I’ll make a reservation right away - this anthology in no way claims to be a manual on lamp circuitry. Schemes (including historical ones) were selected by a combination technical solutions, if possible with "zest". And everyone has different tastes, so don’t exact if you didn’t guess right ... In the old schemes, a number of denominations are reduced to standard ones.

Skeptics claim that some schemes cannot sound at all "by definition". Here is one diagram that gives just that impression. But still it worked!

This diagram is taken as starting point. The amplifier is made on then new finger lamps, according to the classical scheme on pentodes without a common OOS. The treble tone control circuit is interestingly solved, but it can really work "on the rise" only with a high-quality output transformer. Since the amplifier was intended for an electric player, they saved on a power transformer. If, apart from the pickup, nothing else is connected to it, electrical safety is observed with some stretch. It's good to live in civilized countries - the sockets are correct. Here is phase, here is neutral, here is zero. And for some reason it is the same in all outlets. And in my apartment, for example, some of the switches were not in the phase wire, but in the zero one. What after that to demand from sockets ...

The pentodes in the first stage were abandoned rather quickly. Two triode cascades coped with this task no worse, and the sound quality increased. Further improvement was brought by ultra-linear circuits of the output stages. In this connection, the screen grid is connected to the tap of the primary winding of the output transformer. The resulting local OOS significantly reduces the output impedance of the cascade and increases its linearity, and the gain does not decrease much. True, the ultra-linear circuit was mainly used in push-pull amplifiers. Below is a diagram of a typical single-ended amplifier with an ultra-linear output stage.

fig.2

The values ​​of the parts in the tone control have been adjusted to meet modern requirements - in the original they only hunched the frequency response at 5 kHz. However, the rise of HF was rarely used at that time. Variants of this scheme blossomed wildly in the era of economic councils, when the party and the government decided to flood the country with cheap radio products. The ultra-linear cascade disappeared, the tone control was simplified, and the power transformer was often abolished altogether or only an incandescent one was installed. Saved on everything, and it is noticeable. Many people remember the sound of players in cardboard suitcases - a good middle, but there is nothing else.

When repeating the circuit, you can abandon the tone control, and with it, eliminate the first stage of the gain. Then in the two-channel version, only one double triode is needed for the driver. It is also possible to introduce a shallow FOS from the amplifier output into the cathode circuit of the first or second stage.

An increase in the depth of the OOS in tube amplifiers is prevented by the phase incursion on coupling capacitors. To eliminate this shortcoming, the interstage communication must be direct. And such a scheme appeared:

fig.3

Since the slope of the lamp decreases at low anode voltage, a pentode had to be used to obtain the necessary gain. Triodes with the necessary characteristics appeared later. Another highlight of the circuit is the inclusion of a bridge tone control in the amplifier's overall OOS circuit. The advantage of this solution is that at the maximum rise in the frequency response, input overload is excluded. If the adjustment is made in the preamplifier, there is a risk of such an overload. Therefore, the inclusion of regulators in the OOS circuit of the power amplifier was used for a long time and in amplifiers based on transistors and microcircuits. The sound quality, by the way, clearly benefits from this.

The direct heir to this scheme is the Gubin amplifier, a constant participant in Hi-End exhibitions. It can work with pentode and triode switching of the lamps of the output stage. For complete happiness, you can also provide an ultra-linear option.

fig.4

However, direct-coupling schemes also have disadvantages. The first is the need to apply anode voltage only after the cathodes have warmed up. Otherwise, high voltage on the grids can damage the lamps or shorten their life. To do this, you need to use devices for delaying the supply of anode voltage, or to make a rectifier on a kenotron with a large thermal inertia of the cathode. At worst, you can use a separate toggle switch for the anode voltage, but this is not very convenient.

The second disadvantage is the contradiction between efficiency and sound quality. When using automatic bias in the output stage, you have to either reduce the anode voltage of the driver, or put up with an increase in power dissipated by the resistor in the cathode circuit.

