The AF power amplifier, the circuit of which is shown in the figure, is made on lamps from old black-and-white televisions or radio tubes. This is a pre-amplifier with a phase inverter on a double 6N2P triode and a push-pull output stage on two 6P14P tubes.
The use of such old components, which are often unnecessary, or obtained through disassembly or disposal of old equipment, makes the cost of this amplifier close to zero. Although, on the other hand, there are not so many tube lamps left now.
Amplifier characteristics
The amplifier develops a power of about 20 W at a load of 8 ohms with a nonlinear distortion of no more than 0.6%. With a nonlinear distortion factor of no more than 0.25%, the power is 14 watts. The operating frequency range with an unevenness of 6 dB is 30 ... 20,000 Hz. Amplifier input sensitivity 250 mV. Volume control with variable resistor R3.
Amplifier schematic diagram
The diagram shows a mono version of the amplifier. A stereo amplifier consists of two of the same amplifiers, powered by one common bridge rectifier on diodes VD1-VD4.
The input signal through the X1 connector and the volume control at R3 goes to the pre-amplification stage, made on the first triode of the H1 tube. The negative feedback signal enters the cathode circuit of this triode from the tap of the secondary winding of the output transformer T1.
The amplified signal is removed from the anode and fed through the capacitor C6 to the grid of the second triode of the H1 lamp. The second triode is a phase-inverted stage, which creates antiphase signals necessary for the operation of the output push-pull power amplifier.
Fig. 1. Schematic diagram of a simple tube power amplifier for 14-20 watts, 6N2P, 6P14P.
The direct signal is removed from the cathode of this triode and through the capacitor C5 is fed to the grid of the H3 pentode. The inverse signal is removed from the anode of the triode and through C4 is fed to the grid of the H2 pentode.
The primary winding of the output transformer T1 is included in the anode circuit of the pentodes. The cascade is powered through a tap of this winding.
Fig. 2. Transformer winding connection diagram.
To exclude self-excitation at high frequencies, resistors R10 and R12 are included in the networks of the H2 and NC grids. The shielding grids of the H2 and H3 pentodes are connected to the plus of the power supply through resistors R15 and R16. Now for the details.
Details
All capacitors except C3 and C6 must be rated for a voltage of at least 350V, capacitors C3 and C6 for a voltage of at least 50V. The diode bridge on VD1-VD4 can be replaced with another one on rectifier diodes that allow a current of at least 1A and a voltage of at least 350V.
Table 1.
Transformers, both output and mains, are made on the same Ш85 cores. The winding of 1-2 network transformer T2 contains 1000 turns of PEV 0.43. Winding 3-4 - 1300 turns of PEV 0.2.
Filament winding 5-6 contains 33 turns of SEV 0.96. Figure 2 shows the winding diagram of the output transformer T1. The letters H and K in the diagram indicate the beginning and end of the winding section, respectively. The other letters indicate the winding sections. Winding data T1 are summarized in Table 1.
And yet, sometimes I wonder how many all kinds of integral audio power amplifiers... There are a great many microcircuits from the TDA series alone. All of them are practically available. There are plenty to choose from. Circuits based on such integrated audio frequency amplifiers are distinguished by their originality and simplicity. They are especially popular among novice radio amateurs and those who do not want to bother with something bulky. True, the sound quality of integrated audio power amplifiers for the most part leaves much to be desired. But still, they meet the expectations of many. Yes, and there are decent copies on which you can assemble a standing speaker system both for home and for a car. For example, the same TDA7294 or TDA2030. Information on such amplifiers is currently available. I now remember the days of our youth, when not only the Internet, but a personal computer was a huge rarity. I had to go to libraries, look for radio engineering literature, which was worth its weight in gold. Yes, and that was, then the 60s, 70s. Triodes, tetrodes, pentodes and other achievements of science and technology of those years looked at you from the pages of such radio amateur books. And in order to find a truly worthwhile design, a circuit, even an audio frequency amplifier, you had to try. Now all the information in its entirety is posted on the Web. I entered, for example, an audio frequency power amplifier circuit in a search engine, and immediately thousands of pages are displayed. You can find colleagues in a hobby, discuss the desired radio amateur circuit or design ... In short, this is what I am surprised and happy about for many radio amateurs. Anyway. It was a lyrical digression. Now on the topic of TDA7240.
So, TDA7240- this is 20 watt audio amplifier, oriented mainly for installation in a car. All kinds of protections are built into the TDA7240 IC, such as protection against short-circuit and overheating. The appearance of the microcircuit is below.
The low frequency amplifier circuit on the TDA7240 is shown in the illustration below. By the way, the circuit is very similar to the amplifier on the TDA2025.
The output power for a load of 4 ohms at a supply voltage of 14.4 volts is 18 ... 20 watts. At 8 ohms - 10 ... 12 watts. The coefficient of nonlinear distortion in the first case is from 0.1 to 0.5%. In the second - from 0.05 to 0.5%. Supply voltage up to 18 volts. Approximate PCB layout:
category Amplifier circuits Materials in category * Subcategory Transistor Amplifier Circuits
The so-called dynamic intermodulation distortion occurs in transistor amplifiers with sharp drops in signal level. These distortions are especially noticeable when playing music programs. In order to reduce these distortions, this amplifier widely uses local feedback in current, a so-called "current mirror" is used, which improves the symmetry of the amplified signal at the input of the final stage, and an advance correction of the frequency response is used.
The main parameters of the amplifier
Nominal frequency range, Hz .... 16... 100 000;
Rated output power into a load of 8 Ohm (at 0.35% harmonic distortion at frequencies
1,000 and 10,000 Hz), W .... 20;
Rated input voltage, V .... 1;
Relative level of noise and background, dB .... -60.
