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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING LF amplifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers The heading "High fidelity of reproduction" appeared in "Radio" No. 11, 1958 and was opened by an article by I.E. Goron "On the visibility of distortions". In publications under this heading, the issues of designing and constructing household electro-acoustic equipment were considered. A number of articles were devoted to electrodynamic and electrostatic loudspeakers, ionophones, and variants of their acoustic design. In subsequent years, dozens of circuits and designs of low-frequency tube and transistor amplifiers, as well as multi-channel sound transmission systems, were presented on the pages of the magazine. The materials of this section were supplemented by interesting translations from foreign journals on this topic, descriptions of instruments and methods for measuring the parameters of the electroacoustic path. The article by V. Ivanov presented in this issue was one of the first in this series of interesting publications. The authors of the best articles in the modern section "Sound Engineering" continue the traditions of this movement. The rapid development of new technologies increases the reliability of sound reproduction in cinema, multimedia, broadcasting and sound recording to a new level: stereo (especially biphonic) and multi-channel sound transmission systems allow you to literally plunge into the acoustic atmosphere of sound images. One of the main elements of any sound reproducing installation is an electrodynamic loudspeaker. As you know, the voice coil of a loudspeaker, together with a diffuser and a suspension system, is an electromechanical oscillatory system that has its own resonant frequency. The presence of resonance, especially in the low-frequency region, significantly degrades the quality of sound reproduction, since at the resonant frequency the output of the loudspeaker increases sharply and the frequency response of the radiation in terms of sound pressure becomes uneven. To the ear, this manifests itself in the form of an unpleasant mumbling! In addition, the input impedance of a loudspeaker varies with frequency, resulting in a mismatch between the tube and the load and increased distortion. The input impedance at the resonant frequency increases especially sharply. They try to reduce the harmful effect of loudspeaker resonance both by acoustic (correct choice of box volume) and electrical measures, for example, by using a combination of negative voltage feedback and positive current feedback. The weakening of the resonant properties of the loudspeaker in this case can be explained as follows: at the resonant frequency, the input impedance of the loudspeaker increases, which leads to an increase in the negative feedback voltage and a decrease in the positive feedback voltage, and there is a tendency to maintain the gain at the same level. The use of combined feedback can significantly improve the sound quality and extend the frequency response, especially in the low frequency region. The figure shows a schematic diagram of a low-frequency amplifier in which the combined feedback described above is applied. Negative feedback is supplied from the secondary winding of the output transformer, and positive feedback from the resistance R24 connected in series with the voice coil of the loudspeaker. The coupling elements are chosen so that the negative feedback is always deeper than the positive one, this ensures stable operation of the amplifier.
The amplifier has two inputs: the voltage from the pickup or detector of the radio receiver is supplied to the grid of the low-frequency preamplifier, made on the right according to the L1 lamp triode circuit, through the switch of the type of work P1, and the general volume control. To work with a dynamic microphone, there is an additional amplification stage, made on the left according to the 6N2P lamp triode circuit. The amplified audio frequency voltage from the load resistance R6, lamp L1b is fed to the input of the phase-inverted cascade. The phase inverter is made according to the scheme with cathode coupling. In the common cathode circuit of the lamp L2, 2 resistances are included: R12 to create the necessary negative bias on the grid of the lamp L2 and resistance R13, which is the connection element between the stages of the phase inverter. The resistance R14 serves as the grid leakage resistance, and the capacitor C6 grounds the grid of the right trnod of the L2 lamp with alternating current. The output stage of the amplifier is made according to an ultra-linear scheme on lamps of the 6P14P type. The R20 potentiometer is used to balance the arms of the output stage in direct current. The inequality of the constant components of the anode currents of the output lamps reduces the inductance of the primary winding of the output