• Low-power amplifiers and printed circuit boards. Low frequency amplifier (LF) on the TDA7250 chip. Amplifier parameters, a few words about TDA7293

    This article addresses special issues in the design and use of printed circuit boards as they apply to power amplifiers, especially those operating in Class B. All power amplifiers contain power amplification stages themselves and associated control and protection circuitry. Most amplifiers also have a small-signal low-pass stage, balanced output amplifiers, a subsonic filter, output meters, etc.

    Other issues related to PCB design are also considered, such as grounding, safety issues, reliability, etc. The performance of a low-frequency power amplifier depends on a large number of factors, in all cases careful design of the printed circuit board is decisive, primarily due to for the danger of distortion caused by inductive interference; Possible interactions between signal paths and power rails can very easily limit the amplifier's linearity, so it is difficult to overestimate the importance of this problem. The selected circuit board layout (component placement and trace pattern) will largely determine both the distortion and crosstalk levels of the amplifier.

    In addition to the above considerations regarding amplifier performance, the circuit board design will have a significant impact on manufacturability, ease of inspection, repairability, and reliability. All of the above aspects of the problem are discussed below.

    Successfully designing an amplifier PCB circuit requires a level of electronics knowledge to understand the intricacies of the effects described below so that the PCB design process can proceed smoothly and efficiently. It is already considered common practice when designing printed circuit boards for various fields of electronics to be left at the mercy of professionals who, while very knowledgeable in the intricacies of working with computer-aided design systems, have a very vague or even complete lack of understanding of the intricacies of how electronic circuits work. For some areas this approach may be acceptable; When designing a power amplifier, it is completely inadequate due to the fact that basic characteristics such as crosstalk and distortion level are highly dependent on the wiring diagram. A little further down, the PCB designer will be able to understand what we are actually talking about.

    Crosstalk

    Crosstalk (or the phenomenon of signal “flowing” from one channel to another, electrical interference caused by the passage of a signal in adjacent wires) is characterized, first of all, by the source of the signal (which can be any complex impedance) and the receiver, which usually has a higher value of complex impedance , or the potential of a virtual, “floating” earth. When crosstalk in communication channels is discussed, typically the transmitting and receiving channels are referred to as voice and non-voice channels, respectively.

    Crosstalk occurs and manifests itself in different forms:

    1. Capacitive crosstalk results from the proximity of two electrical conductors in space and can be represented by using a virtual (or effective) capacitor connecting the two circuits. The capacitance of such a capacitor increases with increasing frequency in proportion to the value of 6 dB/octave, although higher rates of increase in capacitance are possible. Shielding the conductors with any conductive material completely solves the problem, although increasing the distance between such conductors turns out to be a less expensive method.
    2. Resistive crosstalk occurs for the simple reason that the resistance of the ground bars is different from zero. Copper at room temperature is not a superconductor. Resistive crosstalk is independent of frequency.
    3. Inductive crosstalk is rarely a problem in audio design; they can occur when two low-frequency transformers are recklessly installed too close to each other, but other than this case the problem can usually be forgotten. A significant exception to this rule is the low-frequency Class B power amplifier, in which the currents flowing through the power rails are half-sine waves and which can seriously affect the amplifier's distortion level if they are allowed to interact with the input signal circuits, the feedback loop, or the output circuits. cascade.

    In most linear low-frequency circuits, the main cause of crosstalk is unwanted capacitive coupling between the various circuits of the circuit, and in the vast majority of cases this is determined by the pattern of the wires and traces of the printed circuit board. In contrast, Class B power amplifiers suffer little or even negligible crosstalk caused by capacitive effects, since the impedances of the circuits tend to be small and the distances between them sufficiently large; A much bigger problem is the inductive coupling between the buses through which the power currents flow and the circuits through which the signal passes. If such a coupling occurs between the circuits of the same channel, it manifests itself in the form of distortion and can lead to significant non-linearity in the amplifier's characteristics. If this interaction extends to another (non-speech) channel, it will appear as crosstalk of the distorted signal. In any case, such a connection is highly undesirable and special measures must be taken to prevent its occurrence.

