Preliminary studies High quality preamplifier. Economical ULF with three transistors

Good afternoon.

I would like to continue the story about a tube preamp for a hybrid amplifier.

Complete preamp circuit:

The scheme is very simple. We didn't invent anything. The basis chosen last time is a resistive cascade. There is nothing unusual about it.

Active filters on transistors VT1 and VT2 were added to the circuit. They provide additional cleaning nutrition. Since the main filtering will be done external source, then the filter circuits were simplified - they were made single-stage.

We plan to power the filament from an external stabilized source. Using powerful filtering of all voltages will ensure that there is no background.

It's time to collect

With the prototype board, everything is as usual: we draw, print, translate, etch, drill and clean it with fine sandpaper... After that, put a respirator on your face, a can of black heat-resistant paint in your hands... paint the board black. This way it will not be visible in the body of the assembled amplifier.

Set the board aside and let it dry. It's time to shake out the boxes and pick up the parts. Some of the components are new, others are soldered from early prototypes (well, good, almost new components shouldn’t go to waste?!).

Everything is ready for assembly, it's time to turn on the soldering iron.

The soldering iron is hot - solder:

Note: It is more convenient to solder, starting with the lowest profile components and moving to higher ones. Those. First we solder diodes, zener diodes, then resistors, a socket for a lamp, capacitors, etc... We, of course, broke this sequence and soldered as necessary :)

Capacitors installed. IN this project domestic K73-16 were used. Good capacitors. We carried out a series of measurements of their nonlinearity spectra for them in different modes. The results were encouraging. We will definitely write about this someday.

We solder resistors and other small things

We install the socket and electrolytic capacitors.

Note: When soldering a lamp socket, you must insert a lamp into it. If this is not done, then after assembly there may be problems with installing the lamp. In some (the most “severe” cases) you can even damage the lamp base.

All the details are in place. The preamp is ready.

Checking

The scheme is simple and the likelihood of error is minimal. But you need to check. Connect the amplifier to the power source and turn on:

10 seconds - normal flight... 20... 30... everything is fine: nothing exploded or started smoking. The glow glows quietly, the test power supply protections do not operate. You can exhale with relief and check the modes: all deviations are within acceptable limits for an unheated lamp.

After a 10-minute warm-up, all parameters were established and reached the calculated values. The operating point is set.

Since everything is good, we can continue. We connect a test signal source to the input. At the output there is a resistor simulating the input resistance of a power amplifier. We turn on and measure all the main parameters of the cascade.

Everything is within normal limits. The distortion and gain coincided with what was obtained in the previous article. There is no background.

So our tube preamplifier is ready. It's time to move on to creating a powerful transistor output buffer for it. It can be used with the same success in a purely tube design. To do this, you will need to make a powerful tube output for it.

Perhaps it makes sense to make a universal tube preamplifier (maybe in the form of a designer) for use in tube and hybrid designs?

Best regards, Konstantin M.

— The neighbor started knocking on the radiator. I turned up the music 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 century digital technologies are inextricably linked and can become 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 initial training on the basis of school physics on the same table, go from the simplest designs for half an evening (which, nevertheless, “sing” well) to the most complex units, through which even 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. To thoroughly understand audio engineering, you will have to gradually master quite a lot theoretical material and don’t forget to enrich your knowledge 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 permissible 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.

This baby is convenient for learning the basics of setting up a 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 half of its value is set in it, 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 Required additional cooling, are outlined in dotted lines on 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.

This UMZCH reveals 2 more tricks: firstly, in a very powerful amplifiers The power output cascade also needs to be cooled, so VT3 is installed on a radiator of 100 sq. 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, to balance (R6, guessed it?), the amplifier is powered from +/–24 V, and instead of the load it is turned on wirewound resistor 100…200 Ohm. By the way, the squiggles in some resistors in the diagram are Roman numerals indicating them required power heat dissipation.

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 IP switches on ceramic 0.01 μF to prevent self-excitation at ultra audio frequencies ah, capable of instantly burning 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 operates at the appropriate level). Acustic systems, AC). 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 pulse sources food, eg. computer When trying to “drive” them into active mode, necessary 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, eg. from line scan 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 instrument fleet - you need an oscilloscope, an audio frequency generator (AFG) and a millivoltmeter alternating current with the ability to measure the constant 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, coefficient nonlinear distortion(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 the original ones are given printed circuit boards and setup instructions. 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: gaps in the tracks to which the bases are connected powerful transistors– technological, for setting up, 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 R12, R13 will not be needed powerful version or R10 R11 in a less powerful one (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 in the background purest sound 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: cable twisted pair 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, fluoroplastic tape 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 engineering we had to deal with the concept of Hi-Fi (High Fidelity), high fidelity sound playback. There are hi-fi different levels, which are ranked next. 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. SOI on peak power 1% or 2% is accepted 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: attitude absolute levels 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 forest 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 from 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 (CHD). 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.

