- The neighbor got tired of knocking on the battery. He turned the music up louder so that he could not be heard.
(From audiophile folklore).

The epigraph is ironic, but the audiophile is not necessarily “sick in the head” with the physiognomy of Josh Ernest at a briefing on relations with the Russian Federation, who is “rushing” because the neighbors are “happy”. Someone wants to listen to serious music at home as in the hall. The quality of the equipment for this is necessary, which for fans of the decibel of loudness as such simply does not fit where sane people have a mind, but for the latter, this mind comes 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 - the technique of sound reproduction and electronics in general. Which in the digital age are inextricably linked and can become a highly profitable and prestigious profession. The first step in this matter, optimal in all respects, is to make an amplifier with your own hands: it is UMZCH that allows, with initial training based on school physics, on the same table, to go from the simplest structures 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 to cover the first stages of this path for beginners and, perhaps, to tell something new to experienced ones.

Protozoa

So, for starters, let's try to make a sound 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 do not forget to enrich your knowledge base as you progress. But any “smartness” is easier to digest when you see and feel how it works “in hardware”. In this article, further, too, it will not do without theory - in what you need to know at first and what can be explained without formulas and graphs. In the meantime, it will be enough to be able to use the multitester.

Note: if you have not soldered electronics yet, please note that its components must not be overheated! Soldering iron - up to 40 W (better than 25 W), the maximum allowable soldering time without interruption is 10 s. The soldered lead for the heat sink is held 0.5-3 cm from the place of soldering from the side of the device body with medical tweezers. Acid and other active fluxes must not be used! Solder - POS-61.

On the left in fig.- the simplest UMZCH, "which just works." It can be assembled on both germanium and silicon transistors.

On this crumb, it is convenient to master the basics of setting up the UMZCH with direct connections between the cascades, which give the clearest sound:

Before the first power-up, the load (speaker) is turned off;
Instead of R1, we solder a chain of a constant resistor of 33 kOhm and a variable (potentiometer) of 270 kOhm, i.e. first note. four times smaller, and the second approx. twice the face value against the original according to the scheme;
We supply power and, by rotating the potentiometer slider, at the point marked with a cross, set the specified collector current VT1;
We remove the power, solder the temporary resistors and measure their total resistance;
As R1, we set the nominal resistor from the standard row closest to the measured one;
We replace R3 with a constant 470 Ohm chain + 3.3 kOhm potentiometer;
The same as according to paragraphs. 3-5, incl. a set the voltage equal to half the supply voltage.

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

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

In the center in the same Fig.- a simple UMZCH on transistors, which already develops 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 such class D amplifier (see below) in cheap Chinese computer speakers, their sound improves markedly. Here we learn another trick: powerful output transistors must be placed on radiators. Components that require additional cooling are circled in the diagrams with a dotted line; however, not always; sometimes - with an indication of the required dissipating area of the heat sink. Adjustment of this UMZCH - balancing with 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 100 W transistor amplifier. You can listen to music through it, but not Hi-Fi, the work class is AB2. However, for scoring a picnic area or an outdoor meeting, a school assembly or a small trading floor, it is quite suitable. An amateur rock band, having such an UMZCH for an instrument, can perform successfully.

In this UMZCH, 2 more tricks appear: firstly, in very powerful amplifiers, the buildup cascade of a powerful output also needs to be cooled, so VT3 is put on a radiator from 100 sq. see. For output VT4 and VT5, radiators from 400 square meters are needed. see Secondly, UMZCH with bipolar power supply are not balanced at all without load. Either one or the other output transistor goes into cutoff, and the conjugated one goes into saturation. Then, at full supply voltage, current surges during balancing can destroy the output transistors. Therefore, for balancing (R6, did you guess?), the amplifier is powered from +/-24 V, and instead of the load, a 100 ... 200 Ohm wire resistor is included. By the way, the squiggles in some of the resistors in the diagram are Roman numerals, denoting their required heat dissipation power.

Note: a power source for this UMZCH needs a power of 600 watts or more. Smoothing filter capacitors - from 6800 uF to 160 V. In parallel with the electrolytic capacitors of the IP, ceramic ones of 0.01 uF are turned on to prevent self-excitation at ultrasonic frequencies, which can instantly burn out the output transistors.

On the field workers

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 draws on 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 much power for buildup, so there is no pre-power cascade. Even powerful field-effect transistors do not burn the speakers under any malfunctions - they themselves burn out faster. Also unpleasant, but still cheaper than changing an expensive bass speaker head (GG). Balancing and generally adjustment to this UMZCH are not required. It has only one drawback, like a design for beginners: powerful field-effect transistors are much more expensive than bipolar ones for an amplifier with the same parameters. IP requirements are the same as before. occasion, but its power is needed from 450 watts. Radiators - from 200 sq. cm.

Note: no need to build powerful UMZCH on field-effect transistors for switching power supplies, for example. computer. When trying to “drive” them into the active mode necessary for the UMZCH, they either simply burn out, or they give a weak sound, but “none” in quality. The same applies to powerful high-voltage bipolar transistors, for example. from the horizontal scanning of old TVs.

Right up

If you have already taken the first steps, then it will be quite natural to want to build UMZCH class Hi-Fi, without going too deep into the theoretical jungle. To do this, you will have to expand the instrument park - you need an oscilloscope, an audio frequency generator (GZCH) and an AC millivoltmeter with the ability to measure the DC component. It is better to take the UMZCH E. Gumeli, described in detail in Radio No. 1 for 1989, as a prototype for repetition. To build it, you will need a few inexpensive affordable components, but the quality meets very high requirements: power up to 60 W, bandwidth 20-20,000 Hz, frequency response unevenness 2 dB, non-linear 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 circumstances now known greatly simplify the establishment of this UMZCH, see below. Bearing this in mind and the fact that not everyone succeeds in getting into the Radio archives, it would be appropriate to repeat the main points.