An interesting solution to this problem was found at http://www.svetlana.com/. You can apply a signal to the screen grid circuit of the output pentode, the constant voltage on it is usually close to the anode voltage of the driver. The auto-bias resistor can then have a relatively small resistance. True, the slope on the screen grid is much lower, but the linearity is better. At the same time, the first grid is grounded, and the pentode turns into a kind of triode that works with the grid current (A2 mode). But the driver will have to be powered by a cathode follower.

fig.5

By the way, if the first grid of the output pentode is not directly grounded, it can be used to supply a local OOS signal, including a frequency-dependent one. And this is the way to create a bandpass amplifier without a separate crossover.

A similar driver solution is used in another amplifier. He got here because of the parallel connection of the triodes of the output lamp. However, there are many disadvantages, first of all - monstrous extravagance. Of all the power consumed by the amplifier, almost a third is in the bias circuits. It would be much more reasonable to use separate rectifiers for bias, and in the driver - SRPP on a medium power double triode.

Lovers of high-quality sound will appreciate our homemade product today: the Magnifique Evolution tube single-ended amplifier.

Made in ex-USSR from the old soviet components.

This is a story about where the unconscious desire to create something out of nothing can lead, and what this “monster” can turn into.

Jazz-rock and roll improvisations

There is such a kind of pastime - viewing all kinds of equipment on the network: Maranza, Denons, Yamahas, Rotels, Nedas, and so on. A similar remake surrounds from all sides, hangs over your head, "begging" to buy, change and buy again. Which sometimes happens.

But in parallel with this, I was in the process of finalizing my technique. With audiophile methods, and not only, I tried to get a sound that, in my opinion, corresponded to the level of quality I needed. I wanted a dynamic (driving), warm, detailed, transparent sound with an unshakable stage. Somewhere like this ... And it finally happened. I got the sound I needed on the transistor class AB. So, what is next? A dead end… If the sound is warm and detailed, transparent and driving, then in the conditions of a particular room it is impossible and ridiculous to make it even more of what it already is, because all this is already there. But you always want something more ... And there is a way out! Japanese amplifier Luxman 550A. Transistor class A. Total 20W per channel. But what watts! Distortion - 0.005%. Inside, everything is not just right, but perfectly right. Large arrow indicators are pleasing to the eye. Hull color and finish. In short - Hi-End! 5700$!!! Wait…

In my case, it's more interesting, that's for sure, to get the same sound on a different type of equipment. Lamps! Lamps, lamps! It's inspiring.

For every self-respecting connoisseur of music should have in his arsenal and. This inspired me, especially the creations of those brave gentlemen who presented their projects on the net.

In order not to reinvent the "bicycle", it was decided to create an amplifier from ready-made circuit solutions with the introduction of their own ideas about the "correct".

Scheme

It is located on a separate board and contains a double electronic choke with an anode voltage delay according to Chugunov's design. On the same board there is a bias voltage source and a constant voltage regulator for the filament of the driver lamp. The food spectrum is very clean.

The whole circuit is powered by two transformers: the main one - TAN-43 and an additional one, 10V, to illuminate the indicators and to heat the driver lamp. As the second, you can use the TN-30.

The amplifier itself is made in a separate block, according to the scheme of Oleg Chernyshev, with a fixed offset of the output lamps. Capacitors C4 provide treble correction. For a linear characteristic, 100 nF is sufficient. I have 200 nF - 1db boost.

Type-setting volume control, based on the Soviet instrument paired switch for ten positions. I disassembled it, cleaned it, lubricated it with thick silicone grease and installed a softer spring. The entire structure is surrounded by a screen made of copper sheet. The total resistance of the regulator directly affects the level of the lowest frequencies and the noise level. I recommend its resistance in the range of 15 - 24 kOhm.

Output transformers TV-2Sh (TVZ-1-9). They were selected from seven pieces according to the maximum signal level and frequency range.

The input selector on the toggle switch for two sources. Of course, there is an opportunity and a place to build a relay system if you want in the future.

The wires from the input jacks to the selector are wrapped in a thick silver-plated copper screen from some kind of military radio cable.

The body wires from all blocks and screens and from the front panel converge in the center of the power supply at the minus of the capacitors.

The mutual arrangement of all nodes, blocks and connectors was determined by the minimum noise level and controlled by a spectrometer with subsequent comparative measurements of RMAA.

Instead of Oleg Chernyshov's block, if you don't like his sound or for an experiment, you can put an amplifier block according to such a common scheme http://cxem.net/sound/amps/amp46.php or any other single cycle of the corresponding current consumption.