    PCB routing is only one element of this battle, since the crosstalk must somehow not only be emitted, but also be received somewhere. Typically, the source of maximum radiation will be the internal electrical wires due to their overall length and abundance, the wire routing pattern will probably be the most critical to achieving best performance, so various clamps, cable clamps, etc. will need to be used to secure them. The receiving device is most often the input circuits and feedback circuits, which are also located on the printed circuit board. For good operation of the device, it is necessary to study these issues from the point of view of maximum protection from radiation.

    Distortion caused by power supply interference

    The power rails of a Class B power amplifier carry very large and highly distorted currents. As already emphasized earlier, if due to induction their interaction is allowed on the circuit through which the acoustic signal passes, then the level of distortion will increase sharply. This applies to the PCB conductors and similarly to the cable connections, the sad truth is that it is quite easy to make an amplifier PCB that is absolutely perfect in every way except for this one requirement, and the only solution is to use a second fees. However, to obtain optimal results, the following requirements should be followed:

    1. It is necessary to minimize electromagnetic radiation from the power rails by placing the positive and negative voltage rails as close to each other as physically possible. They should be located as far as possible from the input circuits of the amplifier stage and the connecting output terminals; The best method is to connect the power bus wires to the output stage on one side, and the rest of the amplifier wires on the other. Then you need to run wires from the output to power the rest of the amplifier; half-wave current will no longer pass through them, so it will not cause problems.
    2. It is necessary to minimize the absorption of electromagnetic radiation from the power buses by minimizing the area of ​​​​the circuits covered by the input and feedback circuit wires. They form closed loops through the ground, so the area of ​​the loops covered by them should be minimal. Quite often, the best result can be obtained by maximizing the spatial separation and routing the input circuits and feedback loop wires across the LF ground path, which runs through the center of the printed circuit board from the input to the output ground loop point. Inductive distortion can also occur when interacting with output wires and output ground wires. The latter case presents a rather serious problem, since it is usually difficult to change its position in space without updating the printed circuit board itself.

    Installing Output Semiconductors

    The most important fundamental decision is whether to install high-power output devices on the amplifier's main circuit board. There are a number of strong arguments in favor of such a decision, but, nevertheless, such a choice is not always the best.

    Advantages:

    1. The amplifier's printed circuit board can be designed to form a complete unit that can be thoroughly tested before it is installed in the chassis. This approach greatly facilitates testing, since access to various points of the circuit is provided from all sides; it also eliminates the possibility of surface damage to the PCB itself (scratches, etc.) during inspection.
    2. There is no possibility of incorrect connection of output semiconductor devices, provided that the required semiconductor devices are installed in the correct positions. This is a fairly significant argument, since such errors usually disable the output semiconductor devices, and also lead to other negative effects that develop like falling dominoes, and which will require a large amount of time (and money) to correct.
    3. All connecting wires leading to the output semiconductor devices should be as short as possible. This helps increase the stability of the output stage and resist the occurrence of RF oscillations.

    Flaws:

    1. If the amplifier's output devices require frequent replacement (which clearly indicates some very serious defect), then the repeated soldering operation will damage the traces of the printed circuit board. However, if the worst happens, the damaged section can always be replaced with a short conductor, so there is no need to scrap the printed circuit board; rest assured, such a repair option is always possible.
    2. It is possible that the output semiconductor devices can become very hot, even when they are operating at nominal conditions; For TO3 type devices, a housing temperature of 90 °C is not unusual. If the mounting method used does not allow for some degree of resilience, thermal expansion may result in mechanical forces that can tear off the PCB mounting gaskets.
    3. The heat sink will usually have significant dimensions and weight. Therefore, it is necessary to use a fairly rigid structure that secures the printed circuit board and radiator. Otherwise, the entire structure, due to the lack of sufficient rigidity, will vibrate during transportation, creating excessive forces in the places where the joints are soldered.

