UMZCH with adjustable output impedance. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

 Comments on the article

I am more and more convinced from my own amateur radio experience of the correctness of the provisions of article [1] on the dependence of the nature of the sound ("tube" or "transistor") on the operation of the UMZCH - loudspeaker system. The sound features of the speakers are not at all connected with the element base of the UMZCH and not with the presence or absence of an OOS, but to a large extent with its (UMZCH) output impedance, so I will share the results of my research.

After many experiments with transistor UMZCH (with various feedback [2] and without it at all), each time comparing the sound with the "standard" in the form of UMZCH tube radio "VEF-Radio" (single-cycle, pentode, with ultra-linear switching on of the lamp and a shallow common OOS), I came to the following conclusion. The generally accepted opinion that a low (preferably "zero" or even negative) output impedance of the UMZCH is favorable for reproducing low frequencies is not always true. If the output impedance of the UMZCH is approximately 20 ... 50% of the loudspeaker impedance (at the same time, there is no need to talk about deep damping), the soft sound of the double bass in jazz is more pleasant (of course, purely subjective). On the other hand, when listening to rock, modern electronic music with "assertive" bass, stronger speaker damping is required. A funny situation arises when it is desirable for each style of music to have its own UMZCH: for jazz, it is better to have a tube one and without a common OOS, for rock, a transistor with a deep voltage OOS (OOSN), which provides a low output impedance.

In this regard, I propose a device tested on a mock-up that "reconciles" this contradiction. The adjustable output impedance of the UMZCH allows you to smoothly turn the transistor amplifier into a "tube triode" or "pentode" amplifier without OOS, but without the third harmonic coefficient characteristically high for pentodes. This transformation of the UMZCH is possible with the help of variable resistors, which translate the OOS by voltage into the OOS by current through the load with a constant transfer coefficient of the device.

The UMZCH layout diagram is shown in fig. one.

The output resistance is adjusted as follows: in the lower extreme position of the engines of the dual variable resistors R4.1, R4.2, only OOSN occurs, the depth of which is determined by the ratio of resistors R3, R1 and the initial gain DA1 without NF. In the other extreme position of the sliders, variable resistors create only current OOS (OOS). In this case, depending on the load resistance (4 or 8 ohms), switch SA1 is closed or opened, which changes the resistance of the current sensor (R5, R6).

When moving the sliders of variable resistors, the overall gain of the device, relatively speaking, does not change. Conditionally, since the voltage transfer coefficient in the OOST mode is related to the frequency dependence of the loudspeaker impedance. Only the output resistance of the UMZCH changes from close to zero to several tens of kilo-ohms. Its desired value is set by ear according to the subjectively more pleasant sound of a musical work of the corresponding style.

Dual variable resistors R4 must be of group A (with a linear control characteristic). Instead of variable resistors, you can use any switch for two directions and 11 positions with a chain of 680 Ohm resistors (2x10 pcs.). True, in this case, in order to prevent sharp clicks and possible failure of the loudspeaker when the output impedance of the UMZCH changes, it is necessary to turn off the UMZCH power each time, which reduces efficiency when selecting the desired sound character.

If desired, you can change the UMZCH gain, but the following relationships must be met:

Ku \u3d R1 / R2 \u1d Rn / RdxRXNUMX / RXNUMX;

Rн/Rд=10; R2 = 5R4.

Here, Rн is understood as the passport value of the resistance of the loudspeaker at a frequency of 1000 Hz; Rd is the resistance of the current sensor.

And one more thing: variable depth feedback is a subtle and capricious "thing", and the combination of different types of OOS is even more so. Therefore, before using any other UMZCH in the circuit under consideration (analogues of MDA2020 are TDA2020, K174UN11 [3]), it is desirable to achieve stability (no self-excitation) by selecting correction circuits when the amplifier is turned on according to the repeater circuit (Fig. 2). Here A1 is UMZCH without a chain of general environmental protection.

The main disadvantage of the circuit in Fig. 1 is the presence of mechanical contacts in the feedback circuit (resistors R4 or a switch replacing them). In addition, the resistance of the current sensor (Rd) limits the maximum value of the damping factor even with a "zero" output resistance of the UMZCH.

As an UMZCH, it is advisable to choose a microcircuit with a low intrinsic harmonic coefficient without negative feedback.

The power and type of resistors R5, R6, as well as their resistance can be changed within certain limits depending on the output power of the UMZCH, the given gain and the input resistance of the UMZCH.

Such an amplifier can be successfully used to study the properties of electrodynamic heads in the entire operating frequency range at various impedances. For example, some heads are not very sensitive to the output impedance of the amplifier, others are more sensitive (high intermodulation distortion at mid frequencies).

The proposed structure of the feedback controller requires dual variable resistors. It is possible, however, to simplify this circuit by "transforming" the UMZCH feedback with a single variable resistor, as shown in Fig. 3.

