Use EL156 to build a 25W single-ended Class A amplifier circuit diagram

The tone of the tube amplifier is mellow and transparent, and the sound is natural, sweet, and sincere, which makes you listen to it for a long time and is full of humanistic color. In building a harmonious and beautiful life, tube amplifiers have become a good companion for audio enthusiasts and are also becoming a fashion.

Among tube amps, single-ended Class A tube amps have the best sound quality. The single-ended Class A tube amplifier composed of 300B and 845 tubes has become the best among tube amplifiers. It has the most beautiful sound and is unforgettable. However, the single-ended Class A amplifier composed of 300B has a shortcoming in the ointment. The output power is generally only 6.5W, which is too small for a listening space of more than 20m2. The output power of the single-ended Class A amplifier composed of 845 tubes is around 20W, which can meet the listening requirements. However, the 845 is a high-voltage bile duct, and the working voltage needs to be above 1200V. Not only is the production cost high, it is not safe during production, and it is also large in size. So, is there a kind of tube that can output more than 20W and is small in size, suitable for building a home amplifier?

The famous Western European tube EL156 is worthy of this task. Many people don’t know about EL156 gallbladder. It's no wonder that EL156 was only developed in 1968 and is considered a relatively young generation in the Gallbladder family. The 1960s was the peak era of courage development, and it began to decline thereafter. By the 1970s, foreign transistors had fully entered the electronics industry, replacing electron tubes. Therefore, although the EL156 was developed as the latest type of excellent tube, it was overwhelmed by the widespread use of transistors before it could be applied, so that it remained anonymous and little known.

EL156 was developed and put into production by the famous German company T'elefunken. The audio amplifiers produced by this company are exquisite in workmanship, elegant in shape, and have a balanced, sweet, pure and delicate tone. They are the best in audio amplifiers. They are ranked alongside the British Mullard as the number one brand in Western Europe. It's a pity that Telefunken also closed its bile duct production line during the wave of popularization of transistors. EL156 has long been discontinued.

Fortunately, my country's Shuguang Electronic Tube Factory re-developed the EL156 based on the original factory information and became the only manufacturer of this tube. According to the manufacturer, EL156 is exported to Japan in large quantities every year and has become the new favorite of Japanese bold fans.

EL156 is an oxide cathode gall, so it has higher cathode emission current and lower operating voltage. The screen consumption of this tube is as high as 50W, which is 10W higher than the KT88 high-power tube. I'm afraid EL156 can be regarded as the king of side-heated cathode gallbladders. Compared with the 845 direct-heating tube, the filament power consumption of the EL156 is 2/3 lower, the screen electrode operating voltage is only 1/3, and the single-ended output power can reach 25W. The superior performance is impressive. It is true that "the latecomers take the lead".

1. Circuit working principle

The attached picture shows the circuit of the A23I..156T tube amp using EL156 tubes. This machine is a two-channel stereo amplifier. Only the amplification circuit of one channel is shown in the figure, and the other channel is exactly the same.

As can be seen from the picture, this machine uses two levels of amplification. The preamp uses ECF80 (the domestic model is 6F1), and the power amplifier stage uses EL156 tubes, which are single-ended Class A amplification programs. Usually the preamplifier includes two stages of preamplification and push amplification to meet the push requirements of the power amplifier. However, the EL156 has excellent performance and can output full power with only about 20V of push, so it is enough to combine the pre-stage and the push into one-stage amplification. In Hi-Fi power amplifiers, the number of amplification stages should be reduced as much as possible, so as to reduce the distortion and deformation of the signal during the amplification process.

The preamplifier ECF80 is a three-pole and five-pole composite amplifier. The five-pole part is responsible for signal amplification, and the three-pole part is used as the load in the five-pole amplification. The three-pole gallbladder is equivalent to a nonlinear load resistor, which has a certain offsetting effect with the nonlinearity in the pentode gallbladder, thereby reducing the distortion in the output (the working principle is derived from the ZL94212716.1 patented technology). In addition, due to the space charge effect in the bladder, noise can be reduced. Generally, compared with the resistive load, a higher signal-to-noise ratio can be obtained, which can improve the quality of the signal output by the previous stage. The cathode of the triode tube is connected to an adjustable resistor Wk. Adjusting Wk can change the internal resistance of the triode tube, so that the preamplifier is in a suitable working range and ensures that the preamplifier output has a dynamic range of 20V to promote the full output of the power amplifier Ell56. . In fact, the output range of this preamp can reach ±100V, which is enough to drive any power amplifier.

The power amplification of the latter stage adopts a super linear circuit . The super linear circuit has the high efficiency of a pentode tube and the output characteristics of a triode tube. It is an excellent power amplifier circuit unique to bile ducts. Due to the low internal resistance of the triode output, it has strong control over the speaker and can make the speaker well damped, thus making the sound clear and layered. The ultra-linear circuit of this machine is realized through resistor voltage division. The negative feedback voltage of the curtain grid is determined by the ratio of R5 and R6, and does not need to be obtained from the output voltage tap. The advantage of this "resistor voltage dividing super linear circuit" is that the optimal curtain grid dynamic negative feedback voltage can be obtained by changing R5 and R6, so that the performance of the super linear circuit can be optimized. It also reduces the difficulty of winding the output cow (for details, please refer to the relevant content of the 12th issue of this journal in 2004). Due to the low difficulty of winding the output cowl, the output cowl of this machine has achieved high performance, and the frequency response under load can reach more than 120kHz, which lays a solid foundation for the high level of the power amplifier.

