Hi-Fi amplifier QUAD-405. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

 Comments on the article

For decades, the "QUAD-405" has been one of the best known quality amplifiers. With the application of innovations born of technology, its parameters have been repeatedly improved. We will get acquainted with its modified version, in which the emphasis is on increasing power.

The purpose of the modification was to double the power of the "basic version" of "QUAD", i.e. up to 200 W, while maintaining all its output parameters. This task is not an easy one, since it entails, first of all, an increase in the supply voltage. To get 200W sinusoidal power into a 4-ohm load, you need an 80V peak-to-peak signal. This signal level requires a supply voltage of approximately ±50. .55 V. The situation is even more complicated in the case of 8-ohm loudspeakers. when the output signal swing needs to be increased to 115 V. The supply voltage required for it increases to ± 60 ... 65 V.

From the above examples, it is clear that increasing power requires considerable discretion in solving both circuitry and technological problems. The correct choice of transistors is a necessary but not sufficient condition for the correct solution of this problem.

Scheme "QUAD-405/200"" is shown in Fig.1.

(click to enlarge)

The AC voltage gain is determined in the 1C operational amplifier by the ratio of the resistances R6 and R3. Negative feedback, due to the presence of capacitor C3, begins to act above a frequency of 1 Hz. Through the R5-R3 circuit, 100% negative DC feedback is carried out from the output of the amplifier. Since the amplifier has unity gain with respect to DC, the resulting offset (offset) is the same as the bias voltage of the operational amplifier.

The amplification of the AC voltage and the operation of the class "A" amplifier on the transistor T2 at high frequency is determined mainly by the elements of the bridge. Capacitor C9, together with this amplifier, forms a high-speed integrator, while it simultaneously serves as one of the bridge elements. The next element of the bridge is R37. Control over the output stage current (dumper) is carried out by the third element of the bridge - inductance L2. The fourth element of the bridge is the equivalent resistance of the parallel chain of resistors R16-R17, which, with the help of R15, sets the voltage gain of the cascade at T2, contributing to a very good linearity of the characteristic.

In the same way, voltage is supplied to T2, which compensates for the error that occurs due to the voltage drop across L2 due to the output current. This error signal passes through the amplifier and appears at the output with the same amplitude, but with the opposite phase compared to the signal occurring at 12. After the two error signals are canceled out at the loudspeaker, a slight mismatch of the bridge creates an excellent output signal without distortion. . System performance is affected by Class A distortion, bridge mismatch, and NE5534 op amp distortion.

Limiting the frequency range of the signal arriving at T2 is provided by the integrating chain R11-C6. This places an upper limit on the bandwidth of the amplified frequencies and is one of the simplest ways to protect against intermodulation distortion. About the proper phase shift of the amplifier on T2. in addition to C9, the chain C8-R14, as well as the capacitor C10, also "takes care of". Excessive phase shift that occurs during the turn-on of the output stage is compensated by the chains L3-R33 and L1-R36.

Amplifier "QUA0-405/200" is placed on a single-sided printed circuit board, the drawing of which is shown in Fig. 2, and the location of the elements - in Fig. 3.

Mounting parts on the board begins with resistors (parts are installed in ascending order of their height). This prevents the soldered part from moving out of place when the board is turned over. Resistors are recommended to be measured with an ohmmeter, rather than identified by a color code printed on them. Powerful resistors should be installed a few millimeters above the board so that they cool better. Inductors L1...L3 each contain 22 turns of winding wire 01 mm wound on a mandrel 013 mm (L1, L3) and 016 mm (L2).

Next, an operation is carried out that particularly affects the reliability of the amplifier: the installation of terminal transistors. Let's think about the following: with an efficiency of 70% and a sinusoidal signal, approximately 90 W of thermal power must be diverted so that the instantaneous temperature of the semiconductors does not approach a critical value! In catalogs, this temperature is usually indicated in the range of 120 ... 140 ° C. This can only be achieved by installing transistors T7 ... T10 on a radiator with very good heat transfer (with heat-conducting paste).