The result is the same: in the left position of the engine, the output resistance of the UMZCH is minimal, in the right position it is maximum. In relation to the loudspeaker, the minimum output impedance of the signal source is limited by the resistance of the current sensor included in the load circuit and is about a tenth of the load resistance. For damping most loudspeakers, this ratio is sufficient.

To increase the stability of the UMZCH, it is not at all necessary to correct its frequency response for a "single" gain. We recommend using the recommendations contained in the article A Syritso "Features of UMZCH with high output impedance", published in Radio, 2002, No. 2, pp. 16, 17.

Literature

1. Ageev S. Should the UMZCH have a low output impedance? - Radio, 1997, No. 4, p. 14 - 16.
2. Maslov A. Combined feedback in UMZCH. - Radio, 2001, No. 6, p. 16, 17.
3. Microcircuits for household radio equipment. Directory. - M.: Radio and communication, 1989, p. 114.

Author: A.Maslov, Zhukovsky, Moscow Region

See other articles Section Transistor power amplifiers.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Machine for thinning flowers in gardens 02.05.2024

In modern agriculture, technological progress is developing aimed at increasing the efficiency of plant care processes. The innovative Florix flower thinning machine was presented in Italy, designed to optimize the harvesting stage. This tool is equipped with mobile arms, allowing it to be easily adapted to the needs of the garden. The operator can adjust the speed of the thin wires by controlling them from the tractor cab using a joystick. This approach significantly increases the efficiency of the flower thinning process, providing the possibility of individual adjustment to the specific conditions of the garden, as well as the variety and type of fruit grown in it. After testing the Florix machine for two years on various types of fruit, the results were very encouraging. Farmers such as Filiberto Montanari, who has used a Florix machine for several years, have reported a significant reduction in the time and labor required to thin flowers. ... >>

Advanced Infrared Microscope 02.05.2024

Microscopes play an important role in scientific research, allowing scientists to delve into structures and processes invisible to the eye. However, various microscopy methods have their limitations, and among them was the limitation of resolution when using the infrared range. But the latest achievements of Japanese researchers from the University of Tokyo open up new prospects for studying the microworld. Scientists from the University of Tokyo have unveiled a new microscope that will revolutionize the capabilities of infrared microscopy. This advanced instrument allows you to see the internal structures of living bacteria with amazing clarity on the nanometer scale. Typically, mid-infrared microscopes are limited by low resolution, but the latest development from Japanese researchers overcomes these limitations. According to scientists, the developed microscope allows creating images with a resolution of up to 120 nanometers, which is 30 times higher than the resolution of traditional microscopes. ... >>

Air trap for insects 01.05.2024

Agriculture is one of the key sectors of the economy, and pest control is an integral part of this process. A team of scientists from the Indian Council of Agricultural Research-Central Potato Research Institute (ICAR-CPRI), Shimla, has come up with an innovative solution to this problem - a wind-powered insect air trap. This device addresses the shortcomings of traditional pest control methods by providing real-time insect population data. The trap is powered entirely by wind energy, making it an environmentally friendly solution that requires no power. Its unique design allows monitoring of both harmful and beneficial insects, providing a complete overview of the population in any agricultural area. “By assessing target pests at the right time, we can take necessary measures to control both pests and diseases,” says Kapil ... >>

Random news from the Archive Graphene jacket 23.08.2018 Startup Vollebak offers clothes that will obviously be difficult to buy in the nearest mall. At least for now. The company has just released the world's first graphene jacket. Graphene is a transparent layer of carbon one atom thick, first obtained in 2004. It is the strongest of all materials discovered to date, it is highly flexible and highly conductive. And now you can still wear it on your body. Vollebak has constructed a reversible jacket from stretchable nylon coated on one side with a layer of graphene. The benefits of such a graphene coating depend on how a person wears a jacket. If you leave it somewhere with a heat source, and then put it inside with graphene, then the clothes will begin to warm the body. It can also redistribute temperature from warmer parts of the body to less warm parts. The graphene material also produces less moisture on contact with the skin compared to other materials, so your clothes won't stick if you sweat. Bacteria cannot grow on graphene, the material breathes, but at the same time it does not let water through, so you won’t get wet in the rain, but the sweat will still evaporate. This is what Vollebak's press release says. This jacket costs $695. Since graphene is a new material for clothing, the company hopes that customers will experiment with the jacket, discovering properties that no one else knows about in the process.

Other interesting news:

▪ Wireless control of any battery powered devices

▪ Mosquitoes weaned from drinking blood

▪ Come in, you'll be sniffed here

▪ Legumes force bacteria into symbiosis

▪ How to drink tea properly

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the site Car. Article selection

▪ article Joseph the Beautiful (Chasest Joseph). Popular expression

▪ article In which major city do seawater fill most of the toilets? Detailed answer

▪ article Electrician in the house. Directory

▪ article Putty for joining metal with glass. Simple recipes and tips

▪ article Cards with numbers. Focus Secret

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