Increasing the power of the amplifier on the TDA7294 chip. Low frequency amplifier (LF) on the TDA7250 microcircuit. Circuits of homemade amplifiers on microcircuits.

– The neighbor stopped knocking on the radiator. I turned the music up so I couldn't hear him.
(From audiophile folklore).

The epigraph is ironic, but the audiophile is not necessarily “sick in the head” with the face of Josh Ernest at a briefing on relations with the Russian Federation, who is “thrilled” because his neighbors are “happy.” Someone wants to listen to serious music at home as in the hall. For this purpose, the quality of the equipment is needed, which among lovers of decibel volume as such simply does not fit where sane people have a mind, but for the latter it goes beyond reason from the prices of suitable amplifiers (UMZCH, audio frequency power amplifier). And someone along the way has a desire to join useful and exciting areas of activity - sound reproduction technology and electronics in general. Which in the age of digital technology are inextricably linked and can become a highly profitable and prestigious profession. The optimal first step in this matter in all respects is to make an amplifier with your own hands: It is UMZCH that allows, with initial training on the basis of school physics on the same table, to go from the simplest designs for half an evening (which, nevertheless, “sing” well) to the most complex units, through which a good rock band will play with pleasure. The purpose of this publication is highlight the first stages of this path for beginners and, perhaps, convey something new to those with experience.

Protozoa

So, first, let's try to make an audio amplifier that just works. In order to thoroughly delve into sound engineering, you will have to gradually master quite a lot of theoretical material and not forget to enrich your knowledge base as you progress. But any “cleverness” is easier to assimilate when you see and feel how it works “in hardware.” In this article further, too, we will not do without theory - about what you need to know at first and what can be explained without formulas and graphs. In the meantime, it will be enough to know how to use a multitester.

Note: If you haven’t soldered electronics yet, keep in mind that its components cannot be overheated! Soldering iron - up to 40 W (preferably 25 W), maximum allowable soldering time without interruption - 10 s. The soldered pin for the heat sink is held 0.5-3 cm from the soldering point on the side of the device body with medical tweezers. Acid and other active fluxes cannot be used! Solder - POS-61.

On the left in Fig.- the simplest UMZCH, “which just works.” It can be assembled using both germanium and silicon transistors.

On this baby it is convenient to learn the basics of setting up an UMZCH with direct connections between cascades that give the clearest sound:

• Before turning on the power for the first time, turn off the load (speaker);
• Instead of R1, we solder a chain of a constant resistor of 33 kOhm and a variable resistor (potentiometer) of 270 kOhm, i.e. first note four times less, and the second approx. twice the denomination compared to the original according to the scheme;
• We supply power and, by rotating the potentiometer, at the point marked with a cross, we set the indicated collector current VT1;
• We remove the power, unsolder the temporary resistors and measure their total resistance;
• As R1 we set a resistor with a value from the standard series closest to the measured one;
• We replace R3 with a constant 470 Ohm chain + 3.3 kOhm potentiometer;
• Same as according to paragraphs. 3-5, V. and we set the voltage equal to half the supply voltage.

Point a, from where the signal is removed to the load, is the so-called. midpoint of the amplifier. In UMZCH with unipolar power supply, it is set to half its value, and in UMZCH with bipolar power supply - zero relative to the common wire. This is called adjusting the amplifier balance. In unipolar UMZCHs with capacitive decoupling of the load, it is not necessary to turn it off during setup, but it is better to get used to doing this reflexively: an unbalanced 2-polar amplifier with a connected load can burn out its own powerful and expensive output transistors, or even a “new, good” and very expensive powerful speaker.

Note: components that require selection when setting up the device in the layout are indicated on the diagrams either with an asterisk (*) or an apostrophe (‘).

In the center of the same fig.- a simple UMZCH on transistors, already developing power up to 4-6 W at a load of 4 ohms. Although it works like the previous one, in the so-called. class AB1, not intended for Hi-Fi sound, but if you replace a pair of these class D amplifiers (see below) in cheap Chinese computer speakers, their sound improves noticeably. Here we learn another trick: powerful output transistors need to be placed on radiators. Components that require additional cooling are outlined in dotted lines in the diagrams; however, not always; sometimes - indicating the required dissipative area of ​​the heat sink. Setting up this UMZCH is balancing using R2.

On the right in Fig.- not yet a 350 W monster (as was shown at the beginning of the article), but already quite a solid beast: a simple amplifier with 100 W transistors. You can listen to music through it, but not Hi-Fi, operating class is AB2. However, it is quite suitable for scoring a picnic area or an outdoor meeting, a school assembly hall or a small shopping hall. An amateur rock band, having such a UMZCH per instrument, can perform successfully.

There are 2 more tricks in this UMZCH: firstly, in very powerful amplifiers, the drive stage of the powerful output also needs to be cooled, so VT3 is placed on a radiator of 100 kW or more. see. For output VT4 and VT5 radiators from 400 sq.m. are needed. see. Secondly, UMZCHs with bipolar power supply are not balanced at all without load. First one or the other output transistor goes into cutoff, and the associated one goes into saturation. Then, at full supply voltage, current surges during balancing can damage the output transistors. Therefore, for balancing (R6, guessed it?), the amplifier is powered from +/–24 V, and instead of a load, a wirewound resistor of 100...200 Ohms is switched on. By the way, the squiggles in some resistors in the diagram are Roman numerals, indicating their required heat dissipation power.

Note: A power source for this UMZCH needs a power of 600 W or more. Anti-aliasing filter capacitors - from 6800 µF at 160 V. In parallel electrolytic capacitors 0.01 µF ceramic PIs are switched on to prevent self-excitation at ultrasonic frequencies, which can instantly burn out the output transistors.

On the field

On the trail. rice. - another option for a fairly powerful UMZCH (30 W, and with a supply voltage of 35 V - 60 W) on powerful field-effect transistors:

The sound from it already meets the requirements for entry-level Hi-Fi (if, of course, the UMZCH works on the corresponding acoustic systems, speakers). Powerful field workers do not require high power for build-up, therefore there is no pre-power cascade. Even more powerful field-effect transistors do not burn out the speakers in the event of any malfunction - they themselves burn out faster. Also unpleasant, but still cheaper than replacing an expensive loudspeaker bass head (GB). This UMZCH does not require balancing or adjustment in general. As a design for beginners, it has only one drawback: powerful field-effect transistors are much more expensive than bipolar transistors for an amplifier with the same parameters. Requirements for individual entrepreneurs are similar to previous ones. case, but its power is needed from 450 W. Radiators – from 200 sq. cm.

Note: there is no need to build powerful UMZCHs on field-effect transistors for switching power supplies, for example. computer When trying to “drive” them into the active mode required for UMZCH, they either simply burn out, or the sound is weak and “no quality at all.” The same applies to powerful high-voltage bipolar transistors, for example. from line scan of old TVs.

Straight up

If you have already taken the first steps, then it is quite natural to want to build Hi-Fi class UMZCH, without going too deep into the theoretical jungle. To do this, you will have to expand your instrumentation - you need an oscilloscope, an audio frequency generator (AFG) and an AC millivoltmeter with the ability to measure the DC component. It is better to take as a prototype for repetition the E. Gumeli UMZCH, described in detail in Radio No. 1, 1989. To build it, you will need a few inexpensive available components, but the quality meets very high requirements: power up to 60 W, band 20-20,000 Hz, frequency response unevenness 2 dB, nonlinear distortion factor (THD) 0.01%, self-noise level –86 dB. However, setting up the Gumeli amplifier is quite difficult; if you can handle it, you can take on any other. However, some of the currently known circumstances greatly simplify the establishment of this UMZCH, see below. Bearing in mind this and the fact that not everyone is able to get into the Radio archives, it would be appropriate to repeat the main points.

Schemes of a simple high-quality UMZCH

The Gumeli UMZCH circuits and specifications for them are shown in the illustration. Radiators of output transistors – from 250 sq. see for UMZCH in Fig. 1 and from 150 sq. see for option according to fig. 3 (original numbering). Transistors of the pre-output stage (KT814/KT815) are installed on radiators bent from 75x35 mm aluminum plates with a thickness of 3 mm. There is no need to replace KT814/KT815 with KT626/KT961; the sound does not noticeably improve, but setup becomes seriously difficult.

