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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Two AF power amplifiers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers The advantages of the described amplifiers include a low coefficient of harmonic distortion in the entire operating frequency band, a smooth limitation of the maximum signal levels. The high output impedance of one of the amplifiers helps to reduce the intermodulation distortion of the heads in the mid- and high-frequency bands. The low output impedance of the other dampens the loudspeaker over a wide frequency band. Paradoxical as it may seem, but according to subjective estimates, the quality of operation of transistorized UMZCHs, even with the best parameters, is often considered worse than tube ones. And although the auditory perception of different people varies significantly, nevertheless, the final assessment of the quality of audio equipment still remains with the listeners. With the spread of transformerless UMZCH on transistors, sound recorders are faced with the so-called effect of "transistor" sound. The developers, believing the cause of this phenomenon to be non-linear distortions, increased the depth of the overall OOS, used output amplification stages in class A or its more economical varieties - with dynamic displacement of Super Glass A types. New Class A. Non-switching amp, etc. However, for tube For Hi-End class amplifiers at rated power, a non-linear distortion factor of up to 1% or more is considered acceptable, and for dynamic heads - 5% or more [1, 2]. Then they took up the reduction of intermodulation and dynamic distortions, the main cause of which was considered to be deep OOS. Some came to the conclusion that the OOS depth should be limited to 20 dB, others abandoned it altogether, achieving UMZCH linearity due to local OOS. For effective loudspeaker damping, the amplifier is usually designed with a low output impedance. It is believed that the minimum damping factor should be at least 20, and for Hi-Fi systems - at least 40. The output impedance of tube amplifiers reaches ten ohms. However, in [3] it was shown that the output resistance of the UMZCH with a value of not more than 18 ohms is quite sufficient for effective electrical load damping (8 ohms). In [4], it is also stated that an amplifier with a low output impedance does not provide current proportionality due to the complex resistance of the dynamic head and thermodynamic processes in the coil associated with its heating, as well as the non-linearity of the inductance. In addition, at medium frequencies, intermodulation distortion of the head is reduced when operating from UMZCH with a relatively high output impedance. The high-impedance output has a beneficial effect on the reproduction of pulsed signals. The effectiveness of the electrical damping of the loudspeaker heads can only be discussed in the region of the piston action of the diffuser, i.e., at lower frequencies. For a visual assessment of the braking efficiency of the voice coil of the loudspeaker, it is proposed to include a resistor with a resistance of about 0.2 ... 0.4 Ohm in the common wire of the loudspeaker. connect an oscilloscope to it and apply an intermittent signal to the amplifier input in the frequency range of 30 ... 300 Hz. The duration of tonal bursts should be 25...30 ms (to fit the full period of the lowest frequency signal) with pauses of 40...60 ms. Depending on the output impedance of the UMZCH, the damping of the natural oscillations of the head will be more or less long. Note that the stability of the loudspeaker impedance in the operating frequency band has a positive effect on the operation of any tube and transistor amplifier. So, the conclusion suggests itself that it is advisable to use a transistor UMZCH with a low-resistance output only for working on a multi-band speaker woofer. With heads MF and HF it is preferable to use amplifiers with high-resistance, current output. Separate amplification and reproduction in several bands of audio signals has a particularly beneficial effect on reducing intermodulation distortion of the heads even in case of overload. Based on the above features of the operation of the amplifier and loudspeaker, the author has developed two amplifiers. In the first of them (its diagram in Fig. 1) there are two loops of a common OOS: for alternating current - through R5, C6 and for direct voltage - through the integrator on DA1. The use of an integrator eliminates the DC component at the output of the amplifier even if it is present at the input, for example, due to leakage of the transition capacitor at the output of the tone block or linear amplifier. This solution also has a positive effect on the damping of the loudspeaker. The amplifier has almost zero output impedance at infra-low frequencies and at direct current, which is equivalent to damping the loudspeaker by the secondary winding of the transformer UMZCH on lamps. This eliminates the infra-low-frequency oscillations of the low-frequency head that occur with some transistor UMZCH.