As is known, the rated output voltage of modern audio frequency signal sources (3Ch) does not exceed 0.5 V, while the rated input voltage of most 3Ch power amplifiers (UMZCH) is usually 0.7..1 V. To increase the signal voltage to level that ensures normal operation of the UMZCH, as well as to match the output impedances of the signal sources with its input impedance, 3CH pre-amplifiers are used. As a rule, it is in this part of the sound reproduction path that volume, timbre and stereo balance are adjusted. The main requirements for preamplifiers are low nonlinear signal distortion (harmonic distortion - no more than a few hundredths of a percent) and a low relative level of noise and interference (not higher than -66..-70 dB), as well as sufficient overload capacity. All these requirements are largely met by the pre-amplifier of the Muscovite V. Orlov (he took the AU-X1 amplifier circuit of the Japanese company "Sansui" as a basis). The nominal input and output voltages of the amplifier are 0.25 and 1 V, respectively, the harmonic coefficient in the frequency range 20..20000 Hz at the rated output voltage does not exceed 0.05%, and the signal-to-noise ratio is 66 dB. The input impedance of the amplifier is 150 kOhm, tone control limits (at frequencies of 100 and 10000 Hz) from -10 to +6 dB. The device is designed to work with UMZCH, the input impedance of which is at least 5 kOhm.

The amplifier (Fig. 1 shows a schematic diagram of one of its channels) consists of a source follower on transistor VT1, a so-called bridge passive tone control (elements R6-R11.1, C2-C8) and a three-stage symmetrical signal voltage amplifier. The volume control - variable resistor R1.1 - is included at the amplifier input, which reduces the likelihood of its overload. The timbre in the region of lower frequencies of the audio range is controlled by a variable resistor R7.1, in the region of higher frequencies - by a variable resistor R11.1 (resistors R7.2 and R11.2 are used in another channel of the amplifier). The transfer coefficient of a symmetrical amplifier is determined by the ratio of the resistances of resistors R18, R17 and, with the values ​​​​indicated in the diagram, is approximately 16. The operating mode of the final stage transistors (VT6, VT7) is determined by the voltage drop created by the collector currents of transistors VT4, VT5 on diodes VD1 connected in the forward direction - VD3. Trimmer resistor R15 serves to balance the amplifier. The amplifier can be powered either from the source that powers the UMZCH, or from any unstabilized rectifier with output voltages of +18..22 and -18..22 V.

A possible version of the printed circuit board for one channel of the device is shown in Fig. 2.

It is made of foil fiberglass laminate with a thickness of 1.5 mm and is designed for the installation of resistors MLT and SP4-1 (R15), capacitors MBM (C1, C4, C8, C11), BM-2 (C3, C5-C7) and K50-6 , K50-16 (rest). Capacitors MBM and BM-2 are mounted vertically on the board (one of their terminals is extended to the locally required length using tinned wire with a diameter of 0.5..0.6 mm). Double variable resistor R1 of any type of group B, resistors R7 and R11 - group B. Transistors KP303D can be replaced with KP303G, KP303E, transistor KP103M - with KP103L, transistors KT315V and KT361V - with transistors of these series with index G. Field-effect transistors must be selected according to the initial drain current, which at voltage Uс=8 V should not go beyond 5.5..6.5 mA. D104 diodes are completely interchangeable with diodes of the D220, D223, etc. series. The adjustment comes down to setting the trimmer resistor R15 to zero voltage at the output and selecting the resistor R18 until an output voltage equal to 1 V is obtained at an input voltage of 250 mV with a frequency of 1000 Hz (the sliders of resistors R7, R11 are in the middle position, and resistor R1 is in the upper position in the circuit ).

A significant drawback of the one described, and many other similar devices using transistors, is the relatively large number of elements and, as a consequence of this, the rather large dimensions of the circuit board. Pre-amplifiers based on operational amplifiers (op-amps) are much more compact.

