25W × 2 EL34 push-pull amplifier, EL34 Power Amplifier
Keywords: tube, EL34, amplifier, amplifier
Due to the characteristics of the electronic pipe device itself, the output power of amateur production is generally not too large, especially when using single-ended output amplifiers, the output power is smaller, usually only a few watts, generally no more than 10W. For a 10W power amplifier, it is often only available to 1 to 2W average work power. Because when the high-fidelity is replayed, the peak power of the program source is large, the peak power of 1 to 2W has exceeded 10W, thereby increasing the show, especially when using a speaker with a low sensitivity, distortion More serious. In the Minimum performance requirements of the International Electrotechnical Commission (IEC), the minimum performance requirements of the high-fidelity home audio power amplifier are required, and the rated output power of the amplifier should be ≥10W. It can be seen that if we must appreciate various music programs, including large symphony, etc., a 10W × 2 amplifier is the minimum requirement. In general, the rated power of 20 ~ 30W × 2 can reach the basic goal of high fidelity reproduction. This article details an EL34 25W × 2 amplifier production, which requires ambition that requires larger output power.
I. The output power of the basic structural ascending machine depends primarily on the output stage electronic tube model and the circuit program used. In terms of circuit program, there is usually two ways to increase its output power. For single-ended amplification, two output power tubes can be used in parallel, as shown in FIG. 1 (a). Compared with the single tube, the screen current is increased by 1 fold, and the output power is also increased by one-fold, while the input voltage and distortion remain unchanged. In addition, since the bilayer is in parallel, the internal resistance is half, the load resistance is also reduced by half. Therefore, as long as the current capacity is increased, it is easy to reduce the single-tube Class A output to double-tube Class A work mode. In fact, the primary side impedance of the output transformer is not necessarily reduced by half, and it has also been able to obtain a much larger output power than the single tube. For example, when 300b is operated by single-ended Class A, the output transformer is 3 to 3.5 kΩ. When it is changed to two-tube Class A, the output transformer can take 1.5 to 1.6 kΩ at one time, and the output power is substantially 2 times when the output power is substantially single. However, if the existing impedance is 1.5 to 1.6 kΩ, an output transformer of 2 to 3 kΩ can be used for an output transformer of 2 to 3kΩ.
Another way to improve the output power is to use a push-pull output, as shown in Figure 1 (b). Since the working state of the push-pull output can be set to Class A, AB classes, and Class B, it can achieve 1-2 times when the output power is reached. In terms of Class A push-pull, the smaller push-pull is generally doubled than that of single-ended output power. In fact, when the push-pull operation, the two electronic tubes are operated in series, thereby the primary side impedance of the output transformer is 1 times when the single-ended output is increased, and the output power is also increased by 1 degree. Compared with the single-tube single-ended output, the output power of the dual-tube single-ended output and the push-pull output is twice. However, the push-pull output is not only efficient, but also due to the opposite direction of the two tube flows over the output transformer, the DC magnetic flux offsets each other, and the iron core of the transformer does not need to leave an air gap, the same conditions. The inductor is much higher than the single tube, which is conducive to simplifying the structure. In addition, the push-pull output transformer has the effect of counteracting the power ripple, thereby reducing the requirements for power supply filtering. Finally, the push-pull circuit can also counteract the tempo harmonic, distortion is significantly lower than the single-ended output. Push-pull circuits, disadvantages, tubes need to be paired, and two drive signals that need two amplitude and phase reverse are needed to reverse phase circuits. In addition, the two primary side windings of the transformer, the number, DC resistance, leakage and distributed capacitors should be guaranteed to ensure symmetrical and balance. If you do it in this regard, the advantage of the push-pull circuit is more obvious. The push-pull output must have three ways shown in Figure 2. Fig. 2 (a) is inverted phase of the transformer, there is no need to use the active device, the circuit is simple, the inverted signal waveform is good, the distortion is small, and the output impedance is low is not easy to be affected by the output level gate stream and the driving ability is strong, but this requires excellent performance. Input transformer. Fig. 2 (b) is a screen tensile load inverted phase, since the signal taken from the screen load resistance and the cathode resistance is just inverted, so as long as the screen resistance is equal to the cathode resistance, 100% is added by the cathode resistance. The current of this stage is negative feedback, which can obtain an inverse drive signal equal to the amplitude of the screen and the cathode. The circuit distortion is smaller, and the gain is ≤1, so it is required to have a sufficiently high gain, or then add a secondary push-pull voltage amplifier, otherwise it is not easy to obtain a sufficient power output.