The PSU components mounted on heatsinks are located upwards with a heat sink. When mounting these components vertically with others , the wiring must be changed .
R24 - adjustment of the filament voltage L1 6.3 V.
R17 - adjusting the output voltage of the electronic throttle 300V.
C14 - determines the time for setting the operating mode of the anode voltage.
R11 - setting the current of the pentodes. At the anode - 300V. On R10 48mV.
R12 - setting the level of the indicators to 0db (2W) 2.85V with a load at the output of 4 ohms.

Frame

Chernyshev called his amplifier Pokemon - a small pocket monster. In my case, due to almost complete ignorance of what I do and what I want, I got an amplifier for a full-format case. Or the pocket should be larger ... I didn’t have the goal of compacting all the details for compactness. Moreover, if you want to change something, then the case will not be a hindrance.

The body was assembled from fiberglass. It didn't work out otherwise. But it turned out to be a fairly stable design. Chassis - 6mm. Back wall - 4mm. Top cover(2mm). Its color and texture, in my opinion, are acceptable and do not require coloring. The front panel is made of foil fiberglass (2mm), sheathed in front with aluminum (2mm). In addition, various aluminum corners were used for reinforcement and decoration. The chassis, of course, will be much more convenient if it is made with windows for boards, as they did in the old days. To do this, you can safely use thick plywood, and not expensive fiberglass. Ventilation grill - soap dish. Support legs - bearings (Primare is resting).

***

Actually, this whole project is a process of creation for the sake of the process itself, and not with the goal of getting some of the best amplifier with a reference sound. The most simple parts were used, especially for output transformers and transition capacitors (few people recommended K73 for this role).

What is the result?

What do we see and hear? Despite the lack of fundamental perfectionism, and somewhere, a formal approach, it turned out to be a very beautiful and stylish, as for me, product that demonstrated a fairly powerful and beautiful sound, even on low-sensitivity (85db) speakers. Perhaps, of course, this is not Luxman-550A, but one can note the high detail and transparency of the sound, warmth and dynamism, as well as the “indecent” absence of even a hint of noise or background. In general, according to individual feelings, after listening, this is a very good result.

Instrument measurements

Electricity consumption: 66 VA, 46W, 0.3A.

Output power limited by visible distortion of the sine wave, with an input of 1.3V: 2.3W.

Maximum output power: 3.6W.

Frequency range at 1.5db linearity: 30Hz - 18kHz.

Your article will be here if you send it to us :) [email protected]

The beam tetrode 6P7S is almost a complete analogue of the “sound” lamps 6PZS, 6L6G, adapted to work in horizontal scanning circuits of TVs.

It is distinguished by improved insulation between the electrodes, a slightly larger anode current pulse, and increased electrical strength. The anode outlet is placed on the dome of the pampa flask in the form of a metal cap (Fig. 1). At the same time, the I–V characteristics of the 6P7S tetrode are very close to those of 6PZS and 6L6.

Rice. 1. Design and pinout of the 6P7S lamp.

The high quality of its sound approaches the sound of a generator tetrode of the G-807 type. The latter noticeably surpasses such generally recognized "classics" as 6PZS / 6L6 and 6P27S / EL34.

When building the output stages of AF amplifiers, you can easily use the electrical modes adopted for 6PZS / 6L6 or 6P27S / EL34 lamps.

• anode voltage Ua = 250 V, screen grid Uc2 = 250 V, cathode Ek = 14 V (automatic bias resistor Rk = 180 Ω 2 W);
• anode current Ia0 = 72 mA, screen grid current Ie0 = 5.8 mA (quenching resistor Rc2 = 2.4 kOhm 0.25 W);
• excitation voltage on the control grid Ucl=10 V.

In this mode, the slope of the lamp S = 5.9 mA, internal resistance R (= 32 kOhm, anode load resistance Ra = 2.5 kOhm, maximum (Kg = 10%) output power 6.5 W.

Filament voltage / current 6.3 V / 900 mA, maximum allowable voltage at the anode 500 V, the power dissipated on the anode for a long time is not more than 20 W.