Upon completion of the assembly, we carefully examine the entire circuit again. Using an ohmmeter, we check the insulation between the transistors and the radiator. If everything is in order, you can make the first inclusion. You should not rush, because in the case of a powerful amplifier it is impossible to unambiguously determine how it will behave when the operating point setting is not yet known. By working with due care, the so-called "smoke effect" can be avoided. To do this, we include ammeters in the positive and negative power circuits. It is necessary in one way or another to limit the maximum current of the power supply so that in the event of a short circuit there is no trouble.

Basically, two cases are possible. In the first of them, the final stage functions normally, in the second it “smokes” due to some kind of malfunction. In the first case, the current consumption is about 100 mA. In the second case, there is some kind of anomaly, the current is much larger (it is limited only by the internal resistance of our power supply). In light of this, it is desirable to have a protection with such a characteristic that the impedance at low currents could be neglected, while at high currents it would increase abruptly. This characteristic has a conventional incandescent lamp.

We will include in the positive and negative branches of the power supply by a lamp (a series chain of lamps), the voltage of which is not less than the supply voltage. The protective ability of an incandescent lamp is based on the property that there is a difference of more than one order of magnitude between its cold and hot resistance. If the amplifier is working well, the quiescent current is about 100 mA. With such a current, an incandescent lamp due to the small "cold" resistance is equivalent to a short circuit, as if it were not there. In other words, when it is off, everything is fine. Otherwise, if the pump is on, this indicates a large current and the presence of some kind of malfunction in the system. However, the disaster did not happen, and there is little chance that any part failed. Experience has shown that high current is usually due to improper resistor placement, board defects, poor soldering, high frequency self-excitation, and, much less frequently, poor parts.

With a lamp, troubleshooting is simplified because the circuit can remain on for a longer time. During this time, the defective part will warm up well, and it is easy to detect it by touch. If this does not help, measurements with instruments will be needed. This method of protection using an incandescent lamp is successfully applicable to any amplifier.

So, we connect the supply voltage to the appropriate contacts. Its value is not critical: ±45 ... 55 V. We look at the lamps; if they are off, we control the current in both branches of the supply voltage using ammeters, and then the voltage at the output of the amplifier. This should be around 0V. Current below 100mA and the presence of zero at the midpoint indicates that the DC operating point is set correctly and dynamic control can be exercised. As a precaution, incandescent lamps with a small signal can be left. It should be borne in mind that they limit the output power, and, depending on the magnitude of the signal, they flash and “put down” the power, as in the event of a malfunction, therefore, they are not used with a large signal.

We control signal transmission without load using an audio frequency generator and an oscilloscope. If, after turning on the amplifier without a signal and load, any lamp lights up, immediately turn off the power and engage in a systematic search for errors. Unfortunately, no exact recipe can be given here, as any mistake can affect nutrition. We examine the amplifier again, paying increased attention to the board tracks (presence of breaks, short circuits, etc.), soldering (shorting neighboring points, "unsoldered"). polarity of installed diodes, capacitors, etc.

It is advisable to supplement such an amplifier with an appropriate protective circuit - a "knock silencer". First of all, this protects the speaker system from voltage surges that occur during turning off and on the amplifier, as well as the appearance of a constant voltage at the output in case of a possible malfunction. When finalizing before the output amplifier, you need to turn on some kind of preamplifier and tone control to adjust the level and tone of the sound.

It is advisable to power the amplifier from a structurally simple supply unit (transformer-bridge-capacitor of a high-capacity filter). To achieve an output power of 200 W with a good approximation, a mains transformer of at least 300 W is required. The amplifier can be connected to the power supply using contact connections. The signal input on the board is made in the form of a soldering patch, since it is more expedient to directly solder the shielded cable from the preamplifier here.