This UMZCH is very critical to power supply, installation topology and general, so it needs to be installed in a structurally complete form and only with a standard power source. When trying to power it from a stabilized power supply, the output transistors burn out immediately. Therefore, in Fig. Drawings of original printed circuit boards and setup instructions are provided. We can add to them that, firstly, if “excitement” is noticeable when you first turn it on, they fight it by changing the inductance L1. Secondly, the leads of parts installed on boards should be no longer than 10 mm. Thirdly, it is extremely undesirable to change the installation topology, but if it is really necessary, there must be a frame shield on the side of the conductors (ground loop, highlighted in color in the figure), and the power supply paths must pass outside it.

Note: breaks in the tracks to which the bases of powerful transistors are connected - technological, for adjustment, after which they are sealed with drops of solder.

Setting up this UMZCH is greatly simplified, and the risk of encountering “excitement” during use is reduced to zero if:

• Minimize interconnect installation by placing the boards on radiators of powerful transistors.
• Completely abandon the connectors inside, performing all installation only by soldering. Then there will be no need for R12, R13 in a powerful version or R10 R11 in a less powerful version (they are dotted in the diagrams).
• Use oxygen-free copper audio wires of minimum length for internal installation.

If these conditions are met, there are no problems with excitation, and setting up the UMZCH comes down to the routine procedure described in Fig.

Wires for sound

Audio wires are not an idle invention. The need for their use at present is undeniable. In copper with an admixture of oxygen, a thin oxide film is formed on the faces of metal crystallites. Metal oxides are semiconductors and if the current in the wire is weak without a constant component, its shape is distorted. In theory, distortions on myriads of crystallites should compensate each other, but very little (apparently due to quantum uncertainties) remains. Sufficient to be noticed by discerning listeners against the background of the purest sound of modern UMZCH.

Manufacturers and traders shamelessly substitute ordinary electrical copper instead of oxygen-free copper - it is impossible to distinguish one from the other by eye. However, there is an area of ​​application where counterfeiting is not clear: twisted pair cable for computer networks. If you put a grid with long segments on the left, it will either not start at all or will constantly glitch. Momentum dispersion, you know.

The author, when there was just talk about audio wires, realized that, in principle, this was not idle chatter, especially since oxygen-free wires by that time had long been used in special-purpose equipment, with which he was well acquainted by his line of work. Then I took and replaced the standard cord of my TDS-7 headphones with a homemade one made from “vitukha” with flexible multi-core wires. The sound, aurally, has steadily improved for end-to-end analogue tracks, i.e. on the way from the studio microphone to the disc, never digitized. Vinyl recordings made using DMM (Direct Metal Mastering) technology sounded especially bright. After this, the interconnect installation of all home audio was converted to “vitushka”. Then completely random people, indifferent to the music and not notified in advance, began to notice the improvement in sound.

How to make interconnect wires from twisted pair, see next. video.

Video: do-it-yourself twisted pair interconnect wires

Unfortunately, the flexible “vitha” soon disappeared from sale - it did not hold well in the crimped connectors. However, for the information of readers, flexible “military” wire MGTF and MGTFE (shielded) is made only from oxygen-free copper. Fake is impossible, because On ordinary copper, tape fluoroplastic insulation spreads quite quickly. MGTF is now widely available and costs much less than branded audio cables with a guarantee. It has one drawback: it cannot be done in color, but this can be corrected with tags. There are also oxygen-free winding wires, see below.

Theoretical Interlude

As we can see, already in the early stages of mastering audio technology, we had to deal with the concept of Hi-Fi (High Fidelity), high fidelity sound reproduction. Hi-Fi comes in different levels, which are ranked according to the following. main parameters:

1. Reproducible frequency band.
2. Dynamic range - the ratio in decibels (dB) of the maximum (peak) output power to the noise level.
3. Self-noise level in dB.
4. Nonlinear distortion factor (THD) at rated (long-term) output power. The SOI at peak power is assumed to be 1% or 2% depending on the measurement technique.
5. Unevenness of the amplitude-frequency response (AFC) in the reproducible frequency band. For speakers - separately at low (LF, 20-300 Hz), medium (MF, 300-5000 Hz) and high (HF, 5000-20,000 Hz) sound frequencies.

Note: the ratio of absolute levels of any values ​​of I in (dB) is defined as P(dB) = 20log(I1/I2). If I1

You need to know all the subtleties and nuances of Hi-Fi when designing and building speakers, and as for a homemade Hi-Fi UMZCH for the home, before moving on to these, you need to clearly understand the requirements for their power required to sound a given room, dynamic range (dynamics), noise level and SOI. It is not very difficult to achieve a frequency band of 20-20,000 Hz from the UMZCH with a roll off at the edges of 3 dB and an uneven frequency response in the midrange of 2 dB on a modern element base.

Volume

The power of the UMZCH is not an end in itself; it must provide the optimal volume of sound reproduction in a given room. It can be determined by curves of equal loudness, see fig. There are no natural noises in residential areas quieter than 20 dB; 20 dB is the wilderness in complete calm. A volume level of 20 dB relative to the threshold of audibility is the threshold of intelligibility - a whisper can still be heard, but music is perceived only as the fact of its presence. An experienced musician can tell which instrument is being played, but not what exactly.

40 dB - the normal noise of a well-insulated city apartment in a quiet area or a country house - represents the intelligibility threshold. Music from the threshold of intelligibility to the threshold of intelligibility can be listened to with deep frequency response correction, primarily in the bass. To do this, the MUTE function (mute, mutation, not mutation!) is introduced into modern UMZCHs, including, respectively. correction circuits in UMZCH.

90 dB is the volume level of a symphony orchestra in a very good concert hall. 110 dB can be produced by an extended orchestra in a hall with unique acoustics, of which there are no more than 10 in the world, this is the threshold of perception: louder sounds are still perceived as distinguishable in meaning with an effort of will, but already annoying noise. The volume zone in residential premises of 20-110 dB constitutes the zone of complete audibility, and 40-90 dB is the zone of best audibility, in which untrained and inexperienced listeners fully perceive the meaning of the sound. If, of course, he is in it.

Power

Calculating the power of equipment at a given volume in the listening area is perhaps the main and most difficult task of electroacoustics. For yourself, in conditions it is better to go from acoustic systems (AS): calculate their power using a simplified method, and take the nominal (long-term) power of the UMZCH equal to the peak (musical) speaker. In this case, the UMZCH will not noticeably add its distortions to those of the speakers; they are already the main source of nonlinearity in the audio path. But the UMZCH should not be made too powerful: in this case, the level of its own noise may be higher than the threshold of audibility, because It is calculated based on the voltage level of the output signal at maximum power. If we consider it very simply, then for a room in an ordinary apartment or house and speakers with normal characteristic sensitivity (sound output) we can take the trace. UMZCH optimal power values:

• Up to 8 sq. m – 15-20 W.
• 8-12 sq. m – 20-30 W.
• 12-26 sq. m – 30-50 W.
• 26-50 sq. m – 50-60 W.
• 50-70 sq. m – 60-100 W.
• 70-100 sq. m – 100-150 W.
• 100-120 sq. m – 150-200 W.
• More than 120 sq. m – determined by calculation based on on-site acoustic measurements.

Dynamics

The dynamic range of the UMZCH is determined by curves of equal loudness and threshold values ​​for different degrees of perception:

1. Symphonic music and jazz with symphonic accompaniment - 90 dB (110 dB - 20 dB) ideal, 70 dB (90 dB - 20 dB) acceptable. No expert can distinguish a sound with a dynamics of 80-85 dB in a city apartment from ideal.
2. Other serious music genres – 75 dB excellent, 80 dB “through the roof”.
3. Pop music of any kind and movie soundtracks - 66 dB is enough for the eyes, because... These opuses are already compressed during recording to levels of up to 66 dB and even up to 40 dB, so that you can listen to them on anything.

The dynamic range of the UMZCH, correctly selected for a given room, is considered equal to its own noise level, taken with the + sign, this is the so-called. signal-to-noise ratio.