The power amplifier circuit adopts self-biasing method. The cathode of EL156 has a voltage regulator tube DWl and a resistor Rn in series. DWl generates a stable cathode voltage of about 20V, giving ELl56 a suitable gate negative voltage operating point. RD is only 47Ω, but it cannot be omitted. It has a stable operating point and signal negative feedback effect, which can effectively reduce the distortion of the power amplifier stage.

Usually, the cathode voltage of the power amplifier stage is achieved by connecting the cathode resistor and the capacitor in parallel. This is a traditional approach, but the capacitor needs to be large enough to ensure the low-frequency lower limit of the power amplifier stage. The low frequency limit of HiFi power amplifier is as low as below 10Hz, and the corresponding parallel capacitance will reach hundreds to thousands of μF. In fact, only electrolytic capacitors can be used, and electrolytic capacitors are of poor quality and have poor high and low frequency performance, seriously damaging the quality of the power amplifier signal.

Zener diode (DWl) is a widely used voltage stabilizing device currently. The low frequency response is to DC (0Hz), the high frequency response is hundreds of kHz, and the performance is excellent, much better than electrolytic capacitors.

However, DWl has no DC stabilizing effect, so Rn must be connected in series to stabilize the DC operating point of EL156.

The gate of the power amplifier stage is connected with Rb and Cb high-frequency boost circuits to compensate for the loss of high-frequency components by the output cow. Although these high-frequency components are beyond the hearing range of the human ear, including these high-frequency components in the playback will create a more realistic and natural feeling. The improvement of high-frequency components can make the overall response of the amplifier broad and flat, achieving the high performance of Hi_Fi. This is a way to consider the problem from an overall perspective, and it is also an effective way for the amplifier to overcome the shortcomings of output frequency. Using this method, the frequency response of the tube amplifier can rival that of the transistor amplifier, reaching over 80kHz, and the sound quality is also superior.

High-quality power amplifiers must pay full attention to the power circuit. In fact, as the amplification circuit is becoming more and more perfect, every step of improvement in the power supply circuit will significantly improve the sound quality of the amplifier. The power circuit of this machine uses field effect tube electronic filtering and electronic voltage stabilizing circuits, which are better than traditional filter circuits. In terms of performance, they are unmatched by traditional circuits. The power supplied to the power amplifier circuit reaches or even exceeds that of the preamplifier. requirements, close to the level of ideal power supply. Another feature of using field effect tubes is that the even harmonics produced when the field effect tubes are working seem to beautify the sound quality of the whole machine, making the sound soft and sweet.

The filament voltage is AC powered for better sound quality. In order to overcome the possible hum sound, a balance potentiometer Wl is connected to the filament circuit. Adjusting wl can eliminate the hum sound.

In the circuit diagram, specific values ​​are not given for individual components. Choosing different values ​​and varieties of these components will significantly change the sound quality and timbre, but it will not affect the working status of the circuit. We expect readers to use their hands and brains to choose and adjust the best sound quality for discussion and communication.

2. Selection and production of components

The traditional tube amp was based on the component form at that time, and the manufacturing process was just "sheltering and welding". This circuit technology is mature and suitable for the level of components at that time. Nowadays, circuit technology has already developed to printed circuits, and the level of components has been qualitatively improved. Small-volume, large-capacity components are quite popular, paving the way for the widespread application of printed circuits. Compared with "scaffolding", printed circuits can be said to be a more advanced form of circuit technology. They are not only highly efficient, but also easier to implement for complex and precise electronic circuits. Due to the use of computer wiring, printed circuits can easily achieve the shortest connection between two points and the ideal one-point grounding form. For Hi-Fi amplifier production that pays attention to wiring, it is more ideal to use printed circuits.

This machine uses 2mm double-sided gold-plated printed circuits, which not only completes the ideal wiring of the circuit, but the gold-plated lines also ensure the high quality of the connections, allowing signals to flow smoothly in the circuit. Structurally, this machine is different from traditional tube amps. It adopts the form of "flip-up cover", and the power supply, output, printed circuit and connectors are all installed in the bottom box. The center of gravity of the whole machine has been moved downwards, making it more stable and in line with the pursuit of audiophiles. During production, all circuit connections are completed by soldering the printed circuit. Install the printed board in the bottom box and connect a few wires to the power supply and output. After the debugging is completed, all production is completed. There is no need to turn over and over to install, weld, and debug like traditional tube amps, which is very laborious. Close the upper cover formed by aluminum alloy, and a beautiful, high-quality amplifier will appear in front of us. The structure of this machine is very suitable for enthusiasts to DIY amplifiers .

All resistors use metal film resistors to ensure the noise suppression background of the whole machine. Among them, R5 and R6 are 3W, and the rest are 1/2 W. R3 generates more heat and uses a 6W small wirewound resistor, which is easy to solder directly on the printed circuit board.

Because the volume control RW is at the signal input end, it uses a Japanese ALPS high-end square potentiometer, which not only ensures the signal-to-noise ratio of the whole machine, but also keeps the two-channel volume control highly synchronized, and will not cause sound field changes due to volume changes. Positioning drift. ALPS potentiometers have excellent synchronization that many potentiometers simply cannot compare to. Semi-adjustable Wk and Ws use high-quality organic solid potentiometers. Wl uses a precision fine-tuning potentiometer to accurately balance the hum, so that the suppression of residual AC noise of this machine is comparable to that of a transistor machine.

The rectifier tube adopts BY wave-sealed high-frequency tube, which is beneficial to reducing power supply noise.

The choice of capacitor has a greater impact on the sound quality and is more sophisticated. Cl and C2 need to use CBB capacitors, C05 and CF need to use ceramic capacitors, and C04 needs to use imported CBB capacitors. The power supply filter capacitor uses imported (ruby) large-capacity electrolytic capacitors.