Amplifier schematic diagram
The amplifier contains an input differential stage on transistors V1, V2, balancing a stage on transistors V3, V5 with a "current mirror" on transistors V4, V6, an output stage on transistors V14-V17 and a short-circuit protection device in the load on transistors V9, V10 ...
Resistors R3, R4 in the emitter circuits of the transistors of the first stage create local feedback in current, increasing the linearity and input resistance of the stage, and also improving its symmetry. Resistors R11, R14 create local feedback in the second stage. The frequency response correction in advance is carried out by capacitors C2 and C6.
The output stage is made according to the traditional scheme with a phase inverter on transistors of different structures V14, V15. The quiescent current of the transistors V16, V17 is set by the trimming resistor R15 and is stabilized when the temperature changes by the transistor V7, which has a thermal connection with one of them. Diodes V18, V19 protect the output stage transistors from overvoltage with an inductive nature of the load.
The amplifier is covered by OOS, the voltage of which is removed from the load and through the R10C4C5R9 circuit enters the input of the first stage (into the base circuit of the transistor V2). The R28C10 circuit increases the amplifier's resistance to self-excitation.
The device for protecting the output stage from a short circuit in the load is made according to the bridge circuit. For the negative half-wave of the amplified signal, the bridge is formed by the load resistance and resistors R26, R20 and R17. The emitter junction of the transistor V9 is included in the diagonal of the bridge.
With a sharp decrease in the load resistance, the balance of the bridge is disturbed, the V9 transistor opens and its low resistance of the emitter-collector section shunts (through the V8 diode) the input of the pre-terminal stage on the V14 transistor. As a result, the output stage current is instantly limited. For the positive half-wave of the signal, the bridge is formed by the load resistance and resistors R27, R21 and R19 in the diagonal of the bridge "the emitter junction of the transistor V10 is turned on.
For good linearity of the amplifier, a pair of transistors V1 and V2, VЗ and V5 V4 and V6, V16 and V17 must be selected according to the static current transfer coefficient h21e.
Amplifier details and setup
Transistors V14, V15 are installed on U-shaped heat sinks bent from a strip of sheet (24mm thick, 20mm wide) aluminum alloy (the dimensions of the heat sink are 20 X 25 X 15 mm). The heatsinks of each of the V16, V17 transistors must have a cooling surface of about 250 cm2. The transistor V7 is glued to one of these heatsinks with 88-H glue.
The adjustment of the amplifier is reduced to the elimination (trimming resistor R7) of the constant voltage at the output and setting (trimming resistor R15) the quiescent current of the output stage within 80 ... 100 mA.
An amplifier intended for use in professional audio systems must meet a number of specific requirements. First of all, this is the increased output power, which the device must develop for a long time, as well as a simple and reliable design.
Modern professional amplifiers typically operate in Class D, which allows high power output with little circuit heat. Also, such models can have an effective switching power supply capable of delivering a significant pulse current to the load. An important indicator of the quality of a professional amplifier circuit is the load damping factor, since this class of equipment usually works on long speaker cables. And for better noise immunity, such a device must have balanced input terminals. A professional power amplifier should be rated for installation in a standard rack-mountable rack, and usually has a forced cooling system. Professional power amplifiers are used both for sounding public institutions and creating audio systems for concert performances. The concert amplifier has both standard and specialized connectors (Speakon, TRS) for connecting acoustic systems and signal sources, and light indication of operating modes is well distinguishable in low light conditions. Due to the high output power, concert amplifiers are usually equipped with a "soft start" circuit, which avoids overloading the mains when it is turned on.
Manufactured using the TDA2003 microcircuit. Now let's try to tackle another, more powerful microcircuit. This LM1876 based audio amplifier can deliver up to 20 watts per channel into 4 ohms and guarantees less than 0.1% total harmonic distortion.
The amplifier is powered by a bipolar current source of ± 15 V. After the diode bridge and smoothing capacitors, approximately ± 20 V DC is obtained, which is used to power the LM1876. Inductors L1 and L2 on the input line of the PSU reduce noise from the mains.
The audio input is connected to the board via a regular 3.5mm stereo jack. The stereo potentiometer adjusts the amplitude of the audio signal. The potentiometer also includes a switch that allows the amplifier to go into standby mode. In this mode, the LM1876 only consumes 4mA. The outputs of the amplifiers on the speakers are connected to the RCA connectors on the board.
In operation, this microcircuit produces a sufficiently large amount of heat, so a radiator is required for cooling with a size of 100 mm2 or more. If the amplifier's output power reaches 20W, the power consumption becomes about 40W into a 4 ohm speaker and 20W into an 8 ohm speaker. The maximum allowable crystal temperature is 165 ° C. therefore the radiator should be chosen large. Fortunately, the LM1876 provides overtemperature shutdown. To reduce the total thermal resistance, thermal grease must be applied between the microcircuit and the heatsink. As for the drawings, m / s datasheet and PCB files, you can download them.
Fully assembled ULF circuit
Two electrolytic capacitors C7 and C8 at 6800uF 50V smooth the rectified voltage. Resistors R7 and R8 are connected between their terminals to discharge the capacitors after power off, in order to avoid electric shock. Plus 20V is labeled VCC and minus is labeled VEE. The D1 LED is placed between the VCC and VEE lines to indicate the status of the power supply. The 100uF and 100nF bypass capacitors are connected to the VCC and VEE pins as close to the IC as possible. Capacitors C9 and C10 block the DC voltage from the IC. The audio outputs of each amplifier are connected to RCA connectors J2 and J3.
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