SOI

Nonlinear distortions (ND) of UMZCH are components of the output signal spectrum that were not present in the input signal. Theoretically, it is best to “push” the NI under the level of its own noise, but technically this is very difficult to implement. In practice, they take into account the so-called. masking effect: at volume levels below approx. At 30 dB, the range of frequencies perceived by the human ear narrows, as does the ability to distinguish sounds by frequency. Musicians hear notes, but find it difficult to assess the timbre of the sound. In people without a hearing for music, the masking effect is observed already at 45-40 dB of volume. Therefore, an UMZCH with a THD of 0.1% (–60 dB from a volume level of 110 dB) will be assessed as Hi-Fi by the average listener, and with a THD of 0.01% (–80 dB) can be considered not distorting the sound.

Lamps

The last statement will probably cause rejection, even fury, among adherents of tube circuitry: they say, real sound is produced only by tubes, and not just some, but certain types of octal ones. Calm down, gentlemen - the special tube sound is not a fiction. The reason is the fundamentally different distortion spectra of electronic tubes and transistors. Which, in turn, are due to the fact that in the lamp the flow of electrons moves in a vacuum and quantum effects do not appear in it. A transistor is a quantum device, where minority charge carriers (electrons and holes) move in the crystal, which is completely impossible without quantum effects. Therefore, the spectrum of tube distortions is short and clean: only harmonics up to the 3rd - 4th are clearly visible in it, and there are very few combinational components (sums and differences in the frequencies of the input signal and their harmonics). Therefore, in the days of vacuum circuitry, SOI was called harmonic distortion (CH). In transistors, the spectrum of distortions (if they are measurable, the reservation is random, see below) can be traced up to the 15th and higher components, and there are more than enough combination frequencies in it.

At the beginning of solid-state electronics, designers of transistor UMZCHs used the usual “tube” SOI of 1-2% for them; Sound with a tube distortion spectrum of this magnitude is perceived by ordinary listeners as pure. By the way, the very concept of Hi-Fi did not yet exist. It turned out that they sound dull and dull. In the process of developing transistor technology, an understanding of what Hi-Fi is and what is needed for it was developed.

Currently, the growing pains of transistor technology have been successfully overcome and side frequencies at the output of a good UMZCH are difficult to detect using special measurement methods. And lamp circuitry can be considered to have become an art. Its basis can be anything, why can’t electronics go there? An analogy with photography would be appropriate here. No one can deny that a modern digital SLR camera produces an image that is immeasurably clearer, more detailed, and deeper in the range of brightness and color than a plywood box with an accordion. But someone, with the coolest Nikon, “clicks pictures” like “this is my fat cat, he got drunk like a bastard and is sleeping with his paws outstretched,” and someone, using Smena-8M, uses Svemov’s b/w film to take a picture in front of which there is a crowd of people at a prestigious exhibition.

Note: and calm down again - not everything is so bad. Today, low-power lamp UMZCHs have at least one application left, and not the least important, for which they are technically necessary.

Experimental stand

Many audio lovers, having barely learned to solder, immediately “go into tubes.” This in no way deserves censure, on the contrary. Interest in the origins is always justified and useful, and electronics has become so with tubes. The first computers were tube-based, and the on-board electronic equipment of the first spacecraft was also tube-based: there were already transistors then, but they could not withstand extraterrestrial radiation. By the way, at that time lamp microcircuits were also created under the strictest secrecy! On microlamps with a cold cathode. The only known mention of them in open sources is in the rare book by Mitrofanov and Pickersgil “Modern receiving and amplifying tubes”.

But enough of the lyrics, let's get to the point. For those who like to tinker with the lamps in Fig. – diagram of a bench lamp UMZCH, intended specifically for experiments: SA1 switches the operating mode of the output lamp, and SA2 switches the supply voltage. The circuit is well known in the Russian Federation, a minor modification affected only the output transformer: now you can not only “drive” the native 6P7S in different modes, but also select the screen grid switching factor for other lamps in ultra-linear mode; for the vast majority of output pentodes and beam tetrodes it is either 0.22-0.25 or 0.42-0.45. For the manufacture of the output transformer, see below.

Guitarists and rockers

This is the very case when you can’t do without lamps. As you know, the electric guitar became a full-fledged solo instrument after the pre-amplified signal from the pickup began to be passed through a special attachment - a fuser - which deliberately distorted its spectrum. Without this, the sound of the string was too sharp and short, because the electromagnetic pickup reacts only to the modes of its mechanical vibrations in the plane of the instrument soundboard.

An unpleasant circumstance soon emerged: the sound of an electric guitar with a fuser acquires full strength and brightness only at high volumes. This is especially true for guitars with a humbucker-type pickup, which gives the most “angry” sound. But what about a beginner who is forced to rehearse at home? You can’t go to the hall to perform without knowing exactly how the instrument will sound there. And rock fans just want to listen to their favorite things in full juice, and rockers are generally decent and non-conflict people. At least those who are interested in rock music, and not shocking surroundings.

So, it turned out that the fatal sound appears at volume levels acceptable for residential premises, if the UMZCH is tube-based. The reason is the specific interaction of the signal spectrum from the fuser with the pure and short spectrum of tube harmonics. Here again an analogy is appropriate: a b/w photo can be much more expressive than a color one, because leaves only the outline and light for viewing.

Those who need a tube amplifier not for experiments, but due to technical necessity, do not have time to master the intricacies of tube electronics for a long time, they are passionate about something else. In this case, it is better to make the UMZCH transformerless. More precisely, with a single-ended matching output transformer that operates without constant magnetization. This approach greatly simplifies and speeds up the production of the most complex and critical component of a lamp UMZCH.

“Transformerless” tube output stage of the UMZCH and pre-amplifiers for it

On the right in Fig. a diagram of a transformerless output stage of a tube UMZCH is given, and on the left are pre-amplifier options for it. At the top - with a tone control according to the classic Baxandal scheme, which provides fairly deep adjustment, but introduces slight phase distortion into the signal, which can be significant when operating an UMZCH on a 2-way speaker. Below is a preamplifier with simpler tone control that does not distort the signal.

But let's get back to the end. In a number of foreign sources, this scheme is considered a revelation, but an identical one, with the exception of the capacitance of the electrolytic capacitors, is found in the Soviet “Radio Amateur Handbook” of 1966. A thick book of 1060 pages. There was no Internet and disk-based databases back then.

In the same place, on the right in the figure, the disadvantages of this scheme are briefly but clearly described. An improved one, from the same source, is given on the trail. rice. on right. In it, the screen grid L2 is powered from the midpoint of the anode rectifier (the anode winding of the power transformer is symmetrical), and the screen grid L1 is powered through the load. If, instead of high-impedance speakers, you turn on a matching transformer with regular speakers, as in the previous one. circuit, the output power is approx. 12 W, because the active resistance of the primary winding of the transformer is much less than 800 Ohms. SOI of this final stage with transformer output - approx. 0.5%

How to make a transformer?

The main enemies of the quality of a powerful signal low-frequency (sound) transformer are the magnetic leakage field, the lines of force of which are closed, bypassing the magnetic circuit (core), eddy currents in the magnetic circuit (Foucault currents) and, to a lesser extent, magnetostriction in the core. Because of this phenomenon, a carelessly assembled transformer “sings,” hums, or beeps. Foucault currents are combated by reducing the thickness of the magnetic circuit plates and additionally insulating them with varnish during assembly. For output transformers, the optimal plate thickness is 0.15 mm, the maximum allowable is 0.25 mm. You should not take thinner plates for the output transformer: the fill factor of the core (the central rod of the magnetic circuit) with steel will fall, the cross-section of the magnetic circuit will have to be increased to obtain a given power, which will only increase distortions and losses in it.

In the core of an audio transformer operating with constant bias (for example, the anode current of a single-ended output stage) there must be a small (determined by calculation) non-magnetic gap. The presence of a non-magnetic gap, on the one hand, reduces signal distortion from constant magnetization; on the other hand, in a conventional magnetic circuit it increases the stray field and requires a core with a larger cross-section. Therefore, the non-magnetic gap must be calculated at the optimum and performed as accurately as possible.