See other articles Section Transistor power amplifiers.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

The world's tallest astronomical observatory opened 04.05.2024

Exploring space and its mysteries is a task that attracts the attention of astronomers from all over the world. In the fresh air of the high mountains, far from city light pollution, the stars and planets reveal their secrets with greater clarity. A new page is opening in the history of astronomy with the opening of the world's highest astronomical observatory - the Atacama Observatory of the University of Tokyo. The Atacama Observatory, located at an altitude of 5640 meters above sea level, opens up new opportunities for astronomers in the study of space. This site has become the highest location for a ground-based telescope, providing researchers with a unique tool for studying infrared waves in the Universe. Although the high altitude location provides clearer skies and less interference from the atmosphere, building an observatory on a high mountain poses enormous difficulties and challenges. However, despite the difficulties, the new observatory opens up broad research prospects for astronomers. ... >>

Controlling objects using air currents 04.05.2024

The development of robotics continues to open up new prospects for us in the field of automation and control of various objects. Recently, Finnish scientists presented an innovative approach to controlling humanoid robots using air currents. This method promises to revolutionize the way objects are manipulated and open new horizons in the field of robotics. The idea of ​​controlling objects using air currents is not new, but until recently, implementing such concepts remained a challenge. Finnish researchers have developed an innovative method that allows robots to manipulate objects using special air jets as "air fingers". The air flow control algorithm, developed by a team of specialists, is based on a thorough study of the movement of objects in the air flow. The air jet control system, carried out using special motors, allows you to direct objects without resorting to physical ... >>

Purebred dogs get sick no more often than purebred dogs 03.05.2024

Caring for the health of our pets is an important aspect of the life of every dog ​​owner. However, there is a common assumption that purebred dogs are more susceptible to diseases compared to mixed dogs. New research led by researchers at the Texas School of Veterinary Medicine and Biomedical Sciences brings new perspective to this question. A study conducted by the Dog Aging Project (DAP) of more than 27 companion dogs found that purebred and mixed dogs were generally equally likely to experience various diseases. Although some breeds may be more susceptible to certain diseases, the overall diagnosis rate is virtually the same between both groups. The Dog Aging Project's chief veterinarian, Dr. Keith Creevy, notes that there are several well-known diseases that are more common in certain breeds of dogs, which supports the notion that purebred dogs are more susceptible to disease. ... >>

Random news from the Archive Plastic skin feels the force of touch 22.10.2015 Stanford University engineers have created a plastic "skin" that detects the force of touch and generates an electrical signal that conveys this information to living brain cells. Seventeen specialists led by Professor Zhenan Bao, who has been working on this direction for ten years, have developed a material that mimics such properties of human skin as plasticity and healing ability. In addition, the invention includes a network of sensors that sense and send information about touch, temperature, and pain to the brain. Ultimately, this development should find application in prosthetic limbs. According to Ms. Bao, for the first time, it was possible to obtain such a material capable of detecting pressure and transmitting signals to the nervous system. The top layer creates a sensitive mechanism. At the same time, the sensitivity of the sensors is the same as that of human skin. That is, artificial skin is able to easily distinguish between a light touch of a finger and a handshake, for example. The underlying layer transports electrical signals and converts them into biochemical stimuli compatible with nerve cells. Artificial skin has billions of carbon nanotubes embedded in it. When pressure is applied to the plastic, the nanotubes "squeeze" closer together, allowing them to conduct electricity (the more pressure, the higher the current value). Flexible electronic components were developed by engineers from PARC. When developing the interface for the interaction of electronics with neurons, the technique of Karl Deisseroth, a well-known specialist in the field of optogenetics, was used.

Other interesting news:

▪ New Benchmark MOSFETs International Rectifier

▪ HTC One X+

▪ Catch the neutrino

▪ The risk of the American diet

▪ Transparent and silent robotic eel developed

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the website Electrotechnical materials. Article selection

▪ article Days of bygone anecdotes. Popular expression

▪ article Which animals have the largest total mass? Detailed answer

▪ article Machinist of a machine for driving and dipping piles. Standard instruction on labor protection

▪ article Contactless presence detector. Encyclopedia of radio electronics and electrical engineering

▪ Article Transistors. Code marking. Housing KT-26 (TO-92). Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages ​​of this page

Arabic	Hebrew	Polish
Bengali	Hindi	Portuguese
Chinese	Italian	Spanish
English	Japanese	Turkish
French	Korean	Ukrainian
German	Malay	Vietnamese

Home page | Library | Articles | Website map | Site Reviews

www.diagram.com.ua
2000-2024