Schemes of a simple high-quality UMZCH

UMZCH Gumeli schemes and specifications for them are given in the illustration. Radiators of output transistors - from 250 sq. see for UMZCH according to fig. 1 and from 150 sq. see for variant according to fig. 3 (numbering is original). The transistors of the pre-output stage (KT814/KT815) are mounted on radiators bent from aluminum plates 75x35 mm 3 mm thick. It is not worth replacing KT814 / KT815 with KT626 / KT961, the sound does not noticeably improve, but it is seriously difficult to establish.

This UMZCH is very critical to the power supply, installation topology and general, therefore, it must be adjusted in a structurally finished form and only with a standard power source. When trying to power from a stabilized IP, the output transistors burn out immediately. Therefore, in fig. drawings of original printed circuit boards and instructions for setting up are given. It can be added to them that, firstly, if “excitation” is noticeable at the first start, they fight with it by changing the inductance L1. Secondly, the leads of the parts installed on the boards must be no longer than 10 mm. Thirdly, it is highly undesirable to change the installation topology, but, if it is very necessary, there must be a frame screen 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 ones, for establishing, after which they are sealed with drops of solder.

The establishment of this UMZCH is greatly simplified, and the risk of encountering "excitation" in the process of use is reduced to zero if:

Minimize interconnect wiring by placing boards on high-power transistor heatsinks.
Completely abandon the connectors inside, performing the entire installation only by soldering. Then you will not need R12, R13 in a powerful version or R10 R11 in a less powerful one (they are dotted on the diagrams).
Use the minimum length of oxygen-free copper audio wires for indoor wiring.

When these conditions are met, there are no problems with excitation, and the establishment of UMZCH is reduced to a routine procedure, described in Fig.

Wires for sound

Audio wires are not idle fiction. The need for their use at the present time is undeniable. In copper with an admixture of oxygen, the thinnest 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 (it seems, due to quantum uncertainties) remains. Enough to be noticed by discerning listeners against the background of the purest sound of modern UMZCH.

Manufacturers and traders without a twinge of conscience slip ordinary electrical copper instead of oxygen-free copper - it is impossible to distinguish one from the other by eye. However, there is a scope where a fake does not go unambiguously: a twisted-pair cable for computer networks. Put a grid with long segments on the left, it will either not start at all, or it will constantly fail. Dispersion of impulses, you know.

The author, when there was still talk about audio wires, realized that, in principle, this was not empty chatter, especially since oxygen-free wires by that time had long been used in special-purpose equipment, with which he was well acquainted with the type of activity. Then I took it and replaced the regular cord of my TDS-7 headphones with a home-made one from a “vitukha” with flexible stranded wires. The sound, by ear, has steadily improved for analog tracks through, i.e. on the way from the studio microphone to the disc, never digitized. Recordings on vinyl made using DMM technology (Direct Meta lMastering, direct metal deposition) sounded especially bright. After that, the interblock editing of all home audio was converted to "vitushny". Then completely random people began to notice the improvement in sound, they were indifferent to music and not forewarned in advance.

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

Video: do-it-yourself twisted-pair interconnect wires

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

Theoretical interlude

As you can see, already at the very beginning of mastering sound engineering, we had to deal with the concept of Hi-Fi (High Fidelity), high fidelity of sound reproduction. Hi-Fi comes in different levels, which are ranked next. main parameters:

Band of reproducible frequencies.
Dynamic range - the ratio in decibels (dB) of the maximum (peak) output power to the level of self-noise.
Self-noise level in dB.
Nonlinear distortion factor (THD) at rated (long-term) output power. SOI at peak power is assumed to be 1% or 2% depending on the measurement technique.
Irregularities in the amplitude-frequency characteristic (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) audio frequencies.

Note: the ratio of the absolute levels of any values of I in (dB) is defined as P(dB) = 20lg(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 home-made Hi-Fi UMZCH for the home, before moving on to these, you need to clearly understand the requirements for their power required for scoring a given room, dynamic range (dynamics), self-noise level and SOI. To achieve a frequency band of 20-20,000 Hz from the UMZCH with a blockage at the edges of 3 dB and a frequency response unevenness at the midrange of 2 dB on a modern element base is not very difficult.

Volume

The power of the UMZCH is not an end in itself, it should provide the optimal volume of sound reproduction in a given room. It can be determined by curves of equal loudness, see fig. Natural noise in residential premises is quieter than 20 dB; 20 dB is the wilderness in complete calm. The volume level of 20 dB relative to the threshold of hearing is the threshold of intelligibility - you can still make out the whisper, but the music is perceived only as a fact of its presence. An experienced musician can tell which instrument is playing, but not exactly what.

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

90 dB is the volume level of a symphony orchestra in a very good concert hall. 110 dB can give out an expanded 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 perceived even as distinguishable in meaning with an effort of will, but already annoying noise. The loudness zone in residential premises of 20-110 dB is the zone of full audibility, and 40-90 dB is the zone of the best audibility, in which unprepared and inexperienced listeners fully perceive the meaning of the sound. If, of course, he is in it.

Power

Calculating the power of the equipment for 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) speakers. In this case, the UMZCH will not noticeably add its distortions to those speakers, they are already the main source of non-linearity in the audio path. But the UMZCH should not be made too powerful: in this case, the level of its own noise may be above the threshold of audibility, because. it is considered from the voltage level of the output signal at maximum power. If we consider it very simply, then for a room of an ordinary apartment or house and speakers with normal characteristic sensitivity (sound output), we can take a 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 watts.
70-100 sq. m - 100-150 watts.
100-120 sq. m - 150-200 watts.
Over 120 sq. m - is determined by calculation according to acoustic measurements on site.

