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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING UMZCH with a unipolar power supply. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers The modern construction of household radio-electronic equipment is based entirely on the use of specialized functional integrated circuits. This undoubted advantage in production turns into some inconvenience for those who like to repair the device on their own when they cannot purchase the required microcircuit. In such a situation, the experience of radio amateurs in creating separate nodes and blocks on discrete elements can help out. This article describes a stereo amplifier for use in music centers with the additional possibility of using a common low-frequency radiator. Schematic diagram of the UMZCH is shown in fig. 1. In it, the sound reproduction channels are built in such a way that for a low-frequency load they represent a bridge amplifier [1]. In one of the channels the signal is inverted, while in the other it is not. A common woofer is included between the channel outputs. The signal is inverted in the input buffer amplifier stage on transistors VT1 and VT2, which are connected according to the composite transistor circuit, which made it possible to obtain a high input resistance of the UMZCH. The inverted signal is allocated on the resistor R6, and the non-inverted one - on the resistor R7. For subsequent amplification in the right channel, the signal is taken from the resistor R7 ', and in the left channel - from the resistor R6.
Since the channel schemes are completely identical, we will only describe the left channel. At the input and output of the buffer stage there are filters R1C2 and R9C6 that suppress signals with frequencies of 100 kHz and higher. If UMZCH will be used without a common low-frequency channel, the signal from the buffer stages in both channels must be removed from the resistors R7 (R7 '). From them you can also remove the signal for feeding to a telephone amplifier. Buffer stages are powered by a common voltage regulator. Its only feature is that the base-emitter junctions of transistors VT3 - VT6 are used as zener diodes. When turned on again, such a transition is a good zener diode with a small stabilization current. At the connection point of resistors R10 and R11, the input signal is added to the OOS signal. The gain of the UMZCH depends on the ratio of the resistances of these resistors and, with the ratings indicated in the diagram, is 26 dB. The summed signal is fed to the base of the composite transistor VT7, VT8, loaded on the low-resistance input circuit R14, R15 of the next amplifying stage, assembled on the transistor VT9, connected according to the OB circuit. A cascade with such a transistor connection is less dependent on parasitic interelectrode feedback, which has a good effect on the frequency response of the entire device. The output stage and the device for stabilizing its quiescent current differ little from those described in [2] and [3]. Such a scheme reduces the distortion of low-level signals and makes the sound more intelligible and transparent. Another feature of this output stage is that the DC voltage at its outputs is somewhat lower than half the supply voltage. This eliminates the need for a voltage regulator, bringing the ripple at the output sockets of the power supply beyond the maximum amplitude of the output signal amplitude. To work with a common low-frequency channel [1], it is necessary that the constant voltage at the outputs of the device be the same and stabilized. In this case, this is ensured by the presence of a common composite zener diode VD1, VD2, included in the emitter circuits of the input stages of both UMZCH channels. In parallel with the composite zener diode, a capacitor C10 of a sufficiently large capacity is connected. Gradually charging after power is supplied to the amplifier, it provides a smooth increase in voltage at its outputs, which eliminates the characteristic "click" that accompanies turning on the UMZCH. Elements L1, C15, C15' select signals with frequencies below 250 Hz for a common low-frequency channel, and capacitors C14 and C14' - signals with frequencies above 250 Hz for MF-HF links. The SA1 switch allows you to use the UMZCH in stereo mode without a common low-frequency channel with conventional broadband speakers. In the upper position of its movable contact, capacitors C13 and C13' of large capacity are switched on, passing the entire band of amplified frequencies. For this design, the author did not develop a printed circuit board and mounted the parts on a breadboard. Almost the entire amplifier was assembled on it, except for the output stages. Transistors VT10, VT11, VT13 - VT16, capacitor C11 and resistors R18 - R21 (as well as parts of another channel, indicated by indices) are installed on a common heat sink with a cooling surface area of 600 cm2. Transistors VT13 and VT15 are fixed on the heat sink with a common M3 screw. Place a mica pad under the transistors. To prevent contact of the screw with the collectors of transistors, a short piece of PVC tube should be put on it. Transistors VT14 and VT16 are fixed without gaskets. Resistor