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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Corrective amplifier AF for multimedia devices. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers When watching amateur videos, old movies, videos with incorrect audio track encoding settings, you often have to deal with poor sound quality. Sometimes it can be so bad that it is difficult to make out speech not only in a foreign language, but also in your native language. In such a situation, in many cases, an appropriate correction of the amplitude-frequency characteristic (AFC) of the audio path can help. For flexible operational control of the frequency response, it is proposed to use an adjustable AF preamplifier connected between the signal source and the AF power amplifier. When connecting to TVs, computer monitors and active speaker systems of various multimedia devices (DVD and MPEG-4 players, camcorders, cameras, tablet computers, e-books, navigators) to improve the sound quality between a digital signal source and a TV or other household appliance containing UMZCH, it is advisable to turn on the AF preamplifier, with which you can quickly adjust the timbre, volume and stereo balance of the sound. Digital multimedia devices often contain a built-in software equalizer for adjusting the tone, but this is usually inconvenient to use, since you have to "get" to it through multi-level menus, which are often difficult to find without instructions. In addition, the "push-button" control of sound parameters has such a pronounced drawback as slowness - the adjustment time from one extreme value to another and back can be up to 30 seconds or more, while the variable resistor knob can be turned back and forth in one give me a sec. The AF preamplifier circuit with electronic volume, stereo balance and timbre controls for high and low frequencies is shown in fig. 1. Compared to passive volume and tone controls, such controls do not require the use of high-quality dual variable resistors, you can completely get by with ordinary single ones, it is also not necessary to use variable resistors of a given resistance for tone controls, the requirements for shielding quality are reduced. On the other hand, analog electronic controls introduce more distortion and are more "noisy" than passive volume controls.
The XP1 plug is connected to the audio signal source. Through a three-winding inductor L2, coupling capacitors C4, C2 and RC filters R8C5, R11C7, the AF signals are fed to the inputs of the integrated circuit DA2 (AN5836), which is a two-channel electronic volume, balance and tone control for bass and treble. The volume level is changed by a variable resistor R4 (resistor R1 limits the minimum volume), the balance of the stereo channels is changed by a variable resistor R2, the bass and treble timbres are changed by resistors R20 and R21, respectively. The volume control depth is about 45 dB, the treble and bass timbres are approximately -12 to +10 dB. Capacitors C10, C11, C18, C19 reduce the sensitivity of the DA2 chip to interference. The supply voltage is supplied to the variable resistors of the regulators through the Rc filter R19C21. The DA2 chip is powered by a stabilized voltage of 12 V through an LC filter L3C17C20. The stereo signal from the outputs of the DA2 chip through the coupling capacitors C15, C16 and RC filters R15C22, R18C25 is fed to a two-channel telephone power amplifier made on the DA3 integrated circuit (CD2025CP). The voltage gain of its channels, which depends on the ratio of the resistance of resistors R15, R22 (R18, R23), is chosen relatively large, so that it is possible to "rock" high-impedance Hi-Fi stereo phones from the weakest signal source - a pocket MP-3 player . Capacitors C27, C28 - voltage boost. Damping circuits R24C30, R25C31 and capacitors C22, C25 prevent self-excitation of the microcircuit. From the outputs of the DA3 microcircuit, through the coupling capacitors C32, C33 and the inductor winding L4, the stereo signal is fed to the sockets XS2, XS3, to which you can connect headphones, an external UMZCH input, a TV, a computer monitor. The output signal is fed to each socket through individual current-limiting resistors R28, R29 and R30, R31. If necessary, the number of sockets for connecting audio devices can be increased [1]. Resistors R26, R27 are necessary to eliminate the click when connected to the load