An example is a device developed by Muscovite Yu. Solntsev based on the general-purpose OS K574UD1A (Fig. 3).

His studies showed that the harmonic coefficient of this op-amp strongly depends on the load: it is quite acceptable when its resistance is more than 100 kOhm, it increases to 0.1% when the load resistance decreases to 10 kOhm. To obtain sufficiently small nonlinear distortions, the author added to the specified op-amp a so-called parallel amplifier, characterized by the practical absence of “step” distortion even without negative feedback (NFB). With OOS, the harmonic coefficient does not exceed 0.03% in the entire audio frequency range with a load resistance of more than 500 Ohms. The remaining parameters of the pre-amplifier are as follows: nominal input and output voltages 250 mV, signal-to-noise ratio at least 80 dB, overload capacity 15..20 dB. As can be seen from the diagram, the device consists of a linear amplifier with a horizontal frequency response using op-amp DA1 and transistors VT1-VT4 ("parallel" amplifier) ​​and a passive bridge tone control (elements R12-R14, R17-R19, C6-C9). If necessary, this regulator can be excluded from the path using relay K1 (the signal in this case is removed from the voltage divider R10R11). The amplifier's transmission coefficient is determined by the ratio of the resistance of resistor R3 to the total resistance of resistors R2, R4. The bridge regulator has no special features. At lower frequencies, the timbre is adjusted with a variable resistor R18.1, at higher frequencies with a resistor R13.1. Resistors R12, R14 prevent monotonous rise and fall of the frequency response outside the nominal frequency range of the amplifier. For normal operation of the tone control, the load resistance must be at least 50 kOhm. When working with a signal source whose output voltage contains a constant component, it is necessary to turn on a separating capacitor at the amplifier input (shown in the diagram with dashed lines).

All parts of the amplifier, with the exception of the tone control elements, are mounted on a printed circuit board made of foil fiberglass (Fig. 4 shows part of it for one channel). The board is designed for mounting resistors MLT, SP4-1 (R4), capacitors K53-1a, K53-18 (C1, C4), KM-6B (C2, C3, C5, C6) and MBM (others). Double variable resistors R13 and R18 - any type of group B. The tone control elements are mounted directly on their terminals and connected to the board with shielded wires. Instead of those indicated in the diagram, transistors KT3107I, KT313B, KT361K (VT1, VT4) and KT312V, KT315V (VT2, VT3) can be used in the amplifier. Relay K1 - brand RES60 (passport RS4.569.436) or any other with suitable dimensions and operating current and voltage. Diode VD1 - any with a permissible reverse voltage of at least 50 V. For connection to the amplification path, a detachable connector MPH14-1 is used (its plug is installed on the board). To power the amplifier, a bipolar power supply is required, capable of delivering a current of about 30 mA to the load at a ripple voltage of no more than 10 mV (otherwise, if the installation is unsuccessful, a noticeable background may appear). Adjusting the amplifier comes down to setting the required transmission ratio with and without a connected tone control. In the first case, the desired result is achieved by changing the resistance of the tuning resistor R4 (and, if necessary, by selecting resistor R2), in the second, by selecting resistor R11. The amplifier is designed to work with UMZCH, described in the article by Yu. Solntsev “High-quality power amplifier” (Radio, 1984, No. 5, pp. 29-34). The volume control (double variable resistor of group B with a resistance of 100 kOhm) is switched on in this case between its input and the output of the pre-amplifier. The same resistor, but group A, is used as a stereo balance regulator (one of its outer terminals and the engine output in each channel is connected to the volume control slider, and the other outer terminal is connected to the UMZCH input).

In recent years, the industry has mastered the production of integrated circuits (ICs KM551UD, KM551UD2), specially designed for operation in the input stages of audio frequency paths of household radio equipment (preamplifiers-correctors of electric players, amplifiers for recording and playback of tape recorders, microphone amplifiers, etc. devices). They are distinguished by a reduced level of self-noise, low harmonic distortion, and good overload capacity.