For transformers operating with magnetization, the optimal type of core is made of Shp (cut) plates, pos. 1 in Fig. In them, a non-magnetic gap is formed during core cutting and is therefore stable; its value is indicated in the passport for the plates or measured with a set of probes. The stray field is minimal, because the side branches through which the magnetic flux is closed are solid. Transformer cores without bias are often assembled from Shp plates, because Shp plates are made from high-quality transformer steel. In this case, the core is assembled across the roof (the plates are laid with a cut in one direction or the other), and its cross-section is increased by 10% compared to the calculated one.

It is better to wind transformers without bias on USH cores (reduced height with widened windows), pos. 2. In them, a decrease in the stray field is achieved by reducing the length of the magnetic path. Since USh plates are more accessible than Shp, transformer cores with magnetization are often made from them. Then the core assembly is carried out cut to pieces: a package of W-plates is assembled, a strip of non-conducting non-magnetic material is placed with a thickness equal to the size of the non-magnetic gap, covered with a yoke from a package of jumpers and pulled together with a clip.

Note:“sound” signal magnetic circuits of the ShLM type are of little use for output transformers of high-quality tube amplifiers; they have a large stray field.

At pos. 3 shows a diagram of the core dimensions for calculating the transformer, at pos. 4 design of the winding frame, and at pos. 5 – patterns of its parts. As for the transformer for the “transformerless” output stage, it is better to make it on the ShLMm across the roof, because the bias is negligible (the bias current is equal to the screen grid current). The main task here is to make the windings as compact as possible in order to reduce the stray field; their active resistance will still be much less than 800 Ohms. The more free space left in the windows, the better the transformer turned out. Therefore, the windings are wound turn to turn (if there is no winding machine, this is a terrible task) from the thinnest possible wire; the laying coefficient of the anode winding for the mechanical calculation of the transformer is taken 0.6. The winding wire is PETV or PEMM, they have an oxygen-free core. There is no need to take PETV-2 or PEMM-2; due to double varnishing, they have an increased outer diameter and a larger scattering field. The primary winding is wound first, because it is its scattering field that most affects the sound.

You need to look for iron for this transformer with holes in the corners of the plates and clamping brackets (see figure on the right), because “for complete happiness,” the magnetic circuit is assembled as follows. order (of course, the windings with leads and external insulation should already be on the frame):

1. Prepare acrylic varnish diluted in half or, in the old fashioned way, shellac;
2. Plates with jumpers are quickly coated with varnish on one side and placed into the frame as quickly as possible, without pressing too hard. The first plate is placed with the varnished side inward, the next one with the unvarnished side to the first varnished, etc.;
3. When the frame window is filled, staples are applied and bolted tightly;
4. After 1-3 minutes, when the squeezing of varnish from the gaps apparently stops, add plates again until the window is filled;
5. Repeat paragraphs. 2-4 until the window is tightly packed with steel;
6. The core is pulled tightly again and dried on a battery, etc. 3-5 days.

The core assembled using this technology has very good plate insulation and steel filling. Magnetostriction losses are not detected at all. But keep in mind that this technique is not applicable for permalloy cores, because Under strong mechanical influences, the magnetic properties of permalloy irreversibly deteriorate!

On microcircuits

UMZCHs on integrated circuits (ICs) are most often made by those who are satisfied with the sound quality up to average Hi-Fi, but are more attracted by the low cost, speed, ease of assembly and the complete absence of any setup procedures that require special knowledge. Simply, an amplifier on microcircuits is the best option for dummies. The classic of the genre here is the UMZCH on the TDA2004 IC, which has been on the series, God willing, for about 20 years now, on the left in Fig. Power – up to 12 W per channel, supply voltage – 3-18 V unipolar. Radiator area – from 200 sq. see for maximum power. The advantage is the ability to work with a very low-resistance, up to 1.6 Ohm, load, which allows you to extract full power when powered from a 12 V on-board network, and 7-8 W when supplied with a 6-volt power supply, for example, on a motorcycle. However, the output of the TDA2004 in class B is not complementary (on transistors of the same conductivity), so the sound is definitely not Hi-Fi: THD 1%, dynamics 45 dB.

The more modern TDA7261 does not produce better sound, but is more powerful, up to 25 W, because The upper limit of the supply voltage has been increased to 25 V. The lower limit, 4.5 V, still allows it to be powered from a 6 V on-board network, i.e. The TDA7261 can be started from almost all on-board networks, except for the aircraft 27 V. Using attached components (strapping, on the right in the figure), the TDA7261 can operate in mutation mode and with the St-By (Stand By) function, which switches the UMZCH to the minimum power consumption mode when there is no input signal for a certain time. Convenience costs money, so for a stereo you will need a pair of TDA7261 with radiators from 250 sq. see for each.

Note: If you are somehow attracted to amplifiers with the St-By function, keep in mind that you should not expect speakers wider than 66 dB from them.

“Super economical” in terms of power supply TDA7482, on the left in the figure, operating in the so-called. class D. Such UMZCHs are sometimes called digital amplifiers, which is incorrect. For real digitization, level samples are taken from an analog signal with a quantization frequency that is no less than twice the highest of the reproduced frequencies, the value of each sample is recorded in a noise-resistant code and stored for further use. UMZCH class D – pulse. In them, the analogue is directly converted into a sequence of high-frequency pulse-width modulated (PWM), which is fed to the speaker through a low-pass filter (LPF).

Class D sound has nothing in common with Hi-Fi: SOI of 2% and dynamics of 55 dB for class D UMZCH are considered very good indicators. And TDA7482 here, it must be said, is not the optimal choice: other companies specializing in class D produce UMZCH ICs that are cheaper and require less wiring, for example, D-UMZCH of the Paxx series, on the right in Fig.

Among the TDAs, the 4-channel TDA7385 should be noted, see the figure, on which you can assemble a good amplifier for speakers up to medium Hi-Fi, inclusive, with frequency division into 2 bands or for a system with a subwoofer. In both cases, low-pass and mid-high-frequency filtering is done at the input on a weak signal, which simplifies the design of the filters and allows deeper separation of the bands. And if the acoustics are subwoofer, then 2 channels of the TDA7385 can be allocated for the sub-ULF bridge circuit (see below), and the remaining 2 can be used for MF-HF.

UMZCH for subwoofer

A subwoofer, which can be translated as “subwoofer” or, literally, “boomer,” reproduces frequencies up to 150-200 Hz; in this range, human ears are practically unable to determine the direction of the sound source. In speakers with a subwoofer, the “sub-bass” speaker is placed in a separate acoustic design, this is the subwoofer as such. The subwoofer is placed, in principle, as conveniently as possible, and the stereo effect is provided by separate MF-HF channels with their own small-sized speakers, for the acoustic design of which there are no particularly serious requirements. Experts agree that it is better to listen to stereo with full channel separation, but subwoofer systems significantly save money or labor on the bass path and make it easier to place acoustics in small rooms, which is why they are popular among consumers with normal hearing and not particularly demanding ones.

The “leakage” of mid-high frequencies into the subwoofer, and from it into the air, greatly spoils the stereo, but if you sharply “cut off” the sub-bass, which, by the way, is very difficult and expensive, then a very unpleasant sound jumping effect will occur. Therefore, channels in subwoofer systems are filtered twice. At the input, electric filters highlight midrange-high frequencies with bass “tails” that do not overload the midrange-high frequency path, but provide a smooth transition to sub-bass. Bass with midrange “tails” are combined and fed to a separate UMZCH for the subwoofer. The midrange is additionally filtered so that the stereo does not deteriorate; in the subwoofer it is already acoustic: a sub-bass speaker is placed, for example, in the partition between the resonator chambers of the subwoofer, which do not let the midrange out, see on the right in Fig.

A UMZCH for a subwoofer is subject to a number of specific requirements, of which “dummies” consider the most important to be as high a power as possible. This is completely wrong, if, say, the calculation of the acoustics for the room gave a peak power W for one speaker, then the power of the subwoofer needs 0.8 (2W) or 1.6W. For example, if S-30 speakers are suitable for the room, then a subwoofer needs 1.6x30 = 48 W.

It is much more important to ensure the absence of phase and transient distortions: if they occur, there will definitely be a jump in the sound. As for SOI, it is permissible up to 1%. Intrinsic bass distortion of this level is not audible (see curves of equal volume), and the “tails” of their spectrum in the best audible midrange region will not come out of the subwoofer.