Dynamics

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

Symphonic music and jazz with symphonic accompaniment - 90 dB (110 dB - 20 dB) ideal, 70 dB (90 dB - 20 dB) acceptable. Sound with dynamics of 80-85 dB in a city apartment will not be distinguished from ideal by any expert.
Other serious musical genres - 75 dB is excellent, 80 dB is over the roof.
Pops of any kind and movie soundtracks - 66 dB for the eyes is enough, because. these opuses are already compressed in levels up to 66 dB and even up to 40 dB during recording, so that you can listen to anything.

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

SOI

Nonlinear distortions (NI) UMZCH are components of the spectrum of the output signal, which were not in the input. 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. 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 it is difficult to assess the timbre of the sound. In people without a musical ear, the masking effect is already observed at 45-40 dB of volume. Therefore, UMZCH with a THD of 0.1% (-60 dB from a volume level of 110 dB) will be assessed as a Hi-Fi by an ordinary listener, and with a THD of 0.01% (-80 dB) can be considered not distorting the sound.

Lamps

The last statement, perhaps, will cause rejection, up to furious, among adherents of tube circuitry: they say that only tubes give real sound, and not just any, but certain types of octal ones. Calm down, gentlemen - a special tube sound is not fiction. The reason is fundamentally different distortion spectra for electronic tubes and transistors. Which, in turn, are due to the fact that the electron flow in the lamp moves in a vacuum and quantum effects do not appear in it. A transistor is a quantum device, where minor charge carriers (electrons and holes) move in a crystal, which is generally impossible without quantum effects. Therefore, the spectrum of tube distortions is short and clean: only harmonics up to the 3rd - 4th are clearly traced in it, and there are very few combination components (sums and differences of the frequencies of the input signal and their harmonics). Therefore, in the days of vacuum circuitry, SOI was called the harmonic coefficient (KH). In transistors, the distortion spectrum (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, the designers of transistorized UMZCH took for them the usual "tube" SOI of 1-2%; a sound with a tube distortion spectrum of this magnitude is perceived by ordinary listeners as clean. By the way, the very concept of Hi-Fi did not exist then. It turned out - they sound dull and deaf. In the process of the development of transistor technology, an understanding was developed of what Hi-Fi is and what is needed for it.

At present, the growing pains of transistor technology have been successfully overcome and side frequencies at the output of a good UMZCH are hardly captured by special measurement methods. And lamp circuitry can be considered to have passed into the category of art. Its basis can be any, why can't electronics go there? An analogy with photography would be appropriate here. No one can deny that a modern digital SLR gives an image immeasurably clearer, more detailed, deeper in terms of brightness and color range than a plywood box with an accordion. But someone with the coolest Nikon "clicks pictures" like "this is my fat cat got drunk like a bastard and sleeps with his paws spread", and someone with Smena-8M on a Svemov b / w film takes a picture in front of which people are crowding at a prestigious exhibition.

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

Experimental stand

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

But enough of the lyrics, let's get down to business. For those who like to tinker with the lamps in fig. - a diagram of a bench lamp UMZCH, designed 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 slight refinement touched only the output transformer: now you can not only “drive” your native 6P7S in different modes, but also select the screen grid switching ratio 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. See below for output transformer manufacturing.

Guitarists and rockers

This is the case when you can not 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 pass through a special prefix - fuser - deliberately distorting its spectrum. Without this, the sound of the string was too sharp and short, because. an electromagnetic pickup reacts only to the modes of its mechanical oscillations in the plane of the soundboard of the instrument.

An unpleasant circumstance soon emerged: the sound of an electric guitar with a fuser gains full strength and brightness only at high volumes. This is especially evident for guitars with a humbucker pickup, which gives the most "evil" sound. But what about a beginner, forced to rehearse at home? Do not go to the hall to perform, not knowing exactly how the instrument will sound there. And just rock lovers 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 outrageous surroundings.

So, it turned out that the fatal sound appears at volume levels acceptable for residential premises, if the UMZCH is tube. The reason is the specific interaction of the signal spectrum from the fuser with a clean 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 contour and the light for viewing.

Those who need a tube amplifier not for experiments, but because of technical necessity, have no time to master the intricacies of tube electronics for a long time, they are passionate about others. UMZCH in this case, it is better to do transformerless. More precisely, with a single-ended matching output transformer that operates without constant bias. This approach greatly simplifies and speeds up the manufacture of the most complex and critical assembly of the lamp UMZCH.

“Transformerless” UMZCH tube output stage and preamplifiers 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 options for a preamplifier for it. Above - with a tone control according to the classic Baksandal scheme, which provides a fairly deep adjustment, but introduces small phase distortions into the signal, which can be significant when operating the UMZCH on a 2-way speaker. Below is a simpler preamplifier with tone control that does not distort the signal.

But let's get back to the end. In a number of foreign sources, this circuit is considered a revelation, however, identical to it, with the exception of the capacity of electrolytic capacitors, is found in the Soviet Radio Amateur's Handbook of 1966. A thick book of 1060 pages. There was no Internet then and databases on disks.

In the same place, on the right in the figure, the shortcomings of this scheme are briefly but clearly described. Improved, from the same source, 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 through the load. If, instead of high-impedance speakers, you turn on a matching transformer with a conventional speaker, as in the previous. 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 a 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 stray 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", buzzes or squeaks. Foucault currents are fought by reducing the thickness of the plates of the magnetic circuit and additionally isolating them with varnish during assembly. For output transformers, the optimal thickness of the plates is 0.15 mm, the maximum allowable is 0.25 mm. Thinner plates should not be taken for the output transformer: the fill factor of the core (the central core 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 distortion and losses in it.