Figure 5 shows the circuit of a pre-amplifier based on the KM551UD2 IC (proposed by Muscovite A. Shadrov). This IC is a dual op-amp with a supply voltage from +5 to +16.5 V. An IC with index A differs from a device with index B in half the input common-mode voltage (4 V) and the normalized noise voltage referred to the input (no more than 1 µV when the signal source resistance is 600 Ohms; for KM551UD2B it is not standardized). The nominal input and output voltages of this amplifier are the same as those of the device according to the circuit in Fig. 1, harmonic distortion in the frequency range 20..20000 Hz no more than 0.02%, signal-to-noise ratio (unweighted) 90 dB, Adjustment range volume and timbre (at frequencies 60 and 16000 Hz) respectively 60 and +10 dB, transition attenuation between channels in the frequency range 100..10000 Hz is not less than 50 dB. The input and output impedances of the amplifier are 220 and 3 kOhm, respectively. The bridge tone control is included in this case in the OOS circuit, covering the op-amp DA1.1 (hereinafter, the pin numbers of the second op-amp of the microcircuit are indicated in parentheses). At the input there is a fine-compensated volume control on a variable resistor R2.1 with a tap from a conductive element. Loudness compensation (raising low-frequency components at low volume levels) can be turned off using switch SA1.1. Stable operation of the KM551UD2 IC (its frequency response has three bends) is ensured by capacitor C7 and circuit R5C5, the values ​​of which are selected for the transfer coefficient Ki = 10 (the rate of rise of the output voltage with such amplification reaches 3..4 V/μs). Capacitors C12, C13 prevent the amplifier from interconnecting with other devices in the path when powered from a common source. The variable resistor R12.1 (in another channel R12.2) regulates the stereo balance.

All parts of the amplifier, except for variable resistors R2, R7, R11 and switch SA1, are mounted on a printed circuit board made of foil fiberglass. It is designed for the installation of MLT resistors, capacitors MBM (C1, C10), BM-2 (C3-C5, C11), KM (C6, C7, C12, C13) and K50-6, K50-16 (others). Capacitors MBM and BM-2 are mounted vertically. Any dual variable resistors of group A are suitable for regulating volume and stereo balance; resistors of group B are suitable for regulating tone. The amplifier does not require adjustment. The frequency response of bridge tone controls, as is known, has fixed inflection frequencies, therefore, in essence, only the slope of the frequency response sections to the left and right of these frequencies is smoothly adjusted, and its maximum value does not exceed 5..6 dB per octave. To obtain the required limits of tone control at higher and lower frequencies of the audio range, the inflection frequencies must be selected in the mid-frequency region. Such a regulator is ineffective if it is necessary to suppress low- or high-frequency interference in the signal spectrum. For example, with a corner frequency of 2 kHz, the tone control can reduce the level of interference at a frequency of 16 kHz by 15 dB, only at the same time attenuating the spectrum components of 8 and 4 kHz by 10 and 5 dB, respectively. It is clear that in such a case this is not a way out, therefore, to suppress interference at the edges of the spectrum, switchable low-pass (LPF) and high-pass (HPF) filters with a large frequency response slope outside the transparency band are sometimes used to suppress interference. However, even in this case, the desired result is not always achieved, since these filters usually have fixed cutoff frequencies. It's a different matter if the filters are made tunable in frequency. Then, by smoothly shifting the boundaries of the transmitted frequency range in the desired direction, it will be possible to “remove” the interference beyond its limits without affecting the shape of the frequency response within the range. By the way, it is advisable to make such filters non-switchable: they will help combat infra-low-frequency interference from the mechanism of an insufficiently advanced electric player.

HIGH QUALITY PRE-AMPLIFIER CIRCUIT

At the turn of 2004 and 2005, a natural desire arises to build amplifiers on a modern element base, taking advantage of the advanced achievements of global electronic technology.
I bring to your attention a high-quality preamplifier based on the EL2125.
The basic materials are FREE and DIYers are free to use them to replicate them in their own designs.
WHY EL2125?
An excellent chip, according to its characteristics it almost ranks 2nd in the top ten op amps according to model reviews in 2004.
This is, of course, not the AD8099 (first place in the world, award from Intel “Innovation of 2004”), but the EL2125 has already appeared on the CIS market and it is quite possible to get it, especially for those who live in capitals and large cities.
JUDGE FOR YOURSELF HOW GOOD THE EL2125'S CHARACTERISTICS ARE:

Ability to operate on loads up to - 500 Ohm
Operating frequency range up to - 180 MHz
Supply voltage - ±4.5 ... ±16.5 V.
Nonlinear distortion coefficient - less than 0.001%
Output slew rate - 190 V/µs
Noise level - 0.86 nV/vHz (better than AD8099!!!)