To avoid phase and transient distortions, the amplifier for the subwoofer is built according to the so-called. bridge circuit: the outputs of 2 identical UMZCHs are switched on back-to-back through a speaker; signals to the inputs are supplied in antiphase. The absence of phase and transient distortions in the bridge circuit is due to the complete electrical symmetry of the output signal paths. The identity of the amplifiers forming the arms of the bridge is ensured by the use of paired UMZCHs on ICs, made on the same chip; This is perhaps the only case when an amplifier on microcircuits is better than a discrete one.

Note: The power of a bridge UMZCH does not double, as some people think, it is determined by the supply voltage.

An example of a bridge UMZCH circuit for a subwoofer in a room up to 20 sq. m (without input filters) on the TDA2030 IC is given in Fig. left. Additional midrange filtering is carried out by circuits R5C3 and R’5C’3. Radiator area TDA2030 – from 400 sq. see. Bridged UMZCHs with an open output have an unpleasant feature: when the bridge is unbalanced, a constant component appears in the load current, which can damage the speaker, and the sub-bass protection circuits often fail, turning off the speaker when not needed. Therefore, it is better to protect the expensive oak bass head with non-polar batteries of electrolytic capacitors (highlighted in color, and the diagram of one battery is given in the inset.

A little about acoustics

The acoustic design of a subwoofer is a special topic, but since a drawing is given here, explanations are also needed. Case material – MDF 24 mm. The resonator tubes are made of fairly durable, non-ringing plastic, for example, polyethylene. The internal diameter of the pipes is 60 mm, the protrusions inward are 113 mm in the large chamber and 61 in the small chamber. For a specific loudspeaker head, the subwoofer will have to be reconfigured for the best bass and, at the same time, the least impact on the stereo effect. To tune the pipes, they take a pipe that is obviously longer and, by pushing it in and out, achieve the required sound. The protrusions of the pipes outward do not affect the sound; they are then cut off. The pipe settings are interdependent, so you will have to tinker.

Headphone Amplifier

A headphone amplifier is most often made by hand for two reasons. The first is for listening “on the go”, i.e. outside the home, when the power of the audio output of the player or smartphone is not enough to drive “buttons” or “burdocks”. The second is for high-end home headphones. A Hi-Fi UMZCH for an ordinary living room is needed with dynamics of up to 70-75 dB, but the dynamic range of the best modern stereo headphones exceeds 100 dB. An amplifier with such dynamics costs more than some cars, and its power will be from 200 W per channel, which is too much for an ordinary apartment: listening at a power that is much lower than the rated power spoils the sound, see above. Therefore, it makes sense to make a low-power, but with good dynamics, a separate amplifier specifically for headphones: the prices for household UMZCHs with such an additional weight are clearly absurdly inflated.

The circuit of the simplest headphone amplifier using transistors is given in pos. 1 pic. The sound is only for Chinese “buttons”, it works in class B. It is also no different in terms of efficiency - 13 mm lithium batteries last for 3-4 hours at full volume. At pos. 2 – TDA’s classic for on-the-go headphones. The sound, however, is quite decent, up to average Hi-Fi depending on the track digitization parameters. There are countless amateur improvements to the TDA7050 harness, but no one has yet achieved the transition of sound to the next level of class: the “microphone” itself does not allow it. TDA7057 (item 3) is simply more functional; you can connect the volume control to a regular, not dual, potentiometer.

The UMZCH for headphones on the TDA7350 (item 4) is designed to drive good individual acoustics. It is on this IC that headphone amplifiers in most middle and high-class household UMZCHs are assembled. The UMZCH for headphones on KA2206B (item 5) is already considered professional: its maximum power of 2.3 W is enough to drive such serious isodynamic “mugs” as TDS-7 and TDS-15.

The TDA7294 microcircuit is an integrated low-frequency amplifier, which is very popular among electronics engineers, both beginners and professionals. The network is full of different reviews about this chip. I decided to build an amplifier on it. I took the diagram from the datasheet.

This “micruha” feeds on a bipolar diet. For beginners, I will explain that it is not enough to have a “plus” and a “minus”.

You need a source with a positive terminal, a negative terminal and a common one. For example, relative to the common wire there should be plus 30 Volts, and in the other arm minus 30 Volts.

The amplifier on the TDA7294 is quite powerful. The maximum rated power is 100 W, but this is with nonlinear distortion of 10% and at maximum voltage (depending on load resistance). You can reliably shoot at 70W. Thus, on my birthday, I listened to two parallel-connected “Radio Engineering S30” speakers on one TDA 7294 channel. The entire evening and half of the night, the speakers sounded, sometimes putting them into overload. But the amplifier withstood it calmly, although it sometimes overheated (due to poor cooling).

Main characteristicsTDA7294

Supply voltage +-10V…+-40V

Peak output current up to 10A

Operating temperature of the crystal up to 150 degrees Celsius

Output power at d=0.5%:

At +-35V and R=8Ohm 70W

At +-31V and R=6Ohm 70W

At +-27V and R=4Ohm 70W

With d=10% and increased voltage (see), you can achieve 100W, but it will be a dirty 100W.

Amplifier circuit for TDA7294

The diagram shown is taken from the passport, all denominations are preserved. With proper installation and correctly selected element values, the amplifier starts the first time and does not require any settings.

Amplifier elements

The values ​​of all elements are indicated in the diagram. Resistor power 0.25 W.

The “microphone” itself should be installed on the radiator. If the radiator is in contact with other metal elements of the case, or the case itself is the radiator, then it is necessary to install a dielectric gasket between the radiator and the TDA7294 case.

The gasket can be silicone or mica.

The radiator area should be at least 500 sq.cm, the larger the better.

Initially, I assembled two channels of the amplifier, since the power supply allowed, but I did not choose the right housing and both channels simply did not fit into the housing in terms of dimensions. I tried to make the PCB smaller, but it didn't work.

After fully assembling the amplifier, I realized that the case was not enough to cool one channel of the amplifier. My case was a radiator. In short, I rolled out the lip into two channels.

When listening to my device at full volume, the crystal began to overheat, but I lowered the volume level and continued testing. As a result, I listened to music at a moderate volume until midnight, periodically causing the amplifier to overheat. The TDA7294 amplifier turned out to be very reliable.

ModeSTAND- BY TDA7294

If 3.5V or more is applied to the 9th leg, the microcircuit exits sleep mode; if less than 1.5V is applied, it will enter sleep mode.

In order to wake the device from sleep mode, you need to connect the 9th leg through a 22 kOhm resistor to the positive terminal (bipolar power source).

And if the 9th leg is connected through the same resistor to the GND terminal (bipolar power source), then the device will enter sleep mode.

The printed circuit board located under the article is routed so that leg 9 is connected via a 22 kOhm resistor to the positive terminal of the power supply. Consequently, when the power source is turned on, the amplifier immediately begins to operate in sleep mode.

ModeMUTE TDA7294

If 3.5V or more is applied to the 10th leg of the TDA7294, the device will exit the muting mode. If you apply less than 1.5V, the device will enter muting mode.

In practice, this is done like this: through a 10 kOhm resistor, connect the 10 leg of the microcircuit to the plus of a bipolar power source. The amplifier will “sing”, that is, it will not be muted. On the printed circuit board attached to the article, this is done using a track. When power is applied to the amplifier, it immediately begins to sing, without any jumpers or toggle switches.

If we connect the TDA7294 leg through a 10 kOhm resistor 10 to the GND pin of the power supply, then our “amplifier” will enter mute mode.

Power supply.

The voltage source for the device was an assembled one, which showed itself very well. When listening to one channel, the keys are warm. Schottky diodes are also warm, although there are no radiators installed on them. IIP without protection and soft start.

The circuit of this SMPS is criticized by many, but it is very easy to assemble. It works reliably without soft start. This circuit is very suitable for novice electronics engineers because of its prostate.

Frame.

The case was purchased.

Ganichev G.
Moscow

This article continues a series of publications devoted to power amplifiers offered to radio amateurs by MASTER KIT. The article includes two recent developments - NM2042 (powerful low-frequency amplifier 140 W) and NM2043 (powerful automotive bridge Hi-Fi low-frequency amplifier 4x77 W). The amplifiers are designed taking into account all the necessary requirements and are made on a modern integrated element base. The offered PAs have high performance characteristics, high reliability, ease of manufacture/connection and an optimal price/quality ratio, which is an important factor today. You can assemble the devices from the MASTER KIT kits NM2042 and NM2043.