    Evgenia Smirnova

    To send light into the depths of the human heart - this is the purpose of the artist

    Connecting speakers to a laptop, TV, or other music source sometimes requires a separate device to amplify the signal. The idea of ​​building your own amplifier is a good one if you are inclined to work with printed circuit boards at home and have some technical skills.

    How to make a sound amplifier

    The beginning of work on assembling an amplification device for speakers of one type or another consists of searching for tools and components. The amplifier circuit is assembled on a printed circuit board using a soldering iron on a heat-resistant support. It is recommended to use special soldering stations. If you assemble it yourself for the purpose of testing the circuit or for use for a short period of time, the “on wires” option is suitable, but you will need more space to place the components. The printed circuit board guarantees the compactness of the device and ease of further use.

    A cheap and widespread amplifier for headphones or small speakers is created on the basis of a microcircuit - a miniature control unit with a pre-wired set of commands for controlling an electrical signal. All that remains to be added to the circuit with the microcircuit is a few resistors and capacitors. The total cost of an amateur-grade amplifier is ultimately significantly lower than the price of ready-made professional equipment from the nearest store, but the functionality is limited to changing the output volume of the audio signal.

    Remember the features of compact single-channel amplifiers that you assemble yourself based on TDA series microcircuits and their analogues. The microcircuit generates a large amount of heat during operation, so you should eliminate or minimize its contact with other parts of the device. A radiator grille for heat dissipation is recommended for use. Depending on the model of the microcircuit and the power of the amplifier, the size of the required heatsink increases. If the amplifier is assembled in a housing, you should first plan a place for the heat sink.

    Another feature of assembling a sound amplifier with your own hands is the low voltage consumption. This allows you to use a simple amplifier in cars (powered by a car battery), on the road or at home (powered by a special unit or batteries). Some simplified audio amplifiers require a voltage of only 3 Volts. Power consumption depends on the degree of audio signal amplification required. The sound amplifier from the player for standard headphones consumes about 3 Watts.

    It is recommended that a novice radio amateur use a computer program to create and view circuit diagrams. Files for such programs can have a *.lay extension - they are created and edited in the popular virtual tool Sprint Layout. Creating a circuit with your own hands from scratch makes sense if you have already gained experience and want to experiment with the knowledge you have gained. Otherwise, look for and download ready-made files that can be used to quickly assemble a replacement for a low-frequency amplifier for a car radio or a digital combo amplifier for a guitar.

    For laptop

    A do-it-yourself sound amplifier for a laptop is assembled in one of two cases: the built-in speakers are out of order, or their volume and sound quality are not enough for your needs. You will need a simple amplifier designed for external speaker power up to 2 Watts, and winding resistance up to 4 Ohms. To assemble it yourself, in addition to standard amateur radio tools (pliers, soldering station), you will need a printed circuit board, a TDA 7231 microcircuit, and a 9-volt power supply. Select your own housing to house the amplifier components.

    Add the following items to the list of purchased components:

    • non-polar capacitor 0.1 µF – 2 pcs.;
    • polar capacitor 100 µF – 1 pc.;
    • polar capacitor 220 µF – 1 pc.;
    • polar capacitor 470 µF – 1 pc.;
    • constant resistor 10 KOhm – 1 pc.;
    • constant resistor 4.7 Ohm – 1 pc.;
    • two-position switch – 1 pc.;
    • jack for loudspeaker output – 1 pc.

    Determine the assembly order yourself depending on which *.lay electrical diagram you downloaded. Select a radiator of such a size that its thermal conductivity allows you to maintain the operating temperature of the microcircuit below 50 degrees Celsius. If the device is constantly used outdoors with a laptop, it will need a homemade case with slots or holes for air circulation. You can assemble such a case with your own hands from a plastic container or the remains of old radio equipment, securing the board with long screws.