Schematic diagram of UMZCH

An example of the practical implementation of an UMZCH with a single-cycle output stage on a 6P7S lamp when operating in a circuit with automatic shift on control grids is shown in fig. 2. The input signal is fed to the resistor R1, which acts as a gain regulator.

Rice. 2. Scheme of a home-made UMZCH with a single-cycle output stage on a 6P7S lamp.

Let's dwell on this element in more detail, since the input circuits largely determine the sound quality of the device. Let's start with the adjustment characteristics.

For volume controls, resistors with an exponential (inverse-logarithmic) dependence of resistance on the angle of rotation of the engine are generally accepted, that is, a characteristic of type “B” is required.

The design of the resistor must provide reliable mechanical contact between the moving electrodes and the conductive element.

The explanation is very simple: in this zone, the strongest degradation of the sound signal occurs, not to mention the fact that wheezing and cods in the process of adjustment simply act on the nerves.

For dual resistors, an important quality indicator is the unbalance of the characteristics. Consider possible options choice.

We immediately reject the "extremist" option - the use of typically "high-end" components like Ricken Ohm - they are not available to anyone. Let's stop the choice on a more common element base.

From imported audio components of sufficient quality and not too expensive, resistors from ALPS, Bourns, Spectroll can be recommended. Of the domestic ones, volumetric composite ones such as SP4-1 or SPO work well.

Advice. Metal-film and varnish-film elements should not be used.

From discrete regulators it is possible to use domestic type RP1-57E. Those who wish can try to install wire-wound potentiometers PTP-21.

The first stage of the amplifier is assembled on one half of the double "sound" triode 6H8C (VL1.1). The 6H8C input amplifier uses both parts of this tube.

It is a standard voltage amplifier with a resistive load and a gain of about 11. The operating mode of the VL1.1 lamp is set by the auto-bias resistor R4, the anode load is resistor R5.

The second stage, like the first, is also a typical voltage amplifier with a resistive load R8 in the anode circuit. Its transmission coefficient is about 5.

Note. The only difference between the second stage and the “classical” circuit is the increased by an order of magnitude automatic bias resistance R9 in the VL 1.2 cathode circuit. This is due to the need to set the correct operating mode at a large positive potential on the control grid of the triode.

The high resistance in the cathode circuit predetermines a large depth of local feedback, which significantly reduces the AC gain. In addition, according to the concept of building high-end equipment, the presence of environmental protection is undesirable.

In this regard, the resistor R9 is shunted by an electrolytic capacitor C2. Increased requirements are imposed on its quality, since this element quite strongly affects the sound of the device. Specialized high quality audio electrolytes such as Elna-Gerafine have at least high price and inaccessible.

Advice. You can use aluminum oxide-electrolytic capacitors such as K50-24, K50-29; slightly worse than K50-35. Of the same type of components with the same electrical characteristics, but different sizes, capacitors with large cases should be preferred. The latter usually sound better, although this sign is not always justified in pre-amplification stages.

An attempt to shunt C2 with film or paper capacitors did not lead to a clearly defined desired effect. It is not recommended to use oxide semiconductors as C2.

However, we will return to the features of the choice of capacitors installed in the cathode circuit of the lamp when studying the final stage. By alternating current, the second and final stages are interconnected by a separating C4.

This element affects the sound quality in the most radical way, so talking about the requirements for its quality deserves special attention.

We note right away that an ideal component that would not spoil the sound at all simply does not exist in nature. These could include vacuum or air condensers.

However, it is very problematic to imagine, and even more so to put into practice, an amplifier with a “through-hole” the size of a pair of tank batteries. Therefore, the choice of type C4 is always a compromise.

Of course, you can simply note the high quality of specialized audiophile products from such companies as Jensen Capacitors or the exotic "spill" of Audio Note, and put an end to this. But the prohibitive price of such components instantly translates them into the category of equally transcendental dreams for almost all radio amateurs.

Let us dwell in more detail on the really accessible elements of the general application of domestic production. According to many developers of audio equipment, paper-oil and paper-foil products of types K40-9-5 (with the 5th acceptance) are considered the best; K40-U9; K40A-2; CBG; OKBG; BM-2; BMT-2.

Slightly worse metal-paper ones like MBM, MBG, K42-... the roll blank is impregnated with ceresin.

