|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Audio frequency amplifier for monitor. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers Not all computer sound cards are designed to accept low-impedance headphones, and those that do are usually not capable of enough output power for some low-impedance (less than 16 ohm) as well as relatively high-impedance stereo phones. In addition, when they are directly connected to the output of a sound card, there is a risk of damage to it. All this forces cautious PC users to connect their headphones to the appropriate output of an audio power amplifier or preamplifier. In pre-amplifiers, made as an independent device, there is usually an amplifier for headphones, assembled, for example, on the popular K157UD1 op-amp. In UMZCH, the signals to the telephone jack usually come from the outputs of amplifiers through current-limiting resistors. Such solutions can not always be considered optimal now, since both pre-amplifiers and powerful UMZCHs, when acoustics are turned off, can consume tens of watts of power from the network, although several tens or hundreds of milliwatts are usually enough for normal operation of headphones. At present, the author operates the main computer in conjunction with the Orbita-002 stereo amplifying complex, consisting of the PU-002S pre-amplifier, where all the controls and the socket for connecting stereo telephones are located on the front panel, and from the UM- 002C". In the summer, both devices heat up noticeably even in the absence of input signals, and the preamplifier heats up noticeably more. All this prompted the creation of a special amplifier for stereo phones, which would consume a minimum of energy, providing good quality and sufficient sound power, especially when using semi-professional and professional phones. In addition, the presence of additional amplifiers allows you to do without switching audio cables when working with two or more computers at the same time. You can also notice that some models of DVD players are equipped with a VGA output, and some models of computer monitors are equipped with a PAL decoder and one or more analog video inputs, which allows you to connect household DVD players, VCRs, camcorders, cameras to the monitor. In such a situation, the presence of a built-in telephone audio frequency amplifier in the monitor turns out to be very convenient, since the number of devices necessary for viewing video information is reduced. Recently, most telephone amplifiers have been forced to operate at a low supply voltage (1,5 ... 5 V), which limits the amplitude of the output signal amplitude, this sharply degrades the sound quality if you need to turn it up "louder". Therefore, the author assembled a telephone amplifier with a relatively high supply voltage, which was built into the case of a computer LCD monitor.
The circuit diagram of the amplifier is shown in fig. 1. The amplifier is assembled on an inexpensive, non-deficient integrated circuit AN7142 from PANASONIC (about $ 1,3), which is a two-channel power amplifier. The maximum supply voltage of this microcircuit reaches 18 V, which is not often found among microcircuits of this class. The audio frequency voltage is supplied to the XS1 connector, then, through the resistive dividers R1R3 and R2R4 and separating capacitors C3, C4, to the inputs of the DA1 microcircuit. The amplified signal is taken from pins 10 and 7 of the microcircuit, after which it is fed through the isolation capacitors C14, C15 and current-limiting resistors R9, R10 to the jack for connecting stereo phones. Resistors R5, R6 set the gain of the amplifier, but their resistance cannot be increased significantly due to the emerging tendency of the amplifier to self-excite at high frequency. With the resistors R1-R6 indicated in the diagram, the voltage gain of the amplifier is close to 5. This amplification allows you to comfortably watch videos at a low volume level, as well as photographs in jpeg files, in which integrated audio comments sound quiet without additional amplification. The increased load resistance and reduced gain of the microcircuit has a positive effect on its sound quality. The microcircuit is powered by a voltage of +15 V through an LC filter consisting of elements L1, SU, C13, C16, C19. With this method of turning on the microcircuit, the amplifier's own noise and interference from the operation of the computer and monitor nodes are below the threshold of audibility. Volume control is performed by means of the operating system. If you have a multimedia keyboard, you can adjust the system volume without leaving the running application, which is convenient when playing games that do not have volume controls in the settings.
The appearance of the mounted amplifier is shown in the photo fig. 2. It is assembled on a fiberglass board with dimensions of 93x27 mm; its drawing and arrangement of elements are shown in fig. 3. The design feature is the use of low-profile oxide capacitors with a height of no more than 8 mm, which makes it easy to mount it in almost any thin LCD monitor case. The author installed a telephone amplifier in the BENQ monitor case, fig. 4, which became morally obsolete during the three-year warranty and, after purchasing a new monitor, was transferred to the rank of auxiliary. The amplifier is installed outside the metal screen.
The switching power supply of the monitor generates two DC voltages: +3,4 and about +15 V. The voltage of +3,4 V for the above reason is not enough to power telephone amplifiers, and the use of +15 V turned out to be very convenient. In the monitor, this voltage is used to power voltage converters for powering fluorescent backlight lamps. The telephone amplifier is connected to the oxide capacitor of the rectified voltage filter. A pleasant feature of the BENQ monitor used was that no matter how (software or hardware) the monitor was turned off, +15 V, +3,4 V voltages were always available at the output of its power supply, if only the power cord was plugged into the outlet. This allows you to use phones connected to the monitor if only one monitor is currently in use without a built-in phone amplifier, and the monitor with a built-in phone amplifier is off. If the monitor uses a voltage of more than +18 V, for example +24 V, to power the backlight lamps, then the amplifier can be powered through a linear integrated regulator, for example, the 7815 series.
Resistors can be of any type of appropriate power - for example, C1-4, C1-14, C2-23, MLT, imported analogues, SMD. Non-polar capacitors - K10-17 or imported analogues, as well as SMD components for an operating voltage of at least 16 V. Capacitor C13 is tantalum, its contribution to noise suppression in the power circuit is noticeably higher than several ceramic capacitors connected in parallel with a capacity of 0,1 uF. Instead of such a capacitor, you can install another one at 13 uF in parallel with C470. However, you should not significantly increase the capacitance of the filter capacitors due to the possible operation of the protection in the monitor's power supply against overload at the moment the power is turned on. Capacitor C19 can be either ceramic or tantalum or aluminum oxide. Inductor L1 small-sized industrial production with a winding resistance of about 1 ohm. The AN7142 microcircuit, when used as a telephone amplifier, is preferably mounted on a small copper heat sink, similar to that shown in fig. 2. The heat sink flange of the microcircuit must be soldered to the common power wire. When mounting the amplifier in the monitor case, it is desirable to place the board in the coolest, lower part of the case. It is very convenient to place plug sockets on the underside of the front or rear decorative panel, which will not change the design of the front panel of the case and will not make it difficult to connect cables "by touch". If necessary, to connect several phones, you can install several jacks by connecting each of them through its own pair of resistors R9, R10. This will allow, for example, without interfering with neighbors at night, to work comfortably on dubbing a video film simultaneously for several people (at low volume during dubbing, a lot of technical and artistic marriage can be allowed). To power the telephone amplifier, you can use any voltage source of 8 ... 18 V DC, which can additionally provide at least 50 mA of load current. The amplifier is connected to the analog line output of the computer sound card. Modern sound cards integrated into the system board allow you to assign any function to any socket - use it as any given input or as any given output. This makes it possible to organize several audio analog or digital line outputs on the computer, which eliminates the need for audio signal splitters. Author: A. Butov, p. Kurba of the Yaroslavl region; Publication: radioradar.net
Machine for thinning flowers in gardens
02.05.2024 Advanced Infrared Microscope
02.05.2024 Air trap for insects
01.05.2024
▪ Flying 4G tower - drone Flying COW ▪ CC3235S is TI's first IoT WiFi-chip to support the 5GHz band
▪ section of the site Labor protection. Selection of articles ▪ article Soft-bodied intellectual. Popular expression ▪ article Which birds can sleep on the fly? Detailed answer ▪ article Workplace radio electronics. HAM Tips ▪ article Car power amplifiers. Directory
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page