetic field to the circuit. Electromagnetic field interference is also the main factor affecting S / N. Therefore, a high-quality preamp should use a power transformer with little electromagnetic interference, the gate signal lead should be as short as possible, and use shielded wire. It is best to add a shield to the input tube. The filament leads should be twisted and away from the grid.
(6) Avoid the interference of the ground loop and improperly grounded amplifier. The various AC interferences that enter the grid circuit through the ground circuit may affect S / N more than other factors, and should be highly valued. For simpler amplifiers, the most effective way to avoid ground wire interference is to use one-point grounding, that is, the input signal is welded to the ground point of the grid cathode, cathode cathode, and cathode capacitor, and then connected to the Connect the grounding point and connect it to the chassis. For multi-stage amplifiers, the ground points of each stage can be collected at one point, and then the ground points of each stage are connected to the power ground in order from the front stage to the rear stage, and then connected to the chassis. The best point to connect with the chassis may be the power ground or the input ground. Which one is better is generally determined by experiment.
â’‹Make sure that the resistor-capacitor has enough power dissipation or withstand voltage
Although the working current of the preamp tube is only a few milliamperes, the power supply voltage of the screen electrode is generally above 200V, and the power consumption is still considerable. Therefore, the power dissipation of the resistor used in the circuit and the withstand voltage of the capacitor should be sufficiently large. When selecting, the value of the two should generally be more than twice the actual power consumption and the applied voltage. Otherwise, reliability will be reduced.
2. Analysis and calculation of transistor preamplifier circuit
Transistor voltage amplifying circuits are usually composed of one-stage or multi-stage resistance-capacitance coupling voltage amplifying circuits and impedance conversion circuits, and some also have negative feedback circuits. Mastering the principles and calculation methods of these basic circuits, you can design a variety of preamplifiers according to actual needs.
Commonly used voltage amplifying circuits are common cathode amplifying circuits and SRPP amplifying circuits. The following is a brief introduction to its working principle and calculation method.
A) Resistance-capacitance coupled common cathode amplifier circuit
The common cathode amplifying circuit can be composed of a triode or a pentode, but the common cathode circuit composed of a five-stage tube is generally only used for the post-stage amplifier because of the large noise.
â‘ Working principle
When the signal voltage is added to the grid of the electron tube, a fat current ia is generated in the screen circuit. When ia flows through Ra, a voltage drop Ua is generated on Ra. This is the amplified signal voltage. The phase change of its amplitude is opposite to ia. When the screen pressure changes from high to low, the capacitor Ca discharges; when the screen pressure changes from low to high, the capacitor Ca charges. When the charging and discharging current is injected through RL, the voltage drop U generated on RL. This is the output signal voltage of the circuit. If the amplifier is composed of a two-stage common cathode circuit, RL is the grid electrode cathode Rg of the second-stage electron tube and outputs the signal voltage U. The grid of the second-stage electron tube will be added for further amplification.
â‘¡ Calculation method
As a high-fidelity tube amplifier, we want its frequency response to be as wide as possible. The low-frequency response of the tube is mainly determined by the input coupling capacitance Cg, the output coupling capacitance Ca and the cathode bypass capacitance Ck, where the values â€‹â€‹of Cg and Ca should meet the requirements of the following formula, namely:
Cg ï¼ˆCaï¼‰ â‰¥1 / 2Ï€fLRg
In the formula, the lower limit frequency of the fL amplifier is generally 20Hz. Rg is the value of the gate bias resistance. When calculating Ca, Rg is the value of the gate bias resistance of the subsequent electron tube. The cathode resistance Ck can be estimated by the following formula:
Ckâ‰¥ (3 ~ 5) / 2Ï€fLRk
The high frequency response is mainly determined by the load resistance R'a. And the distributed capacitance Co is determined. Its high-end cut-off frequency is:
fH = 1 / 2Ï€R'aCo
It can be seen that the smaller the Co or R'a, the wider the frequency response. The value of Co depends on the tube and circuit used, which is approximately equal to the sum of the output capacitance of the screen and the input capacitance of the next-stage gate. Therefore, electron tubes with small input and output capacitances should be selected, and the distributed capacitance formed by wiring should be reduced as much as possible. While R'a is relatively old, although it is good for high-frequency response, it cannot be too small, because the voltage amplification of the tube is KO = SR'a. When R'a is small, KO is equal to the internal resistance Ri, Ra and The parallel value of the first-level gate resistance, namely:
1 / Ugm2 = 1 / Ra + 1 / Ri + 1 / R'a
The value of Ra can be selected between (50 ~ 500) KÎ©, and the maximum allowable value of R'a is generally:
R'a = Ri Â· Ï„a / ï¼ˆCoRiâ€”Ï„aï¼‰
In the formula, Ï„a is the time constant of the tube screen, and its value is:
In the formula, M is the frequency distortion coefficient, generally 1.1 ~ 1.26.