MASTER KIT specialists were given, and successfully solved, the task of preparing technical documentation and producing a line of ULFs for use in Hi-Fi audio equipment. Gradually, the range of these devices is expanded and supplemented with new developments. This article will discuss two new developments - and.

All proposed models of power amplifiers have a minimum level of self-noise, a minimum level of nonlinear distortion and a wide reproducible frequency band. Models differ mainly in maximum output power, supply voltage (bipolar or unipolar “automotive” (14.4 V)), number of amplification channels and external design.

Radio amateurs can wire a printed circuit board themselves, but it must be taken into account that this is a very responsible and serious job. Not everyone knows that, for example, incorrect routing of printed conductors in a powerful amplifier can increase the level of its nonlinear distortion tenfold or even render it inoperable altogether. Therefore, professional designers specializing in this field were involved in the development of printed circuit boards.

. Powerful low frequency amplifier 140 W (TDA7293).

The proposed low-frequency amplifier has a minimum nonlinear distortion coefficient and noise level. The device has small dimensions. A wide range of supply voltages and load resistances expands the scope of application of this PA. It can be used both outdoors for various events and at home as part of your musical audio complex. The amplifier has proven itself well as a ULF for a subwoofer.

The ULF is made on the TDA7293 integrated circuit. This IC is a class AB ULF. Thanks to a wide range of supply voltages and the ability to deliver current to a load of up to 10 A, the microcircuit provides the same maximum output power at loads from 4 Ohms to 8 Ohms. One of the main features of this microcircuit is the use of field-effect transistors in the preliminary and output amplification stages and the ability to parallel connect several ICs to operate with low-impedance loads (< 4 Ом).

The operating mode of the IC is controlled using switch SW1. To turn on the ULF, SW1 must be closed. Switch SW2 is provided for technological purposes. For normal operation, SW2 must be jumpered in position 2-3.

Coil L1 must be made independently. L1 – frameless, three-layer, contains ten turns of PEV-1.0 wire in each layer. Winding must be carried out on a 12 mm mandrel. Approximate inductance – 5 µH.

The supply voltage is supplied to contacts X3(+), X6(-) and X7(common).

The signal source is connected to X1(+) and X2(common).

The load is connected to X4(+) and X5(common).

Structurally, the amplifier is made on a printed circuit board made of foil fiberglass. The design provides for installation of the board into the case; for this purpose, mounting holes are provided along the edges of the board for 2.5 mm screws. For the convenience of connecting the supply voltage, signal source and load, the board has reserved spaces for terminal screw clamps.

Structurally, a dual logical input of control signals MUTE/ST-BY is provided for “soft” activation of the ULF.

The amplifier chip must be installed on a heat sink (not included in the kit) with an area of ​​at least 600 cm2. As a radiator, you can use the metal case or chassis of the device into which the ULF is installed. During installation, it is recommended to use heat-conducting paste type KTP-8 to increase the reliability of the IC.

The general view of the amplifier is shown in Fig. 1, the electrical circuit diagram in Fig. 2, the arrangement of elements on the board and the connection of the amplifier in Fig. 3, the view of the printed circuit board from the side of the conductors in Fig. 4. The list of elements is given in Table 2.

Table 1. Technical characteristics.

Supply voltage, bipolar, V	+/- 12...50
Peak output current, A	10
Current in quiescent mode, mA	30
Current in MUTE/ST-BY mode, mA	0,5
Output power, W at harmonic distortion = 1%, Up = +/- 30 V, Rн = 4 Ohm	80
Output power, W at harmonic distortion = 10%, Up = +/- 45 V, Rн = 8 Ohm	140
Output power, W at harmonic distortion = 10%, Up = +/- 30 V, Rн = 4 Ohm	110
Gain Au, dB	30
Reproducible frequency range, Hz	20...20000
Input impedance, kOhm	22
PCB dimensions, mm	47x55

Table 2. List of elements.

Position	Name	Col.
C1	470 pF
C2	0.47 µF
C3, C10	22 µF/63 V
C4, C5	10 µF/63 V
C6, C7, C11	0.1 µF
C8, C9	1000 µF/63 V
DA1	TDA7293
L1	5 µH
R1	1 kOhm
R2	10 kOhm
R3	30 kOhm
R4, R5, R9...R12	22 kOhm
R6	20 kOhm
R7	680 Ohm
R8, R14	4.7 ohm
R13	270 Ohm
VD1	1N4148

Fig1. General view of the NM2042 amplifier.

Fig.2. Electrical circuit diagram of the NM2042 amplifier.

Fig.3. Layout of elements on the board and connection of the NM2042 amplifier.

Fig.4. View of the printed circuit board from the side of the printed conductors of the NM2042 amplifier.

. Powerful car bridge Hi-Fi low frequency amplifier 4X77 W (TDA7560).

The main purpose of this ULF is to install it in your car radio, instead of an old low-frequency amplifier, to increase its output power or for outdoor events using a 12 V battery as the main power source for the equipment. Thanks to the use of a bridge switching circuit, the amplifier develops power up to 80 W into a 2 Ohm load in each of the four channels. A special feature of the amplifier is the use of field-effect transistors in the output stages. The device has small dimensions, a wide range of supply voltages and load resistances.

The ULF is made on an integrated circuit TDA7560 (DA1). This IC is a class AB ULF and is installed in car audio devices to obtain a high-quality, powerful output music signal. The IC is designed to operate with a load of 4...2 Ohms, signal distortion meets Hi-Fi requirements. The microcircuit has protection against short-circuit load and overheating. Features of the microcircuit include the use of field-effect transistors in the output stages. The microcircuit contains four identical bridge amplifiers with a power of up to 80 W into a 2 Ohm load.

Switches SW1 (ST-BY) and SW2 (MUTE) are designed to control the operating modes of the IC. Closing the contacts in SW1 controls the ST-BY (standby/working) mode, and SW2 controls the MUTE (pause) mode.

Particular attention should be paid to connecting the microcircuit to the power source:

The IC is extremely sensitive to supply voltage - a maximum of 18 V.

Reversing the polarity of the supply voltage source leads to failure of the IC (Urev = 6 V maximum).

The supply voltage is connected to contacts X9(+) and X10(-).

Signal sources are connected to X1(+),X2(-);X3(+),X4(-);X5(+),X6(-);X7(+),X8(-).

The amplified signal is removed from contacts X11, X12; X13, X14; X15, X16; X17, X18.

The general view of the amplifier is shown in Fig. 5, the electrical circuit diagram in Fig. 6, the arrangement of elements on the board and the connection of the amplifier in Fig. 7, the top view of the printed circuit board in Fig. 8, the bottom view of the printed circuit board in Fig. 9. The list of elements is given in Table 3.

Table 3. Technical characteristics.

Table 4. List of elements

A low frequency amplifier (LFA) is a device for amplifying electrical oscillations corresponding to the frequency range audible to the human ear, i.e. the LFA should amplify in the frequency range from 20 Hz to 20 kHz, but some VLFs can have a range of up to 200 kHz. The ULF can be assembled as an independent device, or used in more complex devices - televisions, radios, radios, etc.

The peculiarity of this circuit is that pin 11 of the TDA1552 microcircuit controls the operating modes - Normal or MUTE.

C1, C2 - pass-through blocking capacitors, used to cut off the constant component of the sinusoidal signal. It is better not to use electrolytic capacitors. It is advisable to place the TDA1552 chip on a radiator using heat-conducting paste.

In principle, the presented circuits are bridge ones, because in one housing of the TDA1558Q microassembly there are 4 amplification channels, so pins 1 - 2, and 16 - 17 are connected in pairs, and they receive input signals from both channels through capacitors C1 and C2. But if you need an amplifier for four speakers, then you can use the circuit option below, although the power will be 2 times less per channel.

The basis of the design is the TDA1560Q class H microassembly. The maximum power of this ULF reaches 40 W, with a load of 8 ohms. This power is provided by approximately twice the increased voltage due to the operation of the capacitors.