transformer and is often the cause of distortion, especially in the low frequency region. In the circuit of the control grids of lamps L3 and L4, antiparasitic resistances R17 and R19 are included. From the secondary winding of the output transformer, a voltage of positive current feedback (from resistors R1 and R24 connected in parallel) and negative voltage feedback (from the ungrounded end of the secondary winding) is supplied to the cathode circuit of the L21b lamp. To select the optimal damping of the loudspeaker, the feedback (and hence the output impedance of the loudspeaker) is made adjustable. For this purpose, the axes of the potentiometers R21 and R25 are combined mechanically. As the positive feedback increases (the R21 potentiometer slider moves from right to left according to the diagram), the negative feedback should also increase (the R25 potentiometer slider should also move from right to left). To power the amplifier, you can use any rectifier that gives a voltage of 250-320 V at a current of up to 100 mA. The amplifier is assembled on a chassis made of aluminum or mild steel. Its top panel is 210 X 120 mm. Lamps, all electrolytic capacitors, resistance R20 and an output transformer are strengthened on it. On the front panel with a size of 120x60 mm there are: volume control R5 slots 1, 2 and 3 for turning on the microphone and pickup, switch /7, and resistances R21 and R25, combined on the same axis. On the rear panel there is a six-pin plug for connecting power and loudspeaker. Variable resistance R20 is reinforced on the underside of the panel between lamps L3 and L4. His handle is out. Installation is carried out with a soft insulated wire of the PMVG type. All connecting wires and small parts (resistances, capacitors) are located as close as possible to the chassis during installation. Shielded wire MGBBLE is used for input circuits. Particular attention should be paid to the shielding of sockets 1 and 2 for connecting a microphone, as well as the wire connecting socket 1 to the L1a triode grid. After assembling the amplifier, you need to once again carefully check all the wiring connections and their compliance with the diagram. Before turning on the amplifier, the variable resistance knob R6 should be set to the minimum volume position, and the resistances R21 and R25 to the position of the minimum feedback depth (this corresponds to the extreme, right according to the scheme, positions of the sliders). Input terminals 1 and 2 should be shorted with a jumper. Only after that you can insert the lamps and connect the loudspeaker and power to the amplifier. Usually, the amplifier test is started when working from the pickup. After making sure that the amplifier is working and has a significant volume margin, the R5 knob is returned to the minimum volume position and the lamp modes are checked using a high-resistance voltmeter. Next, the shoulders of the output stage are balanced. To do this, the capacitor C8 is soldered from the resistance and connected to the resistance R10. Listening to the operation of the amplifier from the pickup, set the variable resistance knob R20 to the minimum volume position. Then the circuit is restored and, listening to the operation of the amplifier, turn the knob of the variable resistances R2l and R25 to the position of the maximum feedback depth. If there is a temporary jumper on the resistance R21, turning the resistance knob R25 significantly changes the volume of the sound. If, instead, self-excitation of the amplifier is observed, then the ends of the secondary winding of the output transformer must be swapped. After checking the operation of the negative feedback depth regulator (R25), remove the jumper from the resistance R21 and select the resistance R23 to set the desired limits for adjusting the depth of positive feedback. The largest value of positive feedback corresponds to the leftmost position of the R21 potentiometer slider in the diagram. If the negative feedback circuit is broken in this position (for example, solder the wire from the middle lobe of resistance R25), then the amplifier should operate in a mode close to self-excitation, but not be excited. The output transformer is assembled from Sh-25 plates, the thickness of the set is 40 mm. The frame of the transformer is divided by a cardboard partition into two equal parts and in one of them sections 1-2 and 2-3 are located, and in the other section 3-4 and 4-5 of the primary winding. Sections 1-2 and 4-5 contain 1100 turns each, and sections 2-3 and 3-4 each contain 400 turns of PEL 0,18 wire. After winding the primary winding, the excess partition is cut off, insulation is applied (two layers of varnished fabric) and the secondary winding is wound on top (terminals 6-7), which has 82 turns of PEL 0,86 wire. This winding is designed for a loudspeaker with a voice coil impedance of 6 ohms. Author: V.Ivanov
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