In the output stage in a two-stage current amplifier, LSITs are used. Such transistors are characterized by high transconductance, low residual saturation voltage, fast switching and relatively high current transfer ratio in linear mode. The differential cascades with local feedback used in the amplifier are known to have an increased overload capacity, and distortions in them are largely compensated. Diodes VD3-VD6 achieve the necessary level shifts to ensure the mode of transistors VT10, VT12. Summation of signals from repeaters to VT7, VT9 and VT8. VT13 occurs respectively on transistors VT10 and VT12. Resistors R20. R21 are, on the one hand, local OS for VT10. VT12. on the other hand, the load of emitter followers on transistors VT9.VT13. The signal is limited at the output of the second stage, and, accordingly, the amplifier as a whole, occurs earlier than in conventional amplifiers, by about 3 V (due to the voltage drop across transistors VT9. VT13). In this case, with a further increase in the input voltage, there is no hard limiting of the signal, since the transistors VT10, VT12 go into smooth saturation mode. Thus, the amplitude value of the signal at the output of the amplifier is the same. as in a conventional amplifier, but without a hard limit. This circuitry solution allows you to get the nature of distortion during overload, similar to tube amplifiers. Thermal stabilization of the cascade is provided by the transistor VT14. The quiescent current of each of the output transistors VT17-VT20 at a level of about 80 mA is set by resistor R24. The amplifier has a relatively low input impedance (about 6 kΩ). therefore, the signal source (for example, a tone block) must have an output impedance of no more than 200 ohms. Specifications UMZCH
The amplifier is made according to the "double mono" scheme, i.e. with separate power supplies on transformers with an annular magnetic circuit. This design provides higher dynamic performance and avoids crosstalk between channels, which significantly improves the spatial characteristics of sound transmission. The capacitances of the capacitors at the outputs of the power supply must be at least 20000 microfarads. Coil L1 is wound on resistor R33 (MLT-2) with PEV-2 wire 0.69 turn to turn in one layer until filled. Capacitors C2-C5 - K50-35. Resistors R28-R31 are made of manganin wire with a diameter of 0.3 mm. As DA1, you can use the KR544UD1 microcircuits. K140UD8. as well as KR544UD2 with the connection of pins 1 and 8. Transistors VT15, VT16 are equipped with small heat sinks, and transistors VT14, VT17 - VT20 are mounted on plate heat sinks made of duralumin with a thickness of at least 5 mm. The output transistors of each arm of the amplifier are connected to the board with twisted conductors with a minimum length of 1 mm2. The wires leading to the power supply and to the loudspeaker must also be twisted. It is advisable to pre-select transistors in pairs with a spread of n2|e not more than 20%. With serviceable parts, the adjustment of the amplifier is reduced to setting the quiescent current of each IE of the output transistors within 60 ... 100 mA. Amplifier output stages with low output impedance, more suitable for a woofer. made on a more accessible element base (Fig. 2). The rest of the scheme is practically similar to that considered earlier (in Fig. 1 it is separated by a dash-dotted line).
The push-pull output stage on the VT15-VT18 is made according to the OE-OE scheme with deep OOS. Bias circuit on VD9 diodes. VD10 is supplemented with resistors R23, R24, which provide small changes in the input resistance of the cascade and current through the diodes VD9, VD10 even when the current is cut off in the opposite arm of the cascade. Short circuit protection in the load is made on diodes VD11, VD12. As VT7, VT9, VT13, you can use transistors of the KT3102 type with any letter index. With a supply voltage of up to ± 30 V, transistors such as KT11V and VT16 are suitable as VT626, VT12. VT15 - KT646A. Transistors VT15, VT16 are equipped with small plates - heat sinks. For additional thermal stabilization, diodes VD16, VD17 are mounted together with resistors R33. P34 directly on the terminals of the output transistors. When used in positions VT11, VT12, VT15, VT16 transistors of the KT850 series. KT851 the capacitance of capacitors C10, C11 can be reduced to 150 pF, and C12, C13 - up to 39 pF. To increase the stability of the amplifier, it is desirable to include resistors with a resistance of 10-12 ohms in the bases of transistors VT1, VT10 (see Fig. 13) and VT2-VT50 (Fig. 100). which will reduce the capacitance of capacitors C10-C13 or even abandon them. When setting up the amplifier (at first, without powerful transistors VT17, VT18, see Fig. 2), it is turned on and off. by giving a signal from the generator, they are convinced that the device is working without load. Then, by connecting the output transistors, they check it under a resistive load using both a sinusoidal signal and a meander signal up to a frequency of 20 kHz. The output signal should be clean, without any overshoot or ringing. Particular attention should be paid to the output waveform when the amplifier recovers from overvoltage. On a sinusoidal signal, there should be no signs of even momentary excitation. The parameters of the amplifier shown in fig. 2. can be improved by using higher-frequency composite transistors or individual transistors with a unity gain frequency of at least 20 MHz as output transistors. Literature
Author: A.Petrov, Mogilev, Belarus
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