The output power of the amplifier in the first circuit assembled on the TDA2030 is 60W at a load of 4 Ohms and 80W at a load of 2 Ohms; TDA2030A 80W at 4 ohm load and 120W at 2 ohm load. The second circuit of the considered ULF is already with an output power of 14 Watts.

This is a typical two-channel ULF. With a little wiring of passive radio components, this chip can be used to build an excellent stereo amplifier with an output power of 1 W on each channel.

The TDA7265 microassembly is a fairly powerful two-channel Hi-Fi class AB amplifier in a standard Multiwatt package; the microcircuit has found its niche in high-quality stereo technology, Hi-Fi class. The simple switching circuit and excellent parameters made the TDA7265 a perfectly balanced and excellent solution for building high-quality amateur radio equipment.

First, a test version was assembled on a breadboard exactly as shown in the datasheet in the link above, and successfully tested on S90 speakers. The sound is not bad, but something was missing. After some time, I decided to remake the amplifier using a modified circuit.

The microassembly is a quad class AB amplifier designed specifically for use in car audio devices. Based on this microcircuit, you can build several high-quality ULF options using a minimum of radio components. The microcircuit can be recommended to beginning radio amateurs for home assembly of various speaker systems.

The main advantage of the amplifier circuit on this microassembly is the presence of four channels independent of each other. This power amplifier operates in AB mode. It can be used to amplify various stereo signals. If desired, you can connect it to the speaker system of a car or personal computer.

The TDA8560Q is just a more powerful analogue of the TDA1557Q chip, widely known to radio amateurs. The developers have only strengthened the output stage, making the ULF perfectly suited to a two-ohm load.

The LM386 microassembly is a ready-made power amplifier that can be used in designs with low supply voltage. For example, when powering the circuit from a battery. LM386 has a voltage gain of about 20. But by connecting external resistances and capacitances, the gain can be adjusted up to 200, and the output voltage automatically becomes equal to half the supply voltage.

The LM3886 microassembly is a high quality amplifier with an output power of 68 watts into a 4 ohm load or 50 watts into 8 ohms. At peak moment, the output power can reach 135 W. A wide voltage range from 20 to 94 volts is applicable to the microcircuit. Moreover, you can use both bipolar and unipolar power supplies. The ULF harmonic coefficient is 0.03%. Moreover, this is over the entire frequency range from 20 to 20,000 Hz.

The circuit uses two ICs in a typical connection - KR548UH1 as a microphone amplifier (installed in the PTT switch) and (TDA2005) in a bridge connection as a final amplifier (installed in the siren housing instead of the original board). A modified alarm siren with a magnetic head is used as an acoustic emitter (piezo emitters are not suitable). The modification consists of disassembling the siren and throwing out the original tweeter with an amplifier. The microphone is electrodynamic. When using an electret microphone (for example, from Chinese handsets), the connection point between the microphone and the capacitor must be connected via a ~4.7K resistor to +12V (after the button!). The 100K resistor in the K548UH1 feedback circuit is better set with a resistance of ~30-47K. This resistor is used to adjust the volume. It is better to install the TDA2004 chip on a small radiator.

Test and operate - with the emitter under the hood and the PTT in the cabin. Otherwise, squealing due to self-excitation is inevitable. A trimmer resistor sets the volume level so that there is no strong sound distortion and self-excitation. If the volume is insufficient (for example, a bad microphone) and there is a clear reserve of emitter power, you can increase the gain of the microphone amplifier by several times increasing the value of the trimmer in the feedback circuit (the one according to the 100K circuit). In a good way, we would also need a primabass that would prevent the circuit from self-exciting - some kind of phase-shifting chain or a filter for the excitation frequency. Although the scheme works fine without complications

Making a good power amplifier has always been one of the difficult stages when designing audio equipment. Sound quality, softness of bass and clear sound of mid and high frequencies, detail of musical instruments - all these are empty words without a high-quality low-frequency power amplifier.

Preface

Out of variety homemade amplifiers LF on transistors and integrated circuits that I manufactured, the circuit on the driver microcircuit performed best of all. TDA7250 + KT825, KT827.

In this article I will tell you how to make an amplifier amplifier circuit that is perfect for use in homemade audio equipment.

Amplifier parameters, a few words about TDA7293

The main criteria by which the ULF circuit was selected for the Phoenix-P400 amplifier:

• Power approximately 100W per channel at 4 Ohm load;
• Power supply: bipolar 2 x 35V (up to 40V);
• Low input impedance;
• Small dimensions;
• High reliability;
• Speed ​​of production;
• High sound quality;
• Low noise level;
• Low cost.

This is not a simple combination of requirements. First I tried the option based on the TDA7293 chip, but it turned out that this was not what I needed, and here’s why...

Over all this time, I had the opportunity to assemble and test different ULF circuits - transistor ones from books and publications of Radio magazine, on various microcircuits...

I would like to say my word about the TDA7293 / TDA7294, because a lot has been written about it on the Internet, and more than once I have seen that the opinion of one person contradicts the opinion of another. Having assembled several clones of an amplifier using these microcircuits, I made some conclusions for myself.

The microcircuits are really quite good, although a lot depends on the successful layout of the printed circuit board (especially the ground lines), good power supply and the quality of the wiring elements.

What immediately pleased me about it was the fairly large power delivered to the load. As for a single-chip integrated amplifier, the low-frequency output power is very good; I would also like to note the very low noise level in the no-signal mode. It is important to take care of good active cooling of the chip, since the chip operates in “boiler” mode.

What I didn’t like about the 7293 amplifier was the low reliability of the microcircuit: out of several purchased microcircuits, at various points of sale, only two were left working! I burned one out by overloading the input, 2 burned out immediately when turned on (it seems like a factory defect), another one burned out for some reason when I turned it on again for the 3rd time, although before that it worked normally and no anomalies were observed... Maybe I was just unlucky.

And now, the main reason why I did not want to use modules based on TDA7293 in my project is the “metallic” sound that is noticeable to my ears, there is no softness and richness in it, the mid frequencies are a little dull.

I concluded that this chip is perfect for subwoofers or low-frequency amplifiers that will drone in the trunk of a car or at discos!

I will not touch on the topic of single-chip power amplifiers further; we need something more reliable and of high quality so that it is not so expensive in terms of experiments and errors. Assembling 4 channels of an amplifier using transistors is a good option, but it is quite cumbersome in execution, and it can also be difficult to configure.

So what should you use to assemble if not transistors or integrated circuits? - on both, skillfully combining them! We will assemble a power amplifier using a TDA7250 driver chip with powerful composite Darlington transistors at the output.

LF power amplifier circuit based on TDA7250 chip

Chip TDA7250 in a DIP-20 package is a reliable stereo driver for Darlington transistors (high-gain composite transistors), on the basis of which you can build a high-quality two-channel stereo UMZCH.

The output power of such an amplifier can reach or even exceed 100 W per channel with a load resistance of 4 Ohms; it depends on the type of transistors used and the supply voltage of the circuit.

After assembling a copy of such an amplifier and the first tests, I was pleasantly surprised by the sound quality, power and how the music produced by this microcircuit “came to life” in combination with transistors KT825, KT827. Very small details began to be heard in the compositions, the instruments sounded rich and “light”.

You can burn this chip in several ways:

• Reversing the polarity of power lines;
• Exceeding the maximum permissible supply voltage ±45V;
• Input overload;
• High static voltage.

Rice. 1. TDA7250 microcircuit in a DIP-20 package, appearance.

Datasheet for the TDA7250 chip - (135 KB).

Just in case, I purchased 4 microcircuits at once, each of which has 2 amplification channels. The microcircuits were purchased from an online store at a price of approximately $2 per piece. At the market they wanted more than $5 for such a chip!

The scheme according to which my version was assembled does not differ much from the one shown in the datasheet:

Rice. 2. Circuit of a stereo low-frequency amplifier based on the TDA7250 microcircuit and transistors KT825, KT827.

For this UMZCH circuit, a homemade bipolar power supply of +/- 36V was assembled, with capacitances of 20,000 μF in each arm (+Vs and -Vs).