The EL2125 retail price is usually $3 each, not very cheap, but worth it.
Most often, EL2125 is found in SO-8 type housing (prepare micro-tips for soldering irons).
I should note that I would add “amazing musicality” to the list of characteristics. This indicator cannot be measured with instruments and expressed in numbers; it is felt only by ear.

1. As an amplifier for phones with a wide range of impedances:

2. As a high-quality preamplifier for power amplifiers with bipolar power supply (ranging from ± 22 to ± 35 V.) and sensitivity 20 ... 26 dB:

This op-amp involuntarily suggests itself as a more serious pre-amplifier, created on the basis of the Solntsev amplifier and described on the Soldering Iron website:
The amplifier uses dual variable resistors R11 and R17 of any type of group B, R1 and R21 of any type of group B or A. A 100 kOhm variable resistor (tapped from the middle) can be used as a loud-compensated volume control (R21). Transistors can be replaced with KT3107I, KT313B, KT361V,K (VT1, VT4) and KT312V, KT315V (others). Replacing the K574UD1 op amp with other types of op amp is not recommended. If the DC component is at a significant level (in rare cases) at point A, it is necessary to install a capacitor with a capacity of 2.2 - 5 μF.

The described preamplifier is connected to an AF power amplifier with an input impedance of at least 10 kOhm. With a significant increase in Kg, this control unit can also be loaded onto an UMZCH with Rin up to 2 kOhm (which is extremely undesirable), in such cases (if the Rin of your UMZCH is less than 10 kOhm), you just need to once again power up the output stage (a copy of the circuit section VT1-VT2- VT3-VT4-R4-R5-R6-R7, connect to output DA2), connect resistors R23 and R24 in the same way as resistors R2 and R3, although in this case the noise level may increase. And if Rin of your UMZCH is greater than or equal to 100 kOhm, then it is recommended to use K574UD1A(B) as an operational amplifier DA2, this will reduce the level of distortion and noise.

Possible changes in the scheme (improving):
- To exclude P2K switches (very unreliable in operation) from the audio signal path, it is recommended to exclude switch SA1 from the circuit (together with resistors R8, R9), and move switch SA2 to the last stage by short-circuiting resistor R23 (resistors R13, R14 are excluded in this case from the diagram).

Preamp circuit:

It would also not be useless to use this op-amp in a universal pre-amplifier that can also serve as a headphone amplifier. The circuit diagram is shown below:

Emitter followers VT1-VT2 unload the output of the op-amp, and then follows a circuit with local feedback, which further reduces non-linear distortions. Resistors R19 and R20 set the quiescent current of the window stage of the preamplifier, similar to power amplifiers, within 7-12 mA. In this regard, the last stage must be installed on a small heat sink

The page was prepared based on materials from the site http://yooree.narod.ru and http://cxem.net

Modern digital sources sound (CD players, DACs, etc.) have a very low noise level. Much lower than vinyl or magnetic tape. Because of this, the noise requirements of the subsequent amplification path today have become much higher than in the era of analogue audio. In light of these requirements, the preamplifier described below was designed with the primary goal of achieving high-quality sound at ultra-low noise levels without the use of exotic or expensive components.

In most stages the author used his favorite operational amplifiers NE5532, but in some nodes they are used LM4562, since recently they have become more accessible and allow one to obtain much less distortion when operating with a low-impedance load.

What kind of music lover (and even more so an audiophile) is without vinyl? It is for them that the preamplifier is equipped with two background correctors for different types of pickups. In addition, the design has tone control, visual level indicator And balanced outputs, which today has become almost a standard for high quality audio equipment.

The block diagram of the preamplifier is shown in the figure:

Click to enlarge

All modules are assembled on separate printed circuit boards, which simplifies their placement in the housing and facilitates switching.
This part of the series of articles describes the circuit of the amplifier itself with volume, balance and tone controls, as well as the organization of a symmetrical output.

Schematic diagram of the pre-amplifier module:

Click to enlarge

All resistances (not only resistors, but also the resistance of active components, for example the base resistance of a transistor) generate noises, the level of which depends on the resistance value and temperature. Since it is quite difficult to influence the temperature in the listening room, the only way to reduce the noise of the resistances is to reduce the value of the resistance itself. This implies the main feature of the presented scheme - the use low resistance resistors along the entire path of the sound signal.