In the core of an audio transformer operating with a constant bias (eg, 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 bias; on the other hand, in a conventional magnetic circuit it increases the stray field and requires a larger core. 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 plates (punched), pos. 1 in fig. In them, a non-magnetic gap is formed during the penetration of the core and therefore is 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 closes are solid. Shp plates are often used to assemble transformer cores without magnetization, because Shp plates are made of high quality transformer steel. In this case, the core is assembled in an overlap (the plates are placed with a notch in one direction or the other), and its cross section is increased by 10% against the calculated one.

It is better to wind transformers without magnetization on USh cores (reduced height with widened windows), pos. 2. In them, the reduction of 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 also made from them. Then the assembly of the core is carried out in a cut: a package of W-plates is assembled, a strip of non-conductive non-magnetic material is laid with a thickness equal to the value of the non-magnetic gap, covered with a yoke from a package of jumpers and pulled together by a clip.

Note:"Audio" signal magnetic circuits of the ShLM type for output transformers of high-quality tube amplifiers are of little use, they have a large stray field.

At pos. 3 is a diagram of the dimensions of the core for calculating the transformer, at pos. 4 winding frame design, and on pos. 5 - patterns of its details. As for the transformer for the "transformerless" output stage, it is better to do it on the SLMme with an overlap, because. the bias is negligible (the bias current is equal to the current of the screen grid). 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 turn out to be much less than 800 ohms. The more free space left in the windows, the better the transformer turned out. Therefore, the windings wind turn to turn (if there is no winding machine, this is a terrible machine) from the thinnest possible wire, the anode winding laying coefficient for the mechanical calculation of the transformer is taken as 0.6. The winding wire is of the PETV or PEMM brands, they have an oxygen-free core. It is not necessary to take PETV-2 or PEMM-2, they have an increased outer diameter due to double varnishing and the scattering field will be larger. The primary winding is wound first, because. it is its stray field that most affects the sound.

Iron for this transformer must be looked for with holes in the corners of the plates and clamps (see the figure on the right), because. "For complete happiness" the assembly of the magnetic circuit is carried out in the following. order (of course, the windings with leads and outer insulation should already be on the frame):

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

The core assembled using this technology has very good plate insulation and steel filling. Losses due to magnetostriction are not detected at all. But keep in mind - for the cores of their permalloy, this technique is not applicable, because. from strong mechanical influences, the magnetic properties of permalloy irreversibly deteriorate!

On microchips

UMZCH on integrated circuits (ICs) is most often done by those who are satisfied with sound quality up to average Hi-Fi, but are more attracted by cheapness, speed, ease of assembly and the complete absence of any adjustment procedures that require special knowledge. Simply, an amplifier on microcircuits is the best option for dummies. The classic of the genre here is UMZCH on the TDA2004 IC, standing on the series, God forbid, for 20 years, 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 on a very low-resistance, up to 1.6 Ohm, load, which allows you to remove full power when powered from the 12 V on-board network, and 7-8 W - with a 6-volt power supply, for example, on a motorcycle. However, the TDA2004 output in class B is non-complementary (on transistors of the same conductivity), so the sound is definitely not Hi-Fi: THD 1%, dynamics 45 dB.

The more modern TDA7261 gives no better sound, but more powerful, up to 25 W, because. the upper limit of the supply voltage has been increased to 25V. TDA7261 can be run from almost all on-board networks, except for aircraft 27 V. With the help of hinged components (strapping, on the right in the figure), TDA7261 can operate in mutation mode and with the St-By (Stand By, wait) function, which switches the UMZCH to the minimum power consumption mode when there is no input signal for a certain time. Amenities cost money, so for a stereo you will need a pair of TDA7261 with radiators from 250 sq. see for each.

Note: if you are 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 TDA7482, on the left in the figure, working in the so-called. class D. Such UMZCH are sometimes called digital amplifiers, which is not true. For true digitization, level samples are taken from an analog signal at a quantization frequency of at least twice the highest of the reproducible frequencies, the value of each sample is recorded in an error-correcting code and stored for future use. UMZCH class D - pulsed. In them, the analogue is directly converted into a sequence of high-frequency pulse-width modulated (PWM) pulses, which is fed to the speaker through a low-pass filter (LPF).

Class D sound has nothing to do with Hi-Fi: THD of 2% and dynamics of 55 dB for UMZCH class D are considered very good indicators. And TDA7482 here, I must say, the choice is not optimal: other companies specializing in class D produce UMZCH ICs cheaper and require less strapping, for example, the Paxx D-UMZCH series, on the right in Fig.

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

UMZCH for subwoofer

A subwoofer, which can be translated as a "subwoofer" or, literally, "a subwoofer" reproduces frequencies up to 150-200 Hz, in this range, human ears are practically unable to determine the direction to the sound source. In speakers with a subwoofer, the “subwoofer” speaker is placed in a separate acoustic design, this is the subwoofer as such. The subwoofer is placed, in principle, as it is more convenient, 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. Connoisseurs agree that it is still 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 with consumers with normal hearing and not particularly demanding.

“Leakage” of midrange-high frequencies into the subwoofer, and from it into the air, greatly spoils the stereo, but if you sharply “cut off” the subbass, which, by the way, is very difficult and expensive, then a very unpleasant sound jump effect will occur. Therefore, channel filtering in subwoofer systems is done twice. At the input, MF-HF with bass "tails" are distinguished by electric filters, which do not overload the MF-HF 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, it is already acoustic in the subwoofer: the subwoofer is placed, for example, in the partition between the resonator chambers of the subwoofer that do not let the midrange out, see on the right in Fig.

A number of specific requirements are imposed on the UMZCH for a subwoofer, of which the "dummies" consider the greatest possible power to be the main one. This is completely wrong, if, say, the calculation of acoustics for a room gave peak power W for one speaker, then the power of the subwoofer needs 0.8 (2W) or 1.6W. For example, if speakers S-30 are suitable for the room, then a subwoofer is needed 1.6x30 \u003d 48 watts.

