Method of designing beautiful sound tube preamplifier based on 6n2

Method of designing beautiful sound tube preamplifier based on 6n2

Assembling a tube preamplifier is the first choice for those who want to DIY tube amplifiers.

1. Key points of tube preamplifier design
The tube is an amplifier with high input impedance, high operating voltage and easy aging. It is more difficult to design and manufacture a tube preamplifier that meets Hi-Fi standards than a crystal preamplifier.

Pay special attention to the following points when designing:
1. Reasonable selection of electronic tubes: There are many varieties and models of electronic tubes. Each type of electronic tube has different performance, structure, working conditions and occasions of use, and has certain differences in timbre. Because of this, different electronic tubes cannot be simply used interchangeably. In the preamplifier, triodes should generally be used, because triodes have good linearity, low noise, and the amplification factor will not be too high. They can be used for audio amplification to obtain better comprehensive performance and sound quality, and the circuit is relatively simple. There are many types of triodes, and the specific type to be selected should be determined according to the circuit structure and gain. The gain of a triode amplifier is generally around 0.6 μ, and its μ value is mostly between 20 and 100, so the amplification factor generally does not exceed 70. If the circuit only needs one stage of amplification, a triode with a higher μ value can be selected. For multi-stage amplifiers, medium μ or low μ tubes are preferred to obtain better stability. Some old-fashioned electronic tubes of European and American brands have been hyped up by the media, and their prices have doubled several times. It seems that only these brands of tubes have the best sound. In fact, there are many artificial hype factors here.

my country also has decades of experience in producing electronic tubes. It is a major country in the world in producing electronic tubes. The life of the tubes can reach 10,000 or even 100,000 hours. Many brands are also regarded as treasures by foreign audiophiles. Therefore, when choosing, we should look at this issue scientifically and rationally, and avoid blindly following others. As far as triodes are concerned, domestic 6N1, 6N2, 6N3, 6N4, 6N6, 6N11, etc. are all tubes with quite good performance. 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 now have almost reached the end of their life.

The tube used in this production is 6N2, which is an excellent audio amplifier tube. It has basically the same parameters as 12AX7, similar timbre, and only has different filament pins. It is also very easy to replace 12AX7 in use. 6N2-PDF file

2. Correctly select the working point: Like transistors, the working point of the tube amplifier should also be selected appropriately to achieve the lowest distortion and a larger dynamic range. Since the characteristic curves of each tube are different, they have their own optimal working points. When selecting, the characteristic curves and established methods given in the manual should be followed to ensure that the tube can work in the best state within the dynamic range. However, audio equipment is used to restore music, and being faithful to the original is its mission. Completing the technical specifications of the product is the only criterion for the designer, but its manufacturing is different from general electrical design. On the basis of technical specifications, the "artistic spirituality" of the product is also required.

The voltage amplification of the electron tube will produce a large change in the screen when there is a slight change in the gate voltage. In other words, the influence of the gate voltage on the screen current is very large. When there is no audio signal input to the gate circuit, the screen current is a stable DC, and the more negative the gate voltage is, the smaller the screen current is. When the gate voltage is negative to a certain extent, the screen current will be equal to zero and is in the cut-off state. If an audio signal U is input into the gate circuit, the potential between the gate and the cathode will change continuously, and the screen current will also change accordingly. In addition to the DC voltage drop of its own working power supply, an audio AC voltage drop is added to Rl of the screen circuit. We can get the amplified signal voltage by properly selecting the value of Rl.

In actual production, it is found that the actual characteristics of domestic 6N2 of different brands vary greatly (details are given below), which 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 electron tube is very high, and the cathode must be heated, which is very easy to sense or introduce various AC interference noises. Therefore, it is difficult to improve the signal-to-noise ratio (S/N) of the electron tube amplifier, especially for the preamplifier.