Due to such design and production-technological features, metal-paper capacitors, in comparison with paper-oil and paper-foil ones, have a reduced dielectric strength, which, due to the diffusion of metallization ions into the dielectric, decreases even more during aging.

There is some "viscosity" of the sound of paper capacitors in the high frequencies. At the same time, "slyudyanka", providing the clarity and transparency of the "top", does not allow obtaining the necessary plasticity and relief of sound in the region of mid frequencies and mid-bass, which the "paper" is so famous for.

Note. After a series of experiments, the author was able to establish that the parallel connection of paper and mica capacitors, the capacity of the latter should be 1-7% of the capacity of the main one, makes it possible to combine the sound advantages of both types.

By selecting the ratios of capacitances, it is possible to some extent to change the nature of sound reproduction. Practice has shown the following: for an isolation capacitor with a capacity of more than 0.1 μF, in the case when the input resistance of the subsequent stage is at least 200 kOhm, the mica additional capacitor should have a capacitance in the range of 2-10 thousand pF.

Thus, C4 can be made up of a "wallet", say, of the K40U-9 or BMT-2 type with a capacity of 0.22-0.25 microfarads with an operating voltage of at least 250 V and a mica capacitor, for example, KSO-5, KSO-11 , with a capacity of 3000-6800 pF with the same, or greater, maximum operating voltage.

Note. In the case of building a stereo version of the amplifier, the selection of capacitors that make up the C4 "through" should be approached with particular care.

First of all, from the available stock of the same type of "wallets", and it is desirable that they be from the same batch, using a digital device, it is necessary to select two capacitors with actually the same capacity.

The last requirement is more important than the exact correspondence of the nominal value indicated on the circuit diagram. Since the capacitance of the isolation capacitor is less critical than in the corrective circuits, C4 can lie in the range of 0.17-0.29 microfarads.

The need to use the same elements in both channels of the device is caused by the desire to obtain equal frequency response and phase response, to the mismatch of which stereo systems are very critical. And with single-channel sound reproduction, even very large phase distortions have practically no effect.

It would be useful to measure the intrinsic non-linear distortion coefficient of capacitors using the device and methodology proposed in [Lukin E. "Complex for measuring ultra-low non-linear distortion" - "Radio Hobby" No. 2/2000 p. 40]. It is useful to make sure that the intrinsic mechanical resonance of the capacitor does not fall into the audio frequency region.

!!! Attention. Parts that have a "mechanical" resonance in the audio range are not suitable for audio equipment.

Having completed the selection of paper capacitors, they do the same with mica ones. After that, they can be installed in the circuit. Of the film capacitors, the most suitable for sound path fluoroplastic types FT-... are considered; K72-..., slightly worse than polystyrene PM-...; ON; K70-...; K71-...; polypropylene K78-....

!!! Attention. Do not use polyethylene terephthalate (lavsan) capacitors of the K73-... type in the audio path, which spoil the sound in the most serious way.

This feature allows you to choose the most acceptable character of the device's sound when listening to music programs of various genres and directions. So, for example, for hard rock music performed by bands such as ACDC, tetrode switching is most suitable.

Some deterioration in resolution and transparency does not really harm these genres, especially since it is fully compensated by additional “drive” and aggressiveness of the sound.

The ultra-linear mode is more suitable for chanson, including "Russian", some areas of reggae and jazz, pop music. In general, this inclusion is a kind of reasonable compromise, allowing you to get quite acceptable results for not very aggressive rock, as well as for a number of classical works.

And, finally, the triode inclusion reveals its capabilities to the greatest extent when listening to classical and some varieties of the so-called. "acoustic" music. However, these arguments and observations should not be taken as a dogma, because who better than you to know what is best for you.

Switching modes is carried out by switches SA1.1 and SA1.2. It is best to choose a double biscuit, and double-plated, in a different way, two-biscuit. This is due to the fact that an electrostatic screen must be placed between the biscuits.

Attention. Failure to do so may result in self-excitation.

In the stereo version of the SA1 device, you can make it as a pair of two-board switches separate for each channel, or you can use one four-way switch.

Advice. Install SA1 as close as possible to the final stage and connect it to the appropriate circuits with conductors as short as possible. It is best if these are directly the conclusions of the resistors R12-R15.