output if the VLF power is turned on earlier. The voltage transfer coefficient of the amplifier as a whole with the volume and tone controls set to maximum at a frequency of 1000 Hz is about 25. This is usually enough to match the output of the mobile device and the input of the TV's AF in terms of AF levels. A 1 V voltage stabilizer is assembled on the DA78 (KA12R12) integrated circuit. The VD1 diode protects the device from supplying the supply voltage of the wrong polarity and, in addition, allows you to power the structure from an AC voltage source. The resistance ratio of resistors R6, R7 is chosen so that the stabilizer turns off, de-energizing the load, with an input voltage of less than 9 V. The HL1 LED lights up when the supply voltage is present. Inductor L1 reduces the sensitivity of the device to impulse noise, and also performs the function of a fusible link. Most parts of the amplifier are mounted on a 65x56 mm board (Fig. 2). Installation - bilateral hinged. Fixed resistors - C1-4, C1-14, C2-14, MLT of the power dissipation indicated on the diagram. Variable resistors - SP3-4, SP3-33, SP3-9a, SP3-9b, SP4-1 or similar small-sized imported ones with a resistance of 10 ... 50 kOhm. Their metal screens and rollers of the engines are connected to a common wire (this is necessary to protect the DA2 chip from static electricity). Oxide capacitors - small-sized imported analogues K50-68, K53-19, K53-30. Capacitor C1 - film capacitor with a rated voltage of at least 50 V. Capacitors C6, C9, C11 - small-sized film capacitors, C13, C14 - for surface mounting, other non-polar capacitors - ceramic K10-17, K10-50 or their analogues. Capacitors C13. C14 are installed between the pins C12, and C17, C29 - in the immediate vicinity of the power pins of the respective microcircuits.
When using resistors and capacitors for surface mounting, they are glued to the circuit board after soldering with instant cyanoacrylate adhesive. Diodes TVR10G are replaceable by any of the series 1 N4001 - 1 N4007, UF4001 - UF4007, 1N4933GP-1N4937GP, KD209, KD243, KD247. Instead of the green RL30N-YG414S LED, any general continuous lighting application will do. A possible replacement for the KA78R12 (DA1) chip is any of the ***78R12. This microcircuit is installed on a duralumin heat sink with a cooling surface area (one side) of at least 10 cm2. Instead of the AN5836 chip, the AN5835 is suitable, and instead of the CD2025CP, the TEA2025B (without an additional heat sink). Inductors L1, L3 - industrial production with ferrite H-shaped magnetic circuits. The winding resistance of the first of them should be within 1 ... 2 ohms, the second - 10 ... 20 ohms (with a lower resistance, an additional resistor is connected in series with the winding). Inductors L2, L4 are self-made and are several turns of mounting wire (L2 - shielded) wound on ring magnetic cores with an outer diameter of 8 ... 20 mm from low-frequency ferrite. The amplifier is assembled in a plastic case with dimensions of 82x68x47 mm (Fig. 3) from a 6 V lead-acid battery for a hand lamp. The front panel and the panel on which the variable resistors are fixed are cut out of 3 mm polystyrene sheet.
Accurately made from serviceable parts, the amplifier starts working immediately and does not require adjustment. To power it, any DC voltage source with a stabilized output voltage of 10...28 V (for example, described in [2]), unstabilized 13...28 V DC or 9...20 V AC, capable of delivering the current to the load is not less than 0,12 A. If the amplifier is powered from its own source, then, for example, TS6-2 can be used as a step-down transformer (in this case, the VD1 diode should be replaced with a rectifier bridge). When connecting the amplifier to mobile signal sources with low-voltage battery power (mobile phone, tablet computer), their volume control is set to the maximum gain position, and in stationary sources - to a position close to the maximum, in which there is no overload of the output stages (the effective value of the output signal - about 0,2 ... 0,5 V). If the preamplifier is supposed to be built into any ready-made UMZCH, for example, into active "computer" speakers, then the cascade on the DA3 chip may be unnecessary, in which case the outputs of the DA2 chip are connected to the UMZCH input through isolation capacitors. Literature
Author: A. Butov
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