The above two inverted circuits do not need to be adjusted, and the gain is substantially ≤1. my country is most common in Figure 2 (b). Figure 2 (c) is a cathode coupling inverted, and it is characterized by inverting the input signal to achieve a higher gain. However, the inverting signal amplifier output of this circuit is not strict, that is to say, it is necessary to make a balance adjustment, which is inconvenient for amateur production, so this inverting circuit is more common in the manufacturer production machine. In the past, there were not many inverted circuits in my country's amateur production. For this reason, there should be a general understanding of its working principle, please refer to Figure 2 (c). The input signal is first enlarged (V1) by the first stage voltage, and then enters V on and V for inverting and amplification. The output signal of the V1 directly enters the V-upper gate and the signal amplified from its screen is taken out as a rear push-to-rank of all-line driver signals. Obviously, the signal is inverted to its gate input signal. That is, the working conditions on V are exactly the same as the general voltage amplification level. Another push-pull drive signal is taken from the V. However, the working configuration of the V is different from v. Its cathode is connected to V. The cathode is shared with a cathode resistance. Its gate is connected to the V top gate through a high value resistor R (1 MΩ), and at the same time, the gate of V is grounded by a large capacitance C (0.47 μF). That is, the gate of the V is grounded, that is, the V is operated in the gate grounding state, while the input signal is taken from the cathodic resistance and injected by the cathode. In this way, the V is on the cathode ground amplification state, and the screen output signal is inverted to the gate input signal, and the cathode resistance is inverted with the gate input signal with the screen output signal. The V is operated in the gate ground. The screen output signal is inserted with the cathode input signal, i.e., in reverse phase of the V upper screen output signal, thereby at V, V. The lower screen can take out the phase opposite inverted phase signal. As for the amplitude between the inverted signal, the amplitude of the output is not the same when the amplification amount is different in the two working states, and thus when the two tubes take the same screen resistance. It is usually an enlarged amplification when the cathode is grounded, so it is possible to obtain an inverted phase signal of the magnitude of the magnitude of the magnitude of the web with the R2 in the case where the screen is properly increased.
II. Practical circuit Figure 3 is a circuit schematic diagram of a 25w × 2 power amplifier designed in accordance with the above circuit structure, wherein the power supply portion is shared by left and right channels. From the figure, the entire magnifying portion is composed of an input voltage amplification (V1), inverted phase (V2), and output (V3, V4) 3). The output stage uses EL34, which is stable, and the sound quality is also quite good, and it is easy to buy in the market, no effort to break the goods. The attached table is the main application characteristics of EL34.
As can be seen from the table, when the EL34 is amplified, the maximum output power of the screen load impedance is 11W (the distortion rate is 10%). When it is pushing, the maximum output power in the screen-screen load impedance is 36W (5% distortion rate).
Since the maximum output power requirements of this unit are about 22 to 25W, it can basically use the working parameters in the table in the table, that is, the screen electrical pressure is 350V, the screen current is slightly larger (about 48mA), which is beneficial to reduce distortion. Here to point out that the screen or cord voltage in the table is relative to the cathode potential. Since this output stage uses a cathode self-bias circuit, the voltage drop (about 30 V) of the cathode common resistor R13 is used as the output stage, and therefore, according to the screen voltage in the table, according to the screen Substance can be changed. When designing the circuit, the actual voltage of the screen is about 350 + 30 = 380 (V). According to the measured data in Fig. 3, the output stage cathode (pair) voltage is 29V, and thus the two tube static current is 29V / 300Ω = 96 mA, and the actual screen of EL34 is 350V × 48 mA = 16.8W. This only accounts for 67% of EL34 maximum screen loss (25W), which is very advantageous to extend the work life of the tube due to larger marks. The inverted phase (V2) uses a cathode coupling inverting circuit, and it is known from Figure 3 that the two-tube screen is extremely fixed, only the resistance is slightly different, R7 takes 30KΩ, and R8 takes 33KΩ, so it will be avoidable to balance adjust. If the producer is equipped with a signal generator and an oscilloscope, it can be adjusted as follows. First, R8 is changed to 30kΩ, then one variable resistor (5kΩ) is connected to the screen of the V2 (hereinafter, then input 1 kHz sinusoidal signal, and observed the waveform of the upper two output signals with the oscilloscope, The fine-tunable variable resistance causes the amplitude of the two waveforms, and the fixed electrical power similar to the resistance is of course, at this time, the output power is reduced, and the single tube is not reached twice, but still is more significantly higher than the single tube. Output. Similarly, the output impedance is also low. Since the charted phase level has a large gain, and the local output power is not too large, it is not enough to increase the gain requirements of the input voltage amplifier, and it is enough to use the primary triode voltage. It is also possible to use a double triode to share the left and right channel voltage amplifies, but for the need to isolate the left and right channels, a double triode is connected in parallel with a channel. Power amplifiers with multipolar as an output stage, due to high internal resistance, typically exert a certain overall negative feedback to reduce the output impedance and increase damping coefficient (DF). In general, the negative feedback amount is large, the damping coefficient is low, and the power amplifier is strong for the braking ability of the speaker. However, for a specific speaker, the damping coefficient is too low. To this end, the negative refueling amount of this unit can be changed by a band switch. The negative refueling can be selected as 8DB, 10DB, and 12DB 3 files, respectively, determined by the user according to the actual usage. Since push-pull amplification is relatively low for power supply, the power supply part is also quite simple. In order to improve the rectification efficiency, the crystal diode is used instead of the conventional electronic tube rectification.
Three. Production and commissioning Next page
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