It is much more important to ensure the absence of phase and transient distortions: if they go, there will definitely be a sound jump. As for THD, it is acceptable up to 1%. Bass distortions of this level are not audible (see equal loudness curves), and the “tails” of their spectrum in the best audible midrange region will not get out of the subwoofer.

In order 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 UMZCH are turned on in the opposite direction through the speaker; the signals to the inputs are in antiphase. The absence of phase and transient distortion in the bridge circuit is due to the complete electrical symmetry of the output signal paths. The identity of the amplifiers that form the shoulders of the bridge is ensured by the use of paired UMZCH 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 the 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 the R5C3 and R'5C'3 circuits. Radiator area TDA2030 - from 400 sq. see. Bridge UMZCHs with an open output have an unpleasant feature: when the bridge is unbalanced, a constant component appears in the load current that can disable the speaker, and protection circuits on the subbass often fail, turning off the speaker when not needed. Therefore, it is better to protect the expensive “dubovo” woofer with non-polar batteries of electrolytic capacitors (highlighted in color, and the diagram of one battery is given in the sidebar.

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 sufficiently 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 one. For a specific speaker head, the subwoofer will have to be reconfigured for the best bass and, at the same time, for the least impact on the stereo effect. To tune the pipes, they take obviously longer lengths and, pushing in and out, achieve the desired sound. The outward protrusions of the pipes do not affect the sound, they are then cut off. The pipe settings are interdependent, so you have to tinker.

Headphone Amplifier

A headphone amplifier is made by hand most often for 2 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 build up "buttons" or "burdocks". The second is for high-end home headphones. Hi-Fi UMZCH for an ordinary living room is needed with dynamics up to 70-75 dB, but the dynamic range of the best modern stereo headphones exceeds 100 dB. An amplifier with such dynamics is more expensive than some cars, and its power will be from 200 watts per channel, which is too much for an ordinary apartment: listening at a very low power level 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 a makeweight are obviously too high.

The diagram of the simplest headphone amplifier on transistors is given in pos. 1 fig. Sound - except for Chinese "buttons", works in class B. It also does not differ in efficiency - 13-mm lithium batteries last for 3-4 hours at full volume. At pos. 2 - TDA classic for on-the-go headphones. The sound, however, gives quite decent, up to average Hi-Fi, depending on the parameters of the track digitization. Amateur improvements to the TDA7050 strapping are innumerable, but no one has yet achieved the transition of sound to the next level of class: the “mikruha” itself does not allow. TDA7057 (pos. 3) is simply more functional, you can connect the volume control on a regular, not dual, potentiometer.

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

Preamplifier circuit with tone control.

Hello friends. The article below presents the project of a preamplifier from Maxim Vasiliev, which is essentially a remake of the Sukhov preamplifier by transferring the circuit from the 157 series of microcircuits to import. You can find more detailed information on the KOTE and the vegalab forum on the request "Vassiliev complete amplifier". Schematic diagram:

Click on the image to enlarge the image.

The circuit uses dual operational amplifiers. For example, you can put OPA2134P, TL072 or NE5532, as you like, or which of these is currently at hand. The following figure shows the pinout of the microcircuits, the above ones have the same pinout, so no matter which MS you use, you do not need to make any changes to the board:

We will not write about which microcircuits sound better, you can find a lot of information about this on amateur radio forums, and there are plenty of them on the network.

Power supply bi-polar +/- 12 ... 15 Volts.

Variable resistors of group “A” (imported) are used as volume, balance and timbre controls, if you use domestic variables, choose with group “B”

The printed circuit board is made of double-sided fiberglass. The top layer is not etched, it is used as a screen. The dimensions of the board are 70x158 mm.

The appearance of the printed circuit board is shown in the following two figures:

A bipolar voltage regulator 2 x 15 Volts on 78L15 and 79L15 microcircuits has been added to the board. The figure below shows the pinout of the 2N5551 transistor:

The circuit diagram and printed circuit board in LAY format can be downloaded via a direct link from our website. The size of the archive file for downloading is 0.53 Mb.

Don't dream, act!

Experiments with various preamplifiers, volume and tone controls have shown that the best sound quality is provided with a minimum number of amplifying stages, with passive controls. In this case, adjustments at the input of the power amplifier are undesirable, since they lead to an increase in the level of non-linear distortions of the complex. This effect was recently discovered by the well-known developer of audio equipment Douglas Self.

Thus, the following structure of this part of the sound amplifying path emerges:
- passive bridge regulator of low and high frequencies,
- passive volume control
- preamplifier with a linear frequency response (AFC) and minimal distortion in the operating frequency range.
The obvious disadvantage of adjustments at the input of the preamplifier is that the deterioration of the signal-to-noise ratio is largely offset by the high signal level of modern sound reproduction devices.

Proposed preamplifier can be used in high quality stereo audio amplifiers. The tone control allows you to adjust the amplitude-frequency response (AFC) simultaneously on two channels in two frequency areas: lower and upper. As a result, the features of the room and acoustic systems, as well as the personal preferences of the listener, are taken into account.

And again a little history

The first contender for the role of a preamplifier with a tone control was D. Starodub's circuit (Fig. 1). But the design never "took root" in the power amplifier: careful shielding and a power supply with an extremely low ripple level (about 50 μV) were required. However, the main reason was the lack of sliding variable resistors.

Rice. 1. Diagram of a high-quality tone control block

Through trial and error, I came up with a simple preamplifier circuit (Fig. 2), with which, however, the sound reproduction system far exceeded the sound of mass-produced equipment, at least, that my friends and acquaintances had.

