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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Non-combustible automobile ULF. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automotive power amplifiers Most of all, a car enthusiast appreciates reliability on the road, at the same time, breakdowns of "stamped" imported car radios often occur. Below is a ULF circuit that replaces a burned-out imported one - just connect the input of the circuit to the volume control of the device being repaired. After that, you will not need to understand the installation, change the failed parts - the most "tender" part of the car radio - the power amplifier - will become incombustible! Some redundancy in detail gave the scheme two advantages over other similar schemes: - the output voltage range is almost equal to the voltage in the on-board network,
On fig. 1 shows a schematic diagram of the ULF (in brackets are the pins of the microcircuit involved in the second channel), if the radio tape recorder is really, without deception, stereophonic. From the volume control available in the car radio, the signal is fed to the direct input of the DA1 microcircuit - an operational amplifier with a high gain, a high-frequency margin, an input stage power stabilizer and output overload protection.
These properties of a two-channel microcircuit made it possible to get rid of power interference in the input stage in a simple way and simply stabilize the output at a constant voltage, without worrying about the survivability of the microcircuit. From the output of the IC, the signal is fed to the bases of two germanium transistors of different conductivity of the phase-inverted stage VT1, VT2, in this way distortions of the "step" type are suppressed without selecting elements. The emitters of said transistors are connected to the midpoint of resistors R1, R2 of the supply voltage divider to limit currents and protect all transistors of a given channel from burnout. From the collectors VT1 and VT2, various half-waves of the signal enter the bases of the output transistors VT3 and VT4, mounted on a common radiator. The emitters of the output transistors include resistors R6 and R7, which limit the currents in the circuits of the output and output transistors. The same resistors limit the through current when a faulty - "broken" transistor is installed in the circuit. The variable component of the output signal through the capacitor C4 is supplied to the head B1. It should be noted that it is impossible to "save" on the capacitance of this capacitor! For the operation of all cascades in a linear mode, it is necessary to limit the “bass” according to the available heads B1 by reducing the capacitance of the capacitor C1 so that the playback volume increases, but the lower frequencies are not noticeably suppressed. This operation is best performed while listening to the sound inside the car. The more we limit the low frequencies, which are very poorly reproduced on bad heads, the more the maximum undistorted sound power will increase, and the intermodulation distortion of audible frequencies will decrease by "inaudible" - not reproducible heads. Negative feedback on direct and alternating voltage from the output to the inverting input is transmitted from the divider R3, R4, by selecting R4 we output the amplifier mode to half the supply voltage at point "A".
On fig. 2 shows an improved circuit of this divider, in which R4 is connected by direct voltage to the "body", and by alternating voltage - to the dynamic head. With this feedback scheme, sound distortion at low frequencies is reduced. Resistor R9 is needed here to maintain the amplifier mode when the dynamic heads are turned off. The depth of negative feedback on audio frequencies and the gain of the circuit are adjusted by resistor R5 so that the sound of the receiver, as well as the cassette with a weak recording level, leads, at the upper position of the volume control, to a uniform limitation of the oscillogram taken at point "B" (at this time, you should listen to sound distortion). With this setting, distortion in various operating modes will not be too large and, at the same time, there will be a small margin of volume. Due to the broadband of the microcircuit and the use of low-frequency germanium transistors to suppress excitation at high frequencies, it was necessary to take the OOS signal from the emitters VT1 VT2 and bring it to the inverse input of the microcircuit through capacitor C2 (with the correct layout of the amplifier, the capacitance of this capacitor can be reduced, and if it is incorrect - significant capacitive coupling of the output circuits with the "direct" input of the microcircuit - you will have to increase the capacitance C2). The occurrence of generation is seen on the oscillogram as a "bifurcation" of the voltage curve and is accompanied by a sharp decrease in power and sound quality. To protect the microcircuit from voltage surges in the on-board network (the operation of the ignition system, the generator with a relay-regulator), elements R8, C5, C6 are used. Installation Due to the protection of the circuit from harmful influences, there are no big restrictions in installation. One condition must be met: since the amplifier does not invert the signal, it is necessary to shield the wire coming from the volume control, capacitor C1 well and choose the correct common zero point (connection of the radio and power amplifier cases). Installation is carried out in a metal box measuring 100x70x30 or slightly larger. The output transistors are mounted on the top insulated radiator cover, they are connected to the circuit with a flat 5-wire harness. The rest of the installation is made on one side of the board from double-sided fiberglass (the second side - the case is pressed against the wall of the box). Several rectangular sections are cut or etched on the board: direct and inverse inputs of the microcircuit, microcircuit power, on-board network power, microcircuit output and base-cases VT1 and VT2 soldered to it, feedback circuits. The connections between these areas are best made in detail, so that isolated "islands" do not disturb the ground surface of the remaining board. The microcircuit, in which extra pins are bitten off, is installed on one edge of the board, and the output