At present, the relatively effective noise reduction measures are mainly:
(1) Use a stabilized power supply to power the filament. Indirectly heated electron tubes generally use an AC power supply to power the filament. In this way, due to the existence of parasitic capacitance between the filament and the grid, the AC charges and discharges the capacitance, which will produce a voltage drop on the grid resistor, resulting in a large AC noise. In addition, the filament will emit a small amount of electrons (especially at both ends of the filament) at normal operating temperature. These electrons fly to the inner wall of the cathode sleeve and then enter the ground through the cathode resistor to form AC interference. If the cathode capacitor is not large or there is no cathode capacitor, this AC interference cannot be effectively suppressed. If a DC stabilized power supply is used, such as a stabilized power supply composed of a three-terminal voltage regulator block, the AC noise caused by the above reasons can be avoided, and the S/N can be significantly improved.
(2) Reduce the potential between the filament and the grid. If you do not want to use DC voltage regulation to power the filament, you can use the traditional method to connect a (50~100) Ω 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, thereby reducing the impact of AC power supply on S/N. In addition, grounding one of the filament legs of the input stage electron tube can also achieve a similar effect.
(3) Add a certain positive voltage to the center tap of the filament winding. The purpose of adding a positive voltage is to make the filament positive relative to the cathode, pull the electrons emitted by the filament back to the filament, and suppress the AC noise generated by the electrons emitted by the filament. In addition, appropriately reducing the filament supply voltage within the allowable value also has a certain effect.
(4) Improve the filtering effect of the screen power supply. Nowadays, high-voltage and large-capacity electrolytic capacitors are readily available, and the capacity of the screen power supply filter capacitor can be larger, such as 220 μF or more. If it is a multi-stage amplifier, the method of decoupling filtering in stages can be used to improve the filtering effect of the screen power supply. For higher requirements, choke filtering can also be used until a regulated power supply is used.
(5) Reduce the interference of stray electromagnetic fields on the circuit. Electromagnetic field interference is also a major factor affecting S/N. Therefore, high-quality preamplifiers should use power transformers with low electromagnetic interference, and the gate signal lead should be as short as possible and use shielded wires. It is best to add a shielding cover to the input stage electron tube. The filament lead should be twisted and kept away from the gate.
(6) Avoid interference from the ground loop Amplifiers with improper grounding. Various AC interferences that penetrate into the gate loop through the ground loop may have a greater impact on S/N than other factors, and should be taken seriously. For simpler amplifiers, the most effective way to avoid ground interference is to use a single-point grounding, that is, the input signal and the grounding points of the gate electrode, cathode electrode, and cathode capacitor are welded at one point, and then connected to the grounding point of the power filter capacitor, and then connected to the housing. For multi-stage amplifiers, the grounding points of each stage can be first gathered at one point, and then the grounding points of each stage can be connected to the power ground in order from the front stage to the back stage, and then connected to the housing. The best point to connect to the case may be the power ground or the input ground, and which is better is usually determined by experiment.

4. Ensure that the resistors and capacitors have sufficient power dissipation or withstand voltage

Although the working current of the preamplifier tube is only a few milliamperes, the screen power supply voltage is generally above 200V, and the power consumption is still considerable. Therefore, the power dissipation of the resistor and the withstand voltage of the capacitor used in the circuit should be large enough. When selecting, the values ​​of the two should generally be more than twice the actual power consumption and the applied voltage. Otherwise, the reliability will be reduced.

2. Analysis and calculation of triode pre-voltage amplifier circuit

The triode voltage amplifier circuit is usually composed of one or more stages of resistor-capacitor coupled voltage amplifier circuit and impedance conversion circuit, and some are also added with negative feedback circuit. By mastering the principles and calculation methods of these basic circuits, various preamplifiers can be designed according to actual needs.

Commonly used voltage amplifier circuits include common cathode amplifier circuits and SRPP amplifier circuits. The following is a brief introduction to their working principles and calculation methods.

A) RC coupled common cathode amplifier circuit

The common cathode amplifier circuit can be composed of triodes or pentodes, but the common cathode circuit composed of pentodes is generally only used in post-stage amplifiers due to its high noise.

① Working principle

When the signal voltage is added to the gate of the electron tube, the screen circuit generates a dynamic current ia. When ia flows through Ra, a voltage drop Ua is generated on Ra, which is the amplified signal voltage. The phase change of its amplitude is opposite to ia. When the screen voltage changes from high to low, the capacitor Ca discharges; when the screen voltage changes from low to high, the capacitor Ca charges. When the charging and discharging currents are injected through RL, the voltage drop U generated on RL is the output signal voltage of the circuit. If the amplifier is composed of a two-stage common cathode circuit, RL is the gate electrode Rg of the second-stage electron tube, and the output signal voltage U will be added to the gate of the second-stage electron tube for further amplification.

② Calculation method

As a high-fidelity tube amplifier, we hope that its frequency response is as wide as possible. The low-frequency response of the tube is mainly determined by the input coupling capacitor Cg, the output coupling capacitor Ca and the cathode bypass capacitor Ck, where the values ​​of Cg and Ca should meet the following requirements, namely:

Cg（Ca）≥1/2πfLRg

In the formula, fL is the lower frequency limit of the amplifier, which is generally 20Hz, and Rg is the value of the gate bias resistor. When calculating Ca, Rg is the value of the gate bias resistor of the next level of electron tube. The cathode resistance Ck can be estimated using the following formula:

Ck≥（3~5）/2πfLRk

The high frequency response is mainly determined by the load resistance R'a and the distributed capacitance Co. Its high-end cut-off frequency is:

fH =1/2πR'aCo

It can be seen that the smaller Co or R'a is, the wider the frequency response is. The value of Co varies depending on the electron tube and circuit form used. It is approximately equal to the sum of the screen output capacitance and the next-stage grid input capacitance. Therefore, electron tubes with smaller input and output capacitance should be selected, and the distributed capacitance formed by the wiring should be reduced as much as possible. When R'a is relatively large, although it is beneficial to high-frequency response, it cannot be too small, because the voltage amplification factor of the electron tube KO=SR'a, when R'a is small, KO is numerically equal to the parallel value of the internal resistance Ri, Ra and the next-stage grid resistance, that is:

1/Ugm2=1/Ra+1/Ri+1/R'a

The value of Ra can be selected between (50~500)KΩ, and the maximum allowable value of R'a is generally:

R'a=Ri·τa/(CoRi—τa)

In the formula, τa is the electron tube screen time constant, and its value is:

τa =

Where M is the frequency distortion coefficient, which is generally between 1.1 and 1.26.

The grid bias voltage of the electron tube can be calculated using the following formula:

Eg≥1+Ugm2/0.7μ

In the formula, Ugm2 is the maximum input voltage required by the next stage or the output voltage of this stage, and μ is the amplification factor given in the manual. The absolute value of the gate negative voltage should generally be (0.5~1)V larger than the input signal voltage amplitude to prevent the electrons emitted by the cathode from hitting the gate and causing gate current.

In general, the gate resistance of the next stage and the AC screen voltage of the current stage can be taken as:

R'a = (5~10) Ra

Ua = (0.33~0.5) Ea

After the negative grid voltage is determined, static and dynamic load lines can be drawn on the electron tube screen characteristic curve, and Ri, S, and μ decibel values ​​can be calculated at its operating point. If Ri is greatly different from the above set value, R'a should be recalculated.

At this time, the voltage amplification factor Kz in the intermediate frequency region can be calculated using the following formula.

Kz=μ/(1+Ri/Rg+Ri/Ra)

Then, the value of the cathode self-bias resistor is calculated based on the operating point current Io and the gate negative voltage, that is,

Rk=Ez/Io

Due to the nonlinearity of the electron tube characteristic curve, the output current waveform will be disproportionate between Ia and Ug, resulting in nonlinear distortion. At this time, it would be more convenient to use the line segments of the dynamic characteristic curve instead of the ordinate representing the current to analyze its nonlinear distortion. Therefore, these current values ​​can be represented by corresponding line segments, and the asymmetry of the line segments reflects the magnitude of the nonlinear distortion.

B) SRPP voltage amplifier circuit

1) The special circuit structure of this circuit was originally designed for high-frequency amplifiers. Because it has the characteristics of low distortion, low noise, wide frequency response, etc., it can meet the requirements of high fidelity, so it is adopted by many modern vacuum tube audio amplifiers.

The DC paths of the upper tube and the lower tube are connected in series. The lower tube constitutes a triode common cathode voltage amplifier circuit, and the upper tube constitutes a cathode output circuit and serves as a constant current load for the lower tube. Its input signal is provided by the screen 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 amplifier circuit, but its output cathode impedance is very low, the load capacity is greatly improved, and it is easy to match with low-resistance loads. Since the voltages of the upper and lower tubes are output by the cathode of the upper tube, this circuit is also called a parallel adjustment push-pull circuit. Its special structure reduces the influence of the distributed capacitance of the electron tube on the high frequency, and the high-frequency response can be more than three times wider than the general triode circuit, but since the cathode voltage of the upper tube is about 1/2 Ea, it has exceeded the voltage limit between the cathode and the filament of the general electron tube, so it is best to let the filament work with a positive potential of about 70V when applied, otherwise, the reliability is poor.

2) Calculation method

When there is no negative feedback, Ri = △Ua/△Ia; when there is negative feedback, the screen current is △Ia/(1+SRK). Therefore, the internal resistance of the upper tube is

Ri=△Ua/[△Ia/(1+S RK)]= Ra+μRK

This Ri is the load of the lower tube. It is equivalent to being connected in parallel with the screen of the lower tube in AC, so the voltage magnification is

KV=SRa(Ra+μRK)/(Ra+ Ra+μRK)

=μ(Ra+μRK)/(2Ra+μRK)

Considering that the amplification factor will be reduced when the load is connected,

KV=μ(Ra+μRK)/(Ra+RK)(Ra+RL+1)(Ra+μRK)

The output impedance of the circuit is Ro, which is equal to the ratio of the change in output voltage △Uo to the sum of the total current change △I1 of the upper tube and the lower tube, and the current change △I2 generated by the lower tube. That is:

Ro=△Uo/（△I1+△I2）

After a series of derivations, we can get:

Ro=Ra（Ra+RK）/[2Ra+（1+μ）RK]

The calculation of other parameters can be carried out by referring to the calculation method of the transistor voltage amplifier circuit.