The quality of the contact groups of the SA1 switch should be given the closest attention, since they can become a source of severe distortion. It is unacceptable to use products with contact groups made of phosphor bronze or copper, brass, silver-plated metals:

• the first material has a high transition resistance;
• the rest are not suitable due to their low mechanical strength and tendency to oxidize, and in the atmosphere of large industrial cities also the formation of various chemical, primarily sulfur compounds, which are semiconductors.

For the first experiments, you can take components in which the contact groups are made of beryllium bronze or have a coating of an alloy of silver with 40% nickel. All these materials:

• good resistance to abrasion;
• have good electrical characteristics;
• relatively cheap.

A more expensive option is to use switches with gold-plated contacts. The "elite" products include components with contact groups coated with a platinum-iridium alloy or rhodium (the material used is indicated in the manufacturer's specification).

And, finally, even the "best" material will be completely useless if the design of the product does not provide reliable mechanical contact, which should also not be forgotten.

In principle, SA1 can be assembled on the basis of a relay with sealed contacts, for which it is necessary to organize a logical control system. Its circuit design for an experienced radio amateur does not present any difficulties.

Briefly about the circuits associated with SA1. The first switch jack SA1.1 is connected to the screen grid circuit of the final lamp VL2. With its help, the desired scheme for constructing the output stage is selected:

• fixed contacts, fixed directly on the biscuit, are connected to the corresponding terminals of the primary winding of the transformer Tr.1 and the anode voltage source;
• the movable contact mounted on the rotating rotor of the switch is connected through the resistor R15 to the second grid of the VL2 lamp.

In a tetrode connection, R15 serves as a current-limiting element that prevents the danger of electrical overload of the lamp grid.

When operating in ultralinear mode, R15 somewhat equalizes the voltages on the screen grid and the VL2 anode, and also creates a local negative feedback of moderate depth, which increases the linearity of the cascade.

The second section of the SA1.2 switch is connected to the cathode circuit of the same lamp. Cathode automatic bias resistors R12-R14 are connected to the fixed contacts.

Note. In the process of adjusting the circuit, their resistance is selected in such a way that the anode quiescent current of the output lamp in all three inclusions lies within 72-75 mA.

The circuit diagram shows guide values ​​for R12-R14. It is better to select them more accurately only after the new terminal lamps have been “fried” at idle for at least 20-30 hours.

The movable contact SA1.2 is connected to the cathode of the terminal lamp. The positive terminal of the electrolytic capacitor C5 is also connected to the same point.

This element of the circuit eliminates the occurrence of local OOS for alternating current due to the voltage drop across the cathode resistors. Initially, the capacitance of the capacitor C5 can be taken equal to 1000 microfarads.

The exact value depends on a number of factors, not least the characteristics of your loudspeakers. Of course, taking into account their influence in advance in a complex is a very difficult task, so you have to bring the apparatus “to mind” based on the results of control listening.

The generally accepted simplified formula for calculating the capacitance of a capacitor shunting a cathode auto-bias resistor is the following:

where Fn - lowest frequency specified operating range in Hz; Rk is the resistance of the auto-bias resistor in ohms.

Substituting Fn = 10 Hz and Rk = 200 Ohm, we get Sk = 500-1000 uF. After increasing the capacitance of C5 from 500 uF to 1000 uF, the bass becomes deeper and more voluminous, which, in principle, could have been predicted in advance.

But building it up to 2000 microfarads gives a sharply negative effect. In the lower bass region, a rumble and a characteristic “mumbling” appear, and the mid-bass becomes “grainy”. In addition to everything, extremely unpleasant extraneous overtones begin to be heard in the mid-frequency register.

The requirements for the quality of this element have already been considered when describing the pre-amplifier, but in this case there are a number of nuances.

Here, the specificity is associated with the high power of the audio signal, which is developed by the final stage. The author tested small-sized electrolytic capacitors Nippon, Rec and Rubycon with a capacity of 1000 microfarads with an operating voltage of 63 V, no more than a little finger thick, which sounded, firstly, in different ways, which is not surprising, and secondly - somehow “flat ".