    For DIY headphones

    The simplest stereo amplifier for portable headphones should have low power, but the most important parameter will be power consumption. In an ideal example, the design is powered by AA batteries or, in extreme cases, by a simple 3-volt adapter. You will need a high-quality TDA 2822 microcircuit or its equivalent (for example, KA 2209), an electronic circuit for assembling an amplifier with your own hands using a TDA 2822. Additionally, take the following components:

    • capacitors 100 µF (4 pcs.);
    • up to 30 cm of copper wire;
    • headphone socket.

    A heat sink element will be needed if you want to make the amplifier compact and with a closed housing. The amplifier can be assembled on a ready-made or home-made printed circuit board or by surface mounting. The pulse transformer in the power supply may cause interference, so do not use it in this amplifier. The finished amplifier will provide pleasant and powerful sound from the player (record or radio signal), tablet or phone.

    Subwoofer amplifier circuit

    The low-frequency amplifier is assembled with your own hands on the TDA 7294 microcircuit. It is used both to create powerful acoustics with bass in the apartment, and as a car amplifier - in this case, however, you need to purchase a bipolar power supply of 30-35 Volts. The figures below describe the location of components, as well as the values ​​of resistors and capacitors. This subwoofer amplifier will provide an output power of up to 100 watts with outstanding low frequencies.

    Mini sound amplifier for speakers

    The design described above for laptops is suitable as a sound amplification device for domestic or foreign home speakers. Stationary placement of the device will allow you to choose any power adapter from those available. You can ensure the miniature size and acceptable appearance of an inexpensive amplifier by following several rules:

    1. Ready-made high-quality printed circuit board.
    2. Durable plastic or metal case (order from a specialist).
    3. The placement of components is pre-planned.
    4. The amplifier is soldered neatly, without unnecessary drops of solder.
    5. The heatsink only touches the chip.
    6. Ready-made sockets are used for signal output and power input.

    DIY tube sound amplifier

    Tube sound amplifiers are expensive devices, provided that you purchase all the components at your own expense. Old radio amateurs sometimes keep collections of tubes and other parts. Assembling a tube amplifier at home with your own hands is relatively easy if you are willing to spend a few days searching for detailed circuit diagrams on the Internet. The sound amplifier circuit in each case is unique and depends on the sound source (old tape recorder, modern digital equipment), power source, expected dimensions and other parameters.

    Transistor sound amplifier

    Assembling a sound preamplifier with your own hands without using complex microcircuits is possible using transistors. An amplifier based on germanium transistors can be easily integrated into modern audio systems; it does not require additional configuration. The disadvantage of transistor circuits is the larger size of the board assembly. The dependence on the “purity” of the background is also unpleasant - you will need a shielded cable, or an additional circuit for suppressing noise and ripple from the network.

    Video: DIY audio power amplifier

    Found an error in the text? Select it, press Ctrl + Enter and we will fix everything!

    If you are interested in this article, then you have already read a lot of positive reviews on websites and various forums. Quite a few radio amateurs have already repeated this scheme, and, as we understand, they did not regret their choice. It is clear that transistor amplifiers are superior in sound quality to amplifiers implemented on microcircuits. LANZAR has an amazingly low coefficient of nonlinear distortion, and with a fairly wide range of supply voltage it allows you to develop 50...300 Watts of power at a load. And even at three hundred watts, these distortions do not exceed 0.08% over the entire audio range. Briefly about the amplifier parameters:

    Gain coefficient – ​​24 dB;
    Coef. nelin. distortion at 60% power - % 0.04%;
    The slew rate of the output signal is at least 50 V/µS;
    Input impedance – 22 kOhm;
    Signal-to-noise ratio, no less than 90 dB;
    Supply voltage, ± 30…65 V;
    Output power - from 40 to 300 Watts (depending on U power supply)

    Schematic diagram of the Lanzar V3.1 amplifier:

    Pay attention to resistors R3 and R6 - these are current-limiting resistors of parametric stabilizers formed by these resistors and zener diodes VD1 and VD2. The lower the supply voltage, the lower the values ​​of these resistors.