If for constant resistors the choice of low-resistance ratings does not pose a problem, then for variable resistors (for volume, balance and tone controls) the nominal range is significantly limited. Typically in these circuits you can see variable resistors of 47 kOhm, 22 kOhm, or at best 10 kOhm. In this design, Douglas Self used 1kOhm variable resistors - this is perhaps the minimum value available among variable resistors.

By the way, here are the characteristics that we managed to achieve:

(Measurements were carried out at a supply voltage of 17V, with tone controls disabled, using balanced inputs and outputs)

Harmonic distortion + noise (input signal 0.2V, output signal - 1V)	0.0015% (1 kHz, B = 22 Hz to 22 kHz) 0.0028% (20 kHz, B = 22 Hz to 80 kHz)
Harmonic distortion + noise (input signal 2V, output signal - 1V)	0.0003% (1 kHz, B = 22 Hz to 22 kHz) 0.0009% (20 kHz, B = 22 Hz to 80 kHz)
Signal-to-noise ratio (at 0.2V input signal)	96 dB (B = 22 Hz to 22 kHz) 98.7 dBA
Reproducible frequency band:	0.2 Hz to 300 kHz
Maximum output signal level (at 0.2V input): 1.3V
Balance adjustment	+3.6 dB to -6.3 dB
Bass adjustment	±8 dB (100 Hz)
Treble adjustment	±8.5 dB (10 kHz)
Channel separation (R->L)	-98 dB (1 kHz) -74 dB (20 kHz)
Channel separation (L->R)	-102 dB (1 kHz) -80 dB (20 kHz)

The use of low-impedance resistors also reduces the biasing of op-amps by input currents, which also reduces noise caused by fluctuations in op-amp currents.

To reduce the noise of active components, a parallel connection is used in the circuit cascades. Of course, one could use modern low-noise op-amps like AD797. But this will be much more expensive and more complicated (since one package contains only one op-amp). Please note that we are not talking about parallel connection of microcircuits (when they are soldered on top of each other), but about parallel connection of amplifier stages. Only in this case the noise of the amplifying elements will be uncorrelated, due to which the overall noise level decreases by 3 dB when 2 stages are parallelized. When 4 stages are connected in parallel, the noise decreases by 6 dB, i.e. twice.

If 8 cascades are parallelized, the noise will decrease by 9 dB, but for such a gain the costs are unreasonably high.

Due to the use of low-resistance resistors in the tone control, the capacitor values ​​were much larger than usual. But today this is not a problem for modern element base.

Line input and balance control.

To reduce noise and interference, a filter R1C1 and R2C2 is installed directly at the amplifier input. Buffer stages IC1A and IC1B provide approximately 50kΩ input impedance and improve common mode rejection. The amplification stage itself is assembled on LM4562 (IC2A), the gain of which is adjusted by potentiometer P1A. The same potentiometer in the right channel is turned on “out of phase” with the left one, due to which the balance is adjusted. Feedback in the cascade is implemented through two parallel buffers IC3A and IC3b, due to which the cascade gain remains constant regardless of load changes. In addition, this solution reduces noise and provides low output impedance.

A typical implementation of a balance control usually negatively affects the stage and the “virtual” arrangement of instruments, which is why it is quite rare in Hi-End equipment. Douglas Self's solution to this node does not have this drawback.

The noise level of this part of the preamplifier is only -109 dB in the middle position of the balance control, -106 dB at the maximum and -116 dB at the minimum position of the control (in the frequency band 22 Hz to 22 kHz).

Tone control.

Despite the fact that the regulator looks somewhat unusual, nevertheless, the classic Baxandall tone control circuit is used here. As noted above, due to the low ratings of variable resistances, the capacitor ratings are significantly larger than the “typical” values.

Capacitor C7 (1 μF) determines the lower tone control frequency, and capacitors C8 and C9 have a value of 100 nF and determine the tone control frequency at HF. If desired, the depth of tone control can be increased to ± 10 dB. Due to the IC4 elements, the mutual influence of the low-frequency and high-frequency circuits when controlling timbres is eliminated.

Despite the large dimensions and high cost, the use of polypropylene capacitors.

The tone control noise level is only -113 dB in the middle position of the controls.

Relay RE1 serves to turn off the tone control if it is not needed. In this case, the signal is taken from the output of IC2A and goes directly to the input of IC9B, bypassing the tone control. To avoid clicks during switching, resistor R18 is used. To reduce crosstalk, switching in each channel is carried out by a separate relay. In this case, the relay contact groups can be parallelized, which will reduce the contact resistance and further increase the reliability of this part of the circuit.