capacitors are on the opposite. It is possible, upon completion of the installation, to cover the microcircuit with the input circuits with a grounded rectangle made of foil fiberglass (insulated side to the parts). The output transistors in a plastic case are pressed against the radiator cap, as usual. Their collector leads are bitten off, and the conclusions of the emitters and bases are soldered to a strip of foil fiberglass (the insulating tracks are cut out or etched). The output of the collectors is carried out through a screw securing one of the transistors, or securing a strip of fiberglass. With this method of installation, it is impossible to break the terminals of the transistors or close them. Resistors R6 and R7 can also be mounted on the lid, or in the main unit - it depends on the dimensions of the box and parts. Establishment If the preparatory operations mentioned above are completed, it remains to select the fuse in the power circuit so that when the amplifier output is short-circuited, the fuse does not blow immediately, but after tens of seconds. The circuit is protected from overloads and short circuits, but it is better if the tape recorder, which works without the control of the car driver, is de-energized in an emergency. In the future, you can connect any heads in any quantity to the output of the amplifier. Silence will indicate a faulty head, and a decrease in volume will indicate an incorrect connection. It is very difficult to burn the circuit, but still during the adjustment period, check the heating of the output transistors, resistors R6 and R7 in maximum power modes and with a short circuit of the output. Recall that the wrong choice of points for connecting the neutral wire to the tape recorder and power amplifier, or too much length of this wire, can ruin all the work. So, I gave one copy to a tuned car enthusiast, who began to "dare", increasing the length of the wires, and then came with a complaint that "suddenly" it became bad to "play"! Powering an existing working radio
If you think that the transistorized ULF of your radio (Fig. 3) gives little power, but has not yet failed :-), then the work is greatly simplified (we will clarify: we mean the ULF version, in which weak output transistors are pressed against the case- heat sink). Indeed, the circuit has an amplifier with a small voltage gain (approximately 2 V undistorted voltage amplitude), feedback circuits, and insufficiently powerful output elements. To get the output voltage range within the supply voltage (as in the circuit of Fig. 1), it is necessary to add an output stage on powerful transistors, use the existing NFB circuits, while supplying only about a third of the alternating output voltage to the latter.
On fig. 4 only the newly introduced elements are indicated. Capacitor C1, which served as a separator, as having a clearly insufficient capacity, is left only in the OOS circuit. In the collector circuits VT1, VT2, it is necessary to cut the current-carrying tracks. To connect the "old" circuit to the new output stage, a flat bundle of 5 wires is suitable if C1 is transferred to a new board, or of 6 if C1 remains in the "old" place. Transistors VT3, VT4, connected with a common emitter, provide current and voltage amplification and should have approximately equal current transfer coefficients. Capacitor C2 is separating in the dynamic head circuit. The voltage divider R5R6 brings through C1 the variable component of the output signal to the OOS circuits. Through R7, a constant potential is supplied to the same point at the connection point of the collectors of the output transistors. You can experiment with reducing the capacitance of C1 to increase intelligibility and subjective loudness while limiting the lower audio frequencies (be sure to check if gain asymmetry occurs). Establishment If the output transistors are heated, or the current consumed in the pause is too high, it may be necessary to install one of the two resistors R8, R9. If the circuit of the E-B of one of the output transistors sharply reduces the quiescent current, then a resistor is required in this particular shoulder (select by increasing the value from the minimum). In a properly tuned amplifier, the average potential of the mid-point of the output does not change much with changes in the input signal level. The second important sign of the correct setting is the symmetry of the limitation of the sinusoid at a high signal level, and the third is the undistortedness of the sinusoid when the load is connected at any level from 0 to the maximum (at which a symmetrical limitation occurs), if necessary, correct R8, R9. It may turn out that small distortions still remain (after all, we did not redo the entire ULF circuit), but even in this case, the volume and sound quality will noticeably improve, and there is still less editing than in the circuit in Fig. one The installation of this circuit in my version (in the radio tape recorder there is one board installed on the side of the tape recorder) easily fit inside the existing radio case. The board made of foil fiberglass occupies the entire space behind the tape drive mechanism and is attached to the bottom of the case. The output transistors are mounted far apart so that the U-shaped strip of aluminum and tin plate connecting their cases captures more air for cooling. To improve heat dissipation, it is better to use two such structures pressed against different sides of the metal part of the transistors. In space, they must be spaced apart, and the "horns" bent so that the largest possible volume of air comes into contact with the radiators. In addition to two attachment points (transistor collectors), for mechanical strength, such a radiator must be soldered to the board in one or two places. Since the board is attached to the case, the assembly of the parts is made without drilling holes [1] - the curved leads of the parts are soldered to the board sections. There must be zero potential in the places where the board is attached. Attention! It is necessary to ensure that the screws securing the upper removable cover of the radio do not close the installation of a new part of the ULF to the case. Literature
Author: Nikolay Goreiko, Ladyzhin, Vinnitsa region; Publication: cxem.net
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