Rice. 2. Schematic diagram of one channel of the preamplifier for UMZCH S. Batya and V. Sereda

The pre-amplifier circuit of a stereophonic electrophone by Yu. Krasov and V. Cherkunov, which was demonstrated at the 26th All-Union Exhibition of Radio Amateurs-Designers, was taken as a basis. This is the left side of the circuit, including the tone controls.

The appearance of a cascade on transistors of different conductivity in the preamplifier (VT3, VT4) is associated with a discussion of amplifiers with the teacher of the laboratory of television technology at the Department of Radio Systems A. S. Mirzoyants, with whom I worked as a student. In the course of the work, linear cascades were needed to amplify the television signal, and Alexander Sergeevich reported that, according to his experience, top-down structures, as he put it, have the best characteristics, that is, amplifiers based on transistors of the opposite structure with direct connection. In the process of experimenting with UMZCH, I found out that this applies not only to television equipment, but also to sound amplification. Subsequently, I often used similar circuits in my designs, including a pair of field-effect transistor - bipolar transistor.

An attempt to use transistors of different structures in the first stage (composite emitter follower VT1, VT2) did not bring success, because with all the remarkable characteristics (low noise, low distortion), the circuit had a significant drawback - lower overload capacity compared to the emitter follower.
Pre-Amplifier Specifications:
Input resistance, kOhm = 300
Sensitivity, mV= 250
Depth of tone control, dB:
at a frequency of 40 Hz=± 15
at a frequency of 15 kHz=± 15
Depth of stereo balance adjustments, dB=± 6

Since new ideas arose during the design of amplifiers, I gave old designs to someone, or sold them at a fixed rate of watts of output power / ruble. On one of my trips to Leningrad, I took this amplifier with me to sell it to a friend of mine. Volodya said that this guy had a bunch of all kinds of Western equipment, and took the device to him for listening. In the evening he told me the results: the young man turned on the amplifier, listened to a couple of things and was so satisfied with the sound that he gave the money without a word.

To be honest, when I found out that the comparison would be made with imported equipment, I did not particularly hope that the amplifier would impress. In addition, it was not fully completed - there were no top and side covers.

Consider the schematic diagram of one channel of the preamplifier (Fig. 2). High-impedance volume (R2.1) and balance (R1.1) controls are installed at the input. From the middle output of the resistor R2.1 through the transition capacitor C2, the sound signal is fed to the composite emitter follower VT1, VT2, which is necessary for the normal operation of the passive tone control, made according to the bridge circuit. In order to eliminate the attenuation introduced by the tone block and amplify the signal to the required level, a two-stage amplifier based on transistors VT3, VT4 is installed.

The power supply of the preamplifier is unstabilized, from the positive arm of the power amplifier. The supply voltage is supplied to the stages VT3, VT4 through the filter R17, C10, C13, and to the input emitter follower - R8, C4. An important role is played by the diode VD1: without it, it was not possible to completely eliminate the background of an alternating current with a frequency of 100 Hz at the output of the power amplifier.

Structurally, the pre-amplifier is made in a "line", all parts are installed on a printed circuit board, closed on top with a U-shaped screen made of steel 0.8 mm thick.

--
Thank you for your attention!

The calculation was made according to the following relations: R1 = R3; R2 = 0.1R1; R4 = 0.01R1; R5 = 0.06R1; C1[nF] = 105/R3[Ohm]; C2=15C1; C3=22C1; C4 = 220C1.
With R1=R3=100 kOhm, the tone block will introduce attenuation of about 20 dB at a frequency of 1 kHz. You can take variable resistors R1 and R3 of a different value, let, for definiteness, there were resistors with a resistance of 68 kOhm. It is easy to recalculate the values of fixed resistors and capacitors of the bridge tone control without referring to the program or table. 1: we reduce the resistance values of the resistors by 68/100=0.68 times and increase the capacitances of the capacitors by 1/0.68=1.47 times. We get R1 \u003d 6.8 kOhm; R3=680 Ohm; R4=3.9 kOhm; C2=0.033uF; C3=0.33uF; C4=1500 pF; C5 \u003d 0.022 uF.

For smooth tone control, variable resistors with an inverse logarithmic dependence (curve B) are required.
The program allows you to visually view the work of the designed tone control. Tone Stack Calculator 1.3(Fig. 9).

Rice. 9. Simulation of tone controls for the circuit shown in fig. 8

Program Tone Stack Calculator is designed to analyze seven typical schemes of passive tone controls and allows you to immediately show the frequency response when changing the position of the virtual controls.

Rice. 11. Schematic diagram of the tone block and preamplifier for the "student" UMZCH

An experimental test of several instances of operational amplifiers showed that even without a capacitor in the grounded branch of the negative feedback divider, the constant voltage at the output is a few millivolts. However, for reasons of versatility of application, isolation capacitors (C1, C6) are included at the input of the tone block and the output of the preamplifier.
Depending on the required sensitivity of the amplifier, the resistance value of the resistor R10 is selected from Table. 2. One should strive not for the exact value of the resistances of the resistors, but for their pairwise equality in the channels of the amplifier.

table 2

▼ 🕗 25/02/12 ⚖️ 11.53 Kb ⇣ 149 Hello reader! My name is Igor, I'm 45, I'm a Siberian and an avid amateur electronics engineer. I came up with, created and maintain this wonderful site since 2006.
For more than 10 years, our magazine exists only at my expense.

Good! The freebie is over. If you want files and useful articles - help me!

--
Thank you for your attention!
Igor Kotov, editor-in-chief of Datagor magazine

The main disadvantage of a passive tone control is the low gain. Another drawback is that in order to obtain a linear dependence of the volume level on the angle of rotation, it is necessary to use variable resistors with a logarithmic control characteristic (curve "B").
The advantage of passive tone controls is less distortion than active ones (for example, Baksandal's tone control, Fig. 12).