R21 is soldered to the base terminals of transistors VT15 and VT16, and capacitor C11 is soldered to the terminals of collectors VT13, VT15. Transistor VT11 and resistors R18 - R20 are placed on a textolite strip with contacts. The collector of the transistor VT10 is soldered to the base of the transistor VT13. This is necessary for reliable thermal contact with transistors VT13 and VT15. In UMZCH, domestic transistors KT502B can be used instead of VS640; KT503B - instead of VS639; KT818AM - instead of BD912; KT819AM - instead of BD911, MLT 0,25 resistors, capacitors of any appropriate types and ratings. Coil L1 is frameless, contains 320 turns of PEL 1,2 wire, wound in bulk on a mandrel with a diameter of 45 mm, winding length - 35 mm. The establishment of UMZCH begins with checking the voltage at the output of the voltage regulator (emitter VT3) and at the outputs of buffer amplifiers (emitters VT1, VT2). They should not differ from those indicated in the diagram by more than 10%. At this stage of the setup, fuses FU1 and FU2 must be removed. Then, without connecting the load, instead of the FU1 fuse, turn on the ammeter. After that, gradually reducing the limit of its measurement, you need to make sure that the quiescent current of the upper (according to the circuit) arm of the UMZCH does not exceed 100 mA. The same operations are performed in the other arm of the UMZCH by connecting an ammeter to the place of the FU2 switch. Further, having installed both fuses in place, you should make sure that the constant voltages at the outputs of both channels differ by no more than 150 mV. These voltages themselves should be 5 ... 10% below half the supply voltage. If necessary, they are installed by selecting zener diodes VD1 and VD2. Then, instead of heads BA2 and BA3, resistors with a resistance of 4 ohms and a power of several watts are connected to the UMZCH outputs and the quiescent current of each of the channels is checked again. After that, an AF generator is connected to the inputs of both channels closed to each other, and an oscilloscope is connected to the output of one of the channels. Applying a signal of the order of 15 ... 20 mV to the input and observing the output signal on the oscilloscope screen, we make sure that there is no “step” in it. At a quiescent current of 30 ... 40 mA, it does not exist at all at a frequency of 1 kHz, but at a frequency of 12 kHz, a "step" can still be observed. If you increase the quiescent current to 100 ... 130 mA (by reducing the resistance of the resistor R18), then it does not appear even at frequencies above 20 kHz. Further, by applying a rectangular signal to the input, they make sure that there are no parasitic emissions at its fronts and recessions at the output, as well as that there are no parasitic high-frequency oscillations. If there are none, then the capacitance of the capacitor C8 should be increased until they disappear. All the described operations are performed in another channel. This completes the establishment of the UMZCH. The described UMZCH has the following main technical characteristics: input voltage - 0,5 V; input impedance - 330 kOhm; gain - 26 dB; rated power in each MF-HF channel - 14 W at a load of 8 ohms and 20 W at a load of 4 ohms; rated power in the common low-frequency channel - 36 W at a load of 8 ohms; reproducible frequency range - 20...20 000 Hz; harmonic coefficient at a frequency of 1 kHz - 0,04%, at a frequency of 20 kHz - 0,06%. In the LF channel, it is not recommended to use a load with a resistance of less than 8 ohms due to the difficulty of matching this channel with the MF-HF channels in terms of signal level. To turn off the sound in the speakers, it is desirable to disconnect the capacitors C5 (C5') from the buffer stages. If desired, you can make a telephone amplifier by assembling it according to the scheme shown in fig. 2. This amplifier is similar to the UMZCH described in [4]. It also operates in a linear mode (class A), but at lower quiescent currents, of the order of 15 ... 20 mA through each collector circuit of transistors VT3, VT4 (VT3', VT4'). The quiescent current is set by selecting the resistor R6 (R6'). Transistors VT3, VT4 (VT3', VT4') must be installed on a heat sink with a total area of at least 80 cm2 or on the surface of a metal chassis through insulating gaskets. Transistor S2336 can be replaced by KT602BM.
This UMZCH was designed and manufactured by the author to restore the music center "MARC-NR-75F1". The UMZCH power supply must provide a current of at least 5 A at a voltage of 44 V. At other voltages, the output power will change. This should be taken into account and output transistors with appropriate current and voltage limits should be used. In addition, you will have to select zener diodes VD1, VD2 to provide a constant voltage, 5 ... 10% less than half the supply voltage. If the converted device has a stabilized power supply, for example, "Victoria-001stereo" (Riga Radio Plant), then it is advisable to select the voltage at the outputs 1/2 Upit. With a stabilized power supply, the UMZCH parameters will be higher. Literature
Author: M. Sapozhnikov, Ganei Aviv, Israel
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