Power Amplifier Parts

I’ll tell you more about the features of the amplifier parts. List of radio components for circuit assembly:

Name	Quantity, pcs	Note
TDA7250	1
KT825	2
KT827	2
1.5 kOhm	2
390 Ohm	4
33 Ohm	4	power 0.5W
0.15 ohm	4	power 5W
22 kOhm	3
560 Ohm	2
100 kOhm	3
12 ohm	2	power 1W
10 ohm	2	power 0.5W
2.7 kOhm	2
100 Ohm	1
10 kOhm	1
100 µF	4	electrolytic
2.2 µF	2	mica or film
2.2 µF	1	electrolytic
2.2 nF	2
1 µF	2	mica or film
22 µF	2	electrolytic
100 pF	2
100 nF	2
150 pF	8
4.7 µF	2	electrolytic
0.1 µF	2	mica or film
30 pf	2

The inductor coils at the output of the UMZCH are wound on a frame with a diameter of 10 mm and contain 40 turns of enameled copper wire with a diameter of 0.8-1 mm in two layers (20 turns per layer). To prevent the coils from falling apart, they can be fastened with fusible silicone or glue.

Capacitors C22, C23, C4, C3, C1, C2 must be designed for a voltage of 63V, the remaining electrolytes - for a voltage of 25V or more. Input capacitors C6 and C5 are non-polar, film or mica.

Resistors R16-R19 must be designed for a power of at least 5Watt. In my case, miniature cement resistors were used.

Resistances R20-R23, as well as R.L. can be installed with a power of 0.5W. Resistors Rx - power of at least 1W. All other resistances in the circuit can be set to a power of 0.25W.

It is better to select pairs of transistors KT827 + KT825 with the closest parameters, for example:

1. KT827A(Uke=100V, h21E>750, Pk=125W) + KT825G(Uke=70V, h21E>750, Pk=125W);
2. KT827B(Uke=80V, h21E>750, Pk=125W) + KT825B(Uke=60V, h21E>750, Pk=160W);
3. KT827V(Uke=60V, h21E>750, Pk=125W) + KT825B(Uke=60V, h21E>750, Pk=160W);
4. KT827V(Uke=60V, h21E>750, Pk=125W) + KT825G(Uke=70V, h21E>750, Pk=125W).

Depending on the letter at the end of the marking for KT827 transistors, only the voltages Uke and Ube change, the rest of the parameters are identical. But KT825 transistors with different letter suffixes already differ in many parameters.

Rice. 3. Pinout of powerful transistors KT825, KT827 and TIP142, TIP147.

It is advisable to check the transistors used in the amplifier circuit for serviceability. Darlington transistors KT825, KT827, TIP142, TIP147 and others with a high gain contain two transistors, a couple of resistances and a diode inside, so a regular test with a multimeter may not be enough here.

To test each of the transistors, you can assemble a simple circuit with an LED:

Rice. 4. Transistor testing circuit P-N-P structures and N-P-N for operability in key mode.

In each of the circuits, when the button is pressed, the LED should light up. Power can be taken from +5V to +12V.

Rice. 5. An example of testing the performance of the KT825 transistor, P-N-P structure.

Each of the pairs of output transistors must be installed on radiators, since already at an average ULF output power their heating will be quite noticeable.

The datasheet for the TDA7250 chip shows the recommended pairs of transistors and the power that can be extracted using them in this amplifier:

At 4 ohm load
ULF power	30 W	+50 W	+90 W	+130 W
Transistors	BDW93, BDW94A	BDW93, BDW94B	BDV64, BDV65B	MJ11013, MJ11014
Housings	TO-220	TO-220	SOT-93	TO-204 (TO-3)
At 8 ohm load
ULF power	15 W	+30 W	+50 W	+70 W
Transistors	BDX53 BDX54A	BDX53 BDX54B	BDW93, BDW94B	TIP142, TIP147
Housings	TO-220	TO-220	TO-220	TO-247

Mounting transistors KT825, KT827 (TO-3 housing)

Particular attention should be paid to the installation of output transistors. A collector is connected to the housing of transistors KT827, KT825, so if the housings of two transistors in one channel are accidentally or intentionally shorted, you will get a short circuit in the power supply!

Rice. 6. Transistors KT827 and KT825 are prepared for installation on radiators.

If the transistors are planned to be mounted on one common radiator, then their cases must be insulated from the radiator through mica gaskets, having previously coated them on both sides with thermal paste to improve heat transfer.

Rice. 7. Radiators that I used for transistors KT827 and KT825.

In order not to describe for a long time how to install isolated transistors on radiators, I will give a simple drawing that shows everything in detail:

Rice. 8. Insulated mounting of transistors KT825 and KT827 on radiators.

Printed circuit board

Now I'll tell you about the printed circuit board. It will not be difficult to separate it, since the circuit is almost completely symmetrical for each channel. You need to try to distance the input and output circuits from each other as much as possible - this will prevent self-excitation, a lot of interference, and protect you from unnecessary problems.

Fiberglass can be taken with a thickness of 1 to 2 millimeters; in principle, the board does not need special strength. After etching the tracks, you need to tin them well with solder and rosin (or flux), do not ignore this step - it is very important!

I laid out the tracks for the printed circuit board manually, on a sheet of checkered paper using a simple pencil. This is what I have been doing since the times when one could only dream about SprintLayout and LUT technology. Here is a scanned stencil of the printed circuit board design for the ULF:

Rice. 9. Printed circuit board of the amplifier and the location of the components on it (click to open full size).

Capacitors C21, C3, C20, C4 are not on the hand-drawn board, they are needed to filter the power supply voltage, I installed them in the power supply itself.

UPD: Thank you Alexandru for PCB layout in Sprint Layout!

Rice. 10. Printed circuit board for UMZCH on the TDA7250 chip.

In one of my articles I told how to make this printed circuit board using the LUT method.

Download the printed circuit board from Alexander in *.lay(Sprint Layout) format - (71 KB).

UPD. Here are other printed circuit boards mentioned in the comments to the publication:

As for the connecting wires for power supply and at the output of the UMZCH circuit, they should be as short as possible and with a cross-section of at least 1.5 mm. In this case, the shorter the length and greater the thickness of the conductors, the less current loss and interference in the power amplification circuit.

The result was 4 amplification channels on two small strips:

Rice. 11. Photos of finished UMZCH boards for four channels of power amplification.

Setting up the amplifier

A correctly assembled circuit made from serviceable parts begins to work immediately. Before connecting the structure to the power source, you need to carefully inspect the printed circuit board for any short circuits, and also remove excess rosin using a piece of cotton wool soaked in a solvent.

I recommend connecting speaker systems to the circuit when you first turn it on and during experiments using resistors with a resistance of 300-400 Ohms, this will save the speakers from damage if something goes wrong.

It is advisable to connect a volume control to the input - one dual variable resistor or two separately. Before turning on the UMZCH, we put the switch of the resistor(s) in the left extreme position, as in the diagram (minimum volume), then by connecting the signal source to the UMZCH and applying power to the circuit, you can smoothly increase the volume, observing how the assembled amplifier behaves.

Rice. 12. Schematic representation of connecting variable resistors as volume controls for ULF.

Variable resistors can be used with any resistance from 47 KOhm to 200 KOhm. When using two variable resistors, it is desirable that their resistances be the same.

So, let's check the performance of the amplifier at low volume. If everything is fine with the circuit, then the fuses on the power lines can be replaced with more powerful ones (2-3 Amperes); additional protection during operation of the UMZCH will not hurt.

The quiescent current of the output transistors can be measured by connecting an ammeter or multimeter in current measurement mode (10-20A) to the collector gap of each transistor. The amplifier inputs must be connected to common ground (complete absence of input signal), and speakers must be connected to the amplifier outputs.

Rice. 13. Circuit diagram for connecting an ammeter to measure the quiescent current of the output transistors of an audio power amplifier.

The quiescent current of the transistors in my UMZCH using KT825+KT827 is approximately 100mA (0.1A).

Power fuses can also be replaced with powerful incandescent lamps. If one of the amplifier channels behaves inappropriately (hum, noise, overheating of transistors), then it is possible that the problem lies in the long conductors going to the transistors; try reducing the length of these conductors.

In conclusion

That's all for now, in the following articles I'll tell you how to make a power supply for an amplifier, output power indicators, protection circuits for speaker systems, about the case and front panel...