Rice. 12. Active tone control P. Baksandala

As can be seen from the diagram shown in Fig. 12, the active tone control contains passive elements (resistors R1 - R7, capacitors C1 - C4) included in the 100% parallel negative voltage feedback of the operational amplifier DA1. The transmission coefficient of this regulator in the middle position of the R2 and R6 tone control sliders is equal to one, and variable resistors with a linear regulation characteristic are used for adjustment (curve "A"). In other words, an active tone control is free from the drawbacks of a passive tone control.
However, in terms of sound quality, this regulator is clearly worse than the passive one, which is noticed even by inexperienced listeners.

Rice. 13. Placement of parts on the printed circuit board

Items related to the right channel of the preamplifier are marked with a dash. The same marking is made in the printed circuit board file (with the *.lay extension) - the inscription appears when the cursor is moved to the corresponding element.
First, small-sized parts are installed on the printed circuit board: wire jumpers, resistors, capacitors, ferrite "beads" and a socket for the microcircuit. Lastly, terminal blocks and variable resistors are mounted.
After checking the installation, turn on the power and control the "zero" at the outputs of the operational amplifier. The offset is 2 - 4 mV.
If desired, you can drive the device from a sinusoidal generator and take characteristics (Fig. 14).

Rice. 14. Preamplifier characterization setup

--
Thank you for your attention!
Igor Kotov, editor-in-chief of Datagor magazine

Mentioned sources

1. Digest // Radiohobby, 2003, No. 3, pp. 10, 11.
2. Starodub D. The block of tone controls for a high-quality bass amplifier // Radio, 1974, No. 5, p. 45, 46.
3. Shkritek P. Reference guide to sound circuitry. – M.: Mir, 1991, p. 150 - 153.
4. Shikhatov A. Passive tone controls // Radio, 1999, No. 1, p. 14, 15.
5. Rivkin L. Calculation of tone controls // Radio, 1969, No. 1, p. 40, 41.
6. Solntsev Yu. High-quality pre-amplifier // Radio, 1985, No. 4, pp. 32 - 35.
7. //www.moskatov.narod.ru/ (Program of E. Moskatov "Timbreblock 4.0.0.0").

Vladimir Mosyagin (MVV)

Russia, Veliky Novgorod

I have been interested in amateur radio since the fifth grade of high school.
Diploma specialty - radio engineer, Ph.D.

The author of the books "To a young radio amateur for reading with a soldering iron", "Secrets of amateur radio skill", co-author of a series of books "For reading with a soldering iron" in the publishing house "SOLON-Press", I have publications in the journals "Radio", "Instruments and Experimental Techniques", etc. .

Reader's vote

The article was approved by 70 readers.

To participate in the voting, register and enter the site with your username and password.

No less important part of the ULF than the power amplifier is the preamplifier, in which not only the signal is pre-amplified, but also its frequency correction using the tone control.

Transistor preamplifier

The Radiochip website shows a simple electrical circuit of a preliminary ULF with a tone control for low and high frequencies and a volume control. The transistor VT1 is made not so much a preamplifier as an active tone control.

The timbre at low frequencies is regulated by a variable resistor R2. The timbre at high frequencies is regulated by a variable resistor R4. A frequency-dependent bridge is included between the input and output of the cascade on VT1, turning it into an adjustable active filter.

The input signal goes directly to the tone control circuit without any pre-stages. If the output impedance of the signal source is small, this is quite acceptable. But with a high impedance output, for example, if an old turntable with a piezoelectric pickup is to serve as the signal source, you need to do

a preliminary stage to increase the input resistance, for example, according to the emitter follower circuit, as shown in Figure 2. In this case, the input signal is fed to the base of VT2, and the signal to the input of the active tone control is taken from its emitter. The operation mode of the cascade is set by selecting the resistance of the resistor R10.

Recently, a certain person turned to him with a request to assemble an amplifier of sufficient power and separate amplification channels for low, medium and high frequencies. before that, I had already collected it for myself more than once as an experiment, and, I must say, the experiments were very successful. The sound quality of even inexpensive speakers of a not very high level is noticeably improved compared to, for example, the option of using passive filters in the speakers themselves. In addition, it becomes possible to quite easily change the crossover frequencies and the gain of each individual band and, thus, it is easier to achieve a uniform frequency response of the entire sound amplifying path. In the amplifier, ready-made circuits were used, which had previously been tested more than once in simpler designs.

Structural scheme

The figure below shows the diagram of 1 channel:

As you can see from the diagram, the amplifier has three inputs, one of which provides for a simple possibility of adding a preamplifier-corrector for a vinyl player (if necessary), an input switch, a preamplifier-timbral lock (also three-band, with adjustable HF / MF / LF levels), volume control, filter unit for three bands with adjustable gain level for each band with the ability to turn off filtering, and a power supply for high-power final amplifiers (unstabilized) and a stabilizer for the “low-voltage” part (preliminary amplification stages).

Pre-amplifier tone block

A scheme was used as it, which had been tested more than once before, which, with its simplicity and availability of parts, shows quite good characteristics. The scheme (like all subsequent ones) was once published in the Radio magazine and then published more than once on various sites on the Internet:

The input stage on DA1 contains a gain level switch (-10; 0; +10 dB), which simplifies the coordination of the entire amplifier with signal sources of different levels, and the tone control is directly assembled on DA2. The circuit is not capricious to some variation in the values of the elements and does not require any adjustment. As an op amp, you can use any microcircuits used in the audio paths of amplifiers, for example, here (and in subsequent circuits) I tried imported BA4558, TL072 and LM2904. Any one will do, but it is better, of course, to choose op-amp options with the lowest possible level of intrinsic noise and high speed (input voltage rise ratio). These parameters can be found in reference books (datasheets). Of course, it is not at all necessary to use this particular scheme here; it is quite possible, for example, to make not a three-band, but a regular (standard) two-band timbre block. But not a "passive" circuit, but with amplification-matching stages at the input and output on transistors or op-amps.

