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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Hi-Fi amplifier on complementary transistors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers The amplifier circuit is shown in fig. 1. Through the RC low-pass filter circuit, the signal enters the complementary input stage (T1, T2, T3, T4). If you wish, you can increase the capacitance of the isolation capacitor C1, but it makes sense to do this only in the case of a very low cutoff frequency of the sound emitting system. A linearizing resistor R11 of 100 ohms is included in the emitter circuit of the input stage, while a total negative feedback of about 30 dB is connected to the emitters. "Inside" the cascade, between the collector of the "lower" transistor (T2) and the emitter of the "upper" transistor (T3), there is a second ("internal") feedback loop of about 18 dB. This means that with the exception of transistors T1, T2, both loops have the same effect on all other stages.
Through an emitter follower (whose main role is to shift the DC voltage level), the signal from the input stage is fed to the voltage amplifier (T7, T8). In the emitters of the transistors, linearizing resistors are again installed here. The collector current of these transistors flows through circuits that regulate the quiescent current of the FETs in the final amplifier. Let's stop for a moment! The temperature coefficient Kt of FETs (i.e. gate voltage/drain current ratio) is close to zero. For small currents it is small and negative, for large currents it is small and positive. Sign reversal occurs for high-power transistors at a current of about 100 mA. The final amplifier operates at a quiescent current of 100 mA. Field-effect transistors "swing" through transistor emitter followers, in which, as you know, Km is positive. Therefore, it is necessary to use such a pre-biased circuit that would compensate for the temperature dependence. The temperature dependence of the emitter followers is compensated by diodes D3 and D4. The quiescent current of the field-effect transistors of the final amplifier is set by the potentiometer P at a level of about 100 mA. Resistors (R29, R30) are installed in the gate circuits of field-effect transistors to prevent self-excitation. A circuit consisting of diodes and zener diodes (D5 ... D8) prevents the gate-source voltage that is dangerous for field-effect transistors. In the source circuit of the field-effect transistors, there are resistors (R31 and R32) with a nominal value of 0,47 ohms. Of these, R32 is marked with an asterisk - in the prototype, its value was zero. This resistor smooths out possible differences in the transconductance of the FETs. As a rule, the inclusion of R32 does not have a catastrophic effect on the gain, one can expect an increase in distortion by about 20 ... 30%. As usual, the RCL circuit at the output of the amplifier protects it from self-excitation at an extremely high reactive load impedance. The resistance Rx in the T1 emitter circuit at the input of the amplifier is used to precisely balance the amplifier. If we take R13 and R14 of the same value (6,8 kΩ), and short Rx, then the output bias will be quite satisfactory. But if it is necessary to improve it, then R13 is reduced to 6,2 kOhm, and instead of Rx, a 1 kOhm potentiometer is temporarily connected. After about 30 minutes of "warming up" the amplifier, this potentiometer sets the output voltage level to zero. The resistance of the potentiometer is measured, and a resistor with a value closest to the measured value is soldered as Rx. As a rule, when replacing D1 or D2, it becomes necessary to replace Rx. Capacitor C9 performs frequency correction of the amplifier. It causes a double effect: on the one hand, it performs a "lagging" correction with a capacitive load of the T7 and T8 collectors, and, on the other hand, a "leading" one, being connected not to ground, but to R21. Resistor R34 prevents the occurrence of two different ground loops when two or more UMZCH are powered from one power supply. The ground at the input is connected to the metal case or chassis and to the preamplifier, and the other grounds, which are, in fact, return wires for zero currents, are connected separately to the zero point of the power supply. Installation The amplifier is assembled on a double-sided printed circuit board, on the side of the parts there is a continuous grounding foil. Countersinking in the places of "entry" of the leads of the parts into the board prevents short circuits. The leads of the parts connected to the ground are soldered directly (without holes) to the ground foil. On the assembly drawing, these points are marked in black. The two terminating FETs are mounted on aluminum corners that connect to the heatsink to create a thermal bridge, and both are attached to the board. They must be isolated from the corners and the board. The resistor present in the emitter circuit "hangs in the air", since it is installed by surface mounting. Resistors R29 and R30 for shortening the leads are soldered from the side of the board tracks. The heat sinks must not form a "false ground" with the "null" foil, so the "null" foil is interrupted by a deep scratch running parallel to the heat sinks. For normal cooling of field-effect transistors, a cooling surface of about 400 cm is sufficient2. Transistors T9 and T10 are attached to the "zero" foil through a thin mica plate. A short circuit can very easily occur here, so the installation must be carefully checked with an ohmmeter. Coil L1 with a diameter of 10 mm consists of approximately 15 tightly wound turns of wire with a diameter of 0,5 mm (without core). Resistor R33 is located along the L1 axis, and its leads are soldered together with the coil leads, and then attached to the board. The three wires going to the power supply are twisted together. The two wires leading to the speaker are also twisted into a separate bundle (regardless of the previous ones). Since large currents flow here, their magnetic fields can greatly increase distortion - mainly at high frequencies. Twisting wires together causes the magnetic fields of currents flowing in opposite directions to cancel each other out. The zero point of the power supply and the output of the speaker are not connected to the case, and the wires leading to them do not fit together with other wires. Power supply unit The power supply circuit is the simplest (Fig. 4). The transformer, having a tap from the middle of the secondary winding, feeds a full-wave rectifier, consisting of two groups of 2 diodes. Ripple smoothing is carried out by capacitors with a capacity of at least 4700 microfarads (40 V). Such a unit can provide power to two final amplifiers.
The upper voltage limit of the secondary winding of the transformer is determined by the type of transistors T7, T8 used. In the case of using a pair of BC 546/556, the supply voltage (in the absence of a signal) should not exceed 30 ... 32 V. These transistors "do not tolerate" higher voltages. With a supply voltage of ±30 V, you can use a transformer 220 / 2x22,5 V or 230 / 2x24 V. An amplifier with a supply voltage of ± 30 V can deliver about 24 W (at 8 ohms) to the load. The field effect transistors used in the final amplifier are very expensive. For the price of one such transistor, you can purchase the rest of the set of parts. Involuntarily, the question arises whether the excess costs are compensated by the expected improvement in quality. The answer to this question depends on many circumstances, because: - we are talking about subjectively perceived distortions, so the sound sensations for different people will be different; - the perception of distortion depends on the music being played. When playing purely "author's" electronic music, it makes no sense to talk about distortions, because it is impossible to know whether or not these distortions were in the original material; - It is problematic to play music coming from a CD. According to "critical ears" and the author, this music has a specific coloring. Playing from a good analog record or directly from a concert gives excellent quality. Publication: cxem.net
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