Active volume control.

The volume control was also implemented according to the idea of ​​Peter Baxandall, which firstly made it possible to obtain ultra-low noise level(especially at low volumes), and secondly, to obtain a logarithmic control characteristic when using potentiometers with a linear dependence of resistance on the angle of rotation. The maximum gain is +16 dB, with the 0 dB point obtained at the middle position of the potentiometer.

Four amplifiers connected in parallel, as noted above, serve to reduce the noise level by 6 dB. The self-noise level of such a regulator is -101 dB at maximum gain and -109 dB at 0 dB gain. In practice, the volume control is usually set to -20 dB, then the noise level will be -115 dB, which is significantly below the threshold of audibility.

So that you can evaluate the quality of each cascade, their own noise levels are given. The resulting noise level of a given preamplifier, as you might guess, will vary somewhat depending on the position of the potentiometers.

Symmetrical output implemented using a phase inverter on the op-amp IC9A and has double the signal amplitude compared to an asymmetrical one. However, this is normal for professional audio equipment.

Design and setup.

Placement of amplifier elements on the board:

Click to enlarge

During assembly, the resistors are soldered first, and then the remaining components.
Jumper JP1 is designed to select the optimal ground connection for the vinyl corrector (there are similar jumpers on MC / MD boards). Don't forget to connect them. The connection location is selected experimentally after assembling the structure in the housing.

Photo of the assembled board:

Click to enlarge

This block of settings does not require.
Frequency characteristics of the amplifier and tone control:

Click to enlarge

List of elements:

Resistors:
(1% accuracy; metal-film; 0.25W)
R1,R2,R39,R40 = 100Ohm
R3-R6,R41-R44,R78,R79 = 100kOhm
R7-R12,R16,R17,R21-R24,R33,R34,
R45-R50,R54,R55,R59-R62,R71,R72 = 1kOhm
R13,R51 = 470Ohm
R14,R15,R52,R53 = 430Ohm
R18,R35,R36,R56,R73,R74 = 22kOhm
R19,R20,R57,R58 = 20Ohm
R25-R28,R63-R66 = 3.3kOhm
R29-R32,R67-R70 = 10Ohm
R37,R38,R75,R76 = 47Ohm
R77 = 120Ohm
P1,P2,P3,P4 = 1kOhm, 10%, 1W, stereo potentiometer, linear, for example Vishay Spectrol cermet type 14920F0GJSX13102KA. or, Vishay Spectrol conductive plastic type 148DXG56S102SP.

Capacitors:
C1,C2,C10-C14,C26,C27,C35-C39 = 100pF 630V, 1%, polystyrene, axial
C3,C4,C28,C29 = 47µF 35V, 20%, non-polar, 8mm diameter, 3.5mm pin spacing, e.g. Multicomp p/n NP35V476M8X11.5
C5,C6,C30,C31 = 470pF 630V, 1%, polystyrene, axial
C7,C32 = 1µF 250V, 5%, polypropylene, pin spacing 15mm
C8,C9,C33,C34 = 100nF 250V, 5%, polypropylene, lead spacing 10mm
C15,C16,C40,C41 = 220µF 35V, 20%, non-polar, 13mm diameter, 5mm pin spacing, e.g. Multicomp p/n NP35V227M13X20
C17-C25,C42-C50 = 100nF 100V, 10%, pin spacing 7.5mm
C51 = 470nF 100V, 10%, pin spacing 7.5mm
C52,C53 = 100µF 25V, 20%, diameter 6.3mm, pin spacing 2.5mm

Chips:
IC1,IC3,IC5-IC10,IC12,IC14-IC18 = NE5532, for example ON Semiconductor type NE5532ANG
IC2,IC4,IC11,IC13 = LM4562, for example National Semiconductor type LM4562NA/NOPB

Miscellaneous:
K1-K4 = 4-pin connector, pitch 0.1’’ (2.54mm)
K5,K6,K7 = 2-pin connector, pitch 0.1’’ (2.54mm)
JP1 = 2-pin jumper, pitch 0.1’’ (2.54mm)
K8 = 3-pin screw block, 5mm pitch
RE1,RE2 = relay, 12V/960Ohm, 230VAC/3A, DPDT, TE Connectivity/Axicom type V23105-A5003-A201

To be continued...

The article was prepared based on materials from the magazine “Elector” (Germany)

Happy creativity!

Editor-in-Chief of RadioGazeta