Filter block

Filter circuits, also, if desired, you can find a lot, since there are enough publications on the topic of multiband amplifiers now. To facilitate this task and just as an example, I will give here a few possible schemes found in various sources:

- the circuit that was used by me in this amplifier, since the crossover frequencies turned out to be just the ones that the “customer” needed - 500 Hz and 5 kHz, and nothing had to be recalculated.

- the second scheme, simpler on the OS.

And another possible circuit, on transistors:

As yours already wrote, I chose the first scheme because of the rather high-quality band filtering and the compliance of the band separation frequencies with the given ones. Only at the outputs of each channel (band) were added simple gain level controls (as is done, for example, in the third circuit, on transistors). Regulators can be set from 30 to 100 kOhm. Operational amplifiers and transistors in all circuits can be replaced with modern imported ones (taking into account the pinout!) To obtain better circuit parameters. All these schemes do not require any adjustment, if it is not required to change the crossover frequencies. Unfortunately, I don’t have the opportunity to give information on the recalculation of these section frequencies, since the circuits were searched for “ready-made” examples and detailed descriptions were not attached to them.

In the filter block circuit (the first circuit of three), the ability to disable filtering for the midrange and high-frequency channels was added. For this, two push-button switches of the P2K type were installed, with which you can simply close the connection points of the filter inputs - R10C9 with their corresponding outputs - “high-frequency output” and “mid-range output”. In this case, the full sound signal goes through these channels.

Power Amplifiers

From the output of each filter channel, the HF-MF-LF signals are fed to the inputs of power amplifiers, which can also be assembled according to any of the known schemes, depending on the required power of the entire amplifier. I made UMZCH according to the scheme known for a long time from the Radio magazine, No. 3, 1991, p.51. Here I give a link to the "original source", since there are many opinions and disputes about this scheme about its "quality". The fact is that at first glance this is a class “B” amplifier circuit with the inevitable presence of “step” type distortions, but this is not so. The circuit uses current control of the output stage transistors, which allows you to get rid of these shortcomings with the usual, standard inclusion. At the same time, the circuit is very simple, not critical to the parts used, and even transistors do not require special preliminary selection in terms of parameters. In addition, the circuit is convenient in that powerful output transistors can be placed on one heat sink in pairs without insulating gaskets, since the collector leads are connected at the point " output", which greatly simplifies the installation of the amplifier:

When setting up, it is only IMPORTANT to choose the correct operating modes for the transistors of the final stage (by selecting resistors R7R8) - on the bases of these transistors in the “rest” mode and without load, the output (speaker) should have a voltage within 0.4-0.6 volts. The supply voltage for such amplifiers (there should be 6 of them, respectively) was raised to 32 volts with the replacement of the output transistors with 2SA1943 and 2SC5200, the resistance of the resistors R10R12 should also be increased to 1.5 kOhm (to "make life easier" for the zener diodes in the circuit power input op amps). The op amps were also replaced by the BA4558, and the “zero setting” circuit is no longer needed (outputs 2 and 6 in the diagram) and, accordingly, the pinout changes when soldering the microcircuit. As a result, when checking each amplifier according to this scheme, it gave out power up to 150 watts (for a short time) with a completely adequate degree of heating of the radiator.

ULF power supply

As a power supply, two transformers with rectifiers and filters were used according to the usual, standard scheme. To power the low-frequency band channels (left and right channels) - a 250-watt transformer, a rectifier on diode assemblies of the MBR2560 type or similar, and capacitors 40,000 microfarads x 50 volts in each power arm. For midrange and high-frequency channels - a 350-watt transformer (taken from a burned-out Yamaha receiver), a rectifier - a TS6P06G diode assembly and a filter - two capacitors of 25,000 microfarads x 63 volts for each power arm. All electrolytic filter capacitors are shunted with film capacitors with a capacity of 1 microfarad x 63 volts.

In general, the power supply can be with one transformer, of course, but with its corresponding power. The power of the amplifier as a whole in this case is determined solely by the capabilities of the power source. All preamplifiers (tone block, filters) are also powered from one of these transformers (it is possible from any of them), but through an additional bipolar stabilizer unit assembled on a Kren (or imported) MS or according to any of the typical transistor circuits.

The design of a homemade amplifier

This, perhaps, was the most difficult moment in manufacturing, since there was no suitable ready-made case and I had to invent possible options :-)) In order not to sculpt a bunch of separate radiators, I decided to use a radiator case from a car 4-channel amplifier, quite large, something like this:

All the "insides" were, of course, extracted and the layout turned out to be something like this (unfortunately I did not take a corresponding photo):

- as you can see, six terminal UMZCH boards and a pre-amplifier-tone block board were installed in this radiator cover. The board of the filter block no longer fit, so it was fixed on the then added aluminum corner structure (it can be seen in the figures). Also, transformers, rectifiers and power supply filters were installed in this "framework".

The view (front) with all the switches and controls turned out like this:

Rear view, with speaker output blocks and fuse box (since no electronic protection circuits were made due to lack of space in the design and in order not to complicate the circuit):

In the future, the frame from the corner is supposed, of course, to be covered with decorative panels to give the product a more “tradeable” look, but this will be done by the “customer” himself, according to his personal taste. But in general, in terms of sound quality and power, the design turned out to be quite decent. Material author: Andrey Baryshev (especially for the site website).

Assembling a tube preamplifier. Assembling a tube preamplifier Circuits of preamplifiers on microcircuits