The grid bias voltage of the electron tube can be obtained by the following formula, namely:
Egâ‰¥1 + Ugm2 / 0.7Î¼
In the formula, Ugm2 is the maximum input voltage required by the next stage or the output voltage of this stage, and Î¼ is the amplification factor given in the manual. The absolute value of the negative voltage of the gate should generally be larger than the input signal voltage amplitude (0.5 ~ 1) V to prevent the electrons emitted by the cathode from hitting the gate and causing gate current.
In general, the grid resistance of the next stage and the AC screen voltage of this pole can be taken separately:
R'a = (5 ~ 10) Ra
Ua = (0.33 ~ 0.5) Ea
After the negative voltage of the grid is determined, static and dynamic load lines can be made on the characteristic curve of the tube screen, and the values â€‹â€‹of Ri, S and Î¼ decibels can be obtained at its working point. If Ri differs greatly from the set value above, then R'a should be recalculated.
At this time, the voltage amplification factor Kz in the intermediate frequency region can be calculated by the following formula.
Kz = Î¼ / (1 + Ri / Rg + Ri / Ra)
According to the operating point current Io and the gate negative pressure, the value of the negative self-bias resistor is obtained, that is
Rk = Ez / Io
Due to the non-linearity of the characteristic curve of the electron tube, the output current waveform disproportionate to Ia and Ug will be generated, resulting in non-linear distortion. At this time, it is more convenient to analyze the nonlinear distortion by using the line segment of the dynamic characteristic curve instead of the vertical coordinate representing the current. Therefore, these current values â€‹â€‹can be represented by the corresponding line segment. The degree of asymmetry of the line segment reflects the nonlinear distortion the size of.
B) SRPP voltage amplifier circuit
1) The special circuit structure of this circuit was originally designed for high-frequency amplifiers. Because of its low distortion, low noise, wide frequency response and other characteristics, it can adapt to high fidelity requirements, so it is used by many modern tube audio amplifiers.
The DC channels of the upper and lower tubes are connected in series. The lower tube constitutes a triode common cathode voltage amplifying circuit, and the upper tube constitutes a cathode output circuit, and serves as a constant current load of the lower tube. The input signal is provided by the screen electrode of the lower tube and then output by the cathode of the upper tube. Since the voltage amplification factor of the cathode follower is close to 1, the voltage amplification factor of this circuit depends on the lower tube, which is similar to the general triode amplification circuit, but its output negative impedance is very low, the load carrying capacity is greatly improved, easy to load with low resistance match. Because the voltage of the upper tube and the lower tube is output by the cathode of the upper tube, this circuit is also called a parallel-adjusting push-pull circuit. Its special structure alleviates the influence of the distributed capacitance of the electron tube on the high frequency. The high frequency response can be more than three times the frequency of the general triode circuit, but because the cathode voltage of the upper tube is about 1/2 Ea, it has exceeded the general cathode of the electron tube and the filament. Voltage limit value, it is best to let the positive potential around 70V of the light ribbon work in the application, otherwise, the reliability is poor.
2) Calculation method
When there is no negative feedback, Ri = â–³ Ua / â–³ Ia; when there is negative feedback, the screen flow is â–³ Ia / (1 + S RK). Therefore, the internal resistance of the upper tube
Ri = â–³ Ua / [â–³ Ia / ï¼ˆ1 + S RKï¼‰] = Ra + Î¼RK
This Ri is the load of the lower tube, which is equivalent to the screen of the lower tube in parallel during AC, so the voltage amplification factor
KV = SRa ï¼ˆRa + Î¼RKï¼‰ / ï¼ˆRa + Ra + Î¼RKï¼‰
= Î¼ (Ra + Î¼RK) / (2Ra + Î¼RK)
Considering that when the load is connected, the magnification will be reduced, at this time
KV = Î¼ (Ra + Î¼RK) / (Ra + RK) (Ra + RL + 1) (Ra + Î¼RK)
The output impedance of the circuit, Ro, is equal to the ratio of the change in output voltage Î”Uo to the sum of the total current change in the upper and lower tubes Î”I1, and the amount of current change in the lower tube Î”I2. which is:
Ro = â–³ Uo / ï¼ˆâ–³ I1 + â–³ I2ï¼‰
After a series of derivations, we can get:
Ro = Ra ï¼ˆRa + RKï¼‰ / [2Ra + ï¼ˆ1 + Î¼ï¼‰ RK]
The calculation of other parameters can refer to the calculation method of the transistor voltage amplifier circuit.
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