    ● Supply voltage ±70 Volts – 3.3…3.9 kOhm;
    ● Supply voltage ±60 Volts – 2.7…3.3 kOhm;
    ● Supply voltage ±50 Volts – 3.2…2.7 kOhm;
    ● Supply voltage ±40 Volts – 1.5…2.2 kOhm;
    ● Supply voltage ±30 Volts – 1…1.5 kOhm;
    ● Supply voltage ±20 Volts - it is better to choose a different amplifier circuit for assembly.

    The value of the constant voltage at the output of the amplifier depends on the rating of R1. In the diagram, the nominal value of R1 is 27 kOhm, you can put 22 kOhm. Often it has to be selected in the range from 15 to 47 kOhm.

    2 resistors installed in the emitters of the differential stage (R7, R12 and R9, R13) - the values ​​of these resistors directly depend on how accurately you can select the gains of transistors VT1, VT3 and VT2, VT4. The more accurately the gain factors of these transistors are selected, the lower the value can be used in emitter circuits, and the lower the value of these resistors, the less nonlinear distortion introduced by the differential stage. Resistor values ​​without selecting transistors should be about 82...100 Ohms. If the transistors are selected, the resistor values ​​can be reduced to 10 Ohms.

    The value of resistor R14 determines the gain of the amplifier.
    The resistor located between the emitters of transistors VT8 and VT9 is rated at 47 Ohms. It is not recommended to change.
    Resistors located in the base circuits of the output transistors, their value can be in the range of 1...2.4 Ohms.
    Resistors in the emitter circuits of the output transistors - power of at least 5 Watts, nominal value 0.1...0.3 Ohm. Of course, the values ​​of these resistors must be the same.

    Diodes VD3 and VD4 are designed for a current of 1...1.5 Amperes (the brand does not matter), the main thing is that they are the same.
    At the input, two electrolytic capacitors are connected in series with their positive leads outward; they form a non-polar capacitance. And a film capacitor connected in parallel with them, together with them, creates minimal distortion of the audio signal over the entire frequency range. A similar circuit is found in the feedback circuit of an amplifier.

    Capacitor C4 is noise suppressing. The rating can be from 330 to 680 pF.
    Capacitors C12 and C13 - nominal 33 pF. They serve to reduce the speed of the amplifier, since without them the rise in the output signal is too large, and the amplifier becomes prone to self-excitation. Exactly the same capacitor is connected in parallel to resistor R25, which determines the gain.

    Resistor R13 can also be used to adjust the gain.
    Resistors in the base circuit of transistor VT7 - setting the quiescent current of the final stage. VT7 is installed on a radiator with output transistors for thermal stabilization of the quiescent current of the latter. Trimmer resistor – multi-turn type 3296.

    Coil - 10 turns of wire with a diameter of 0.8 mm on a mandrel with a diameter of 12 mm.

    The amplifier is turned on for the first time after checking the installation for the presence of “snot”. The resistor slider of the quiescent current regulator is in the upper extreme position according to the circuit, this means that the quiescent current of the output stage transistors should be minimal. It is also worth limiting the current developed by the power source; to do this, an incandescent lamp of 40...60 Watt is switched on in series with the power transformer. We apply supply voltage to the circuit, and if after a short flash the light goes out, or glows so that the filament is barely visible, then there are no serious errors in the installation. We check the presence of zero at the output of the amplifier and the voltage at the zener diodes VD1 and VD2. Next, turn off the power and remove the incandescent lamp from the circuit. Turn on the power again. We adjust the quiescent current of the output stage with a variable resistor; it should be in the range of 70...100 mA.

    Lanzar amplifier circuit board:

    There is also an alternative version of the printed circuit board for this amplifier, its appearance is shown in the pictures below (this version of the board has not been tested, so check its correctness before proceeding with its manufacture, errors are possible):

    You can download the diagram and both versions of the printed circuit board in LAY format using a direct link from our website. Also in the archive you will find a file in PDF format, from which you will also get a lot of useful information. The download file size is 0.65 Mb.

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