Replacing them with K50-29 with the same denominations, but having a several times larger geometric volume, led to positive result. The much-desired depth and dynamics immediately appeared, and the bass itself became more collected, resilient and saturated.

The explanation for this effect is as follows. In the final stages, a sound signal of significant power is applied to the cathode capacitor. Therefore, they begin to affect:

• and such a characteristic as the maximum allowable ripple voltage (it must also be taken into account when building pre-amplification stages)
• and allowable reactive power, i.e., the thermal processes of the component have a significant influence.

Note. All the above aspects of the choice of components are relevant not only for this design.

All stages of this amplifier are powered by a single source of anode voltage. Interstage interchanges are made in the form of RC chains.

They include resistors R7 and R16, as well as electrolytic capacitors C1, C3. Compared to circuits in which a sound signal operates, the requirements for the quality of filter elements are simpler. Here it is quite possible to use capacitors of types K50-20, K50-26, K50-27, K50-31, K50-32, K50-35. Suitable components and earlier developments K50-3, K50-6, K50-7, K50-12.

At first glance, it does not matter where exactly to install a higher-quality component in the power circuit, because it does not seem to interact directly with sound signal. But this is far from true.

Let us study the influence of the terminal stage on the previous ones. For a simpler understanding of what is happening, we will assume that there is no interstage decoupling. In the process of amplifying the signal, the total anode current of the lamp is divided into two components: constant and variable. G

the generator of the latter is the lamp itself. If the anode power source had zero internal resistance, then the variable component of the anode current of the output lamp would go through that source completely “transparently”, having no effect on the operation of the previous stages.

However, in practice, any power source has a certain, even if small, internal resistance. Therefore, part of the variable component of the anode current of the terminal lamp branches off into the anode circuits of the previous stages, assembled on triodes VL1.1 and VL1.2.

In this case, this part of the current passes through the quenching resistors R16 and R7 (they are set because the supply voltage of the preliminary stages is usually lower than the output ones), the anode load resistors R8 and R5, the separating elements R6 and C4, and the leakage resistor R10.

The second stage of the amplifier has a similar effect on the first, in addition, the situation here is aggravated by the presence of a quenching resistor R16. Because of this, the equivalent internal resistance of the anode power supply increases significantly.

Note. The current amplitude in the anode circuit of the preliminary stage is many times less than that of the final stage.

Now consider the case when C1 and C3, having good formal characteristics, have unsatisfactory "sound" properties.

Note. In such a situation, they are not only unable to effectively perform their function - to close interference to a common wire, but (which is much worse) can themselves generate additional "dirt".

Spreading along the power bus, all this "garbage" goes through the path described above, is amplified, and, mixed with a useful signal, is clearly not able to decorate a music program.

A very effective way to combat this effect is to separate the power supply of the units of the device - in the ideal case, for each cascade there is a separate rectifier, which is widely used in elite audio equipment. In more simple devices you have to compromise, feeding all the nodes of the circuit from one source.

Now let's draw conclusions. The more gain the whole circuit has with a broken NF loop, the better quality elements should be used in the power circuit.

The first stages of amplification are the most critical to the quality of components, and to a lesser extent, the output ones. Therefore, for the power decoupling elements of the first stage of the UMZCH, components of high, ideally "signal" quality, should be used.

In addition, in some cases good effect gives shunting of the electrolytic capacitor with a high-frequency one, similar to how it is done for a "through-hole".

Note. Particular attention should be paid to the details included in the interstage decoupling of the circuit in the case of using kenotron power supplies.

The latter have increased, in comparison with semiconductor, their own resistance.

A common and fairly effective means of reducing the equivalent resistance of a rectifier is to use a very large capacitance at the output of the filter, at least several times greater than necessary to obtain a given ripple factor.

Switching capacitors are especially good here. They are distinguished from similar products for general use by their increased energy intensity, low series resistance (ESR) and the ability to deliver high impulse currents.

Of the capacitors of domestic arbitrariness in this application, K50-23 has proven itself well, slightly worse than K50-17, K50-21, K50-13. You can use the components of earlier developments - K50-ZF, K50I-3, ​​K50I-1.

Therefore, it is no coincidence that so much attention is paid to the illumination of the processes occurring in the power circuits of the circuit. It remains to add that the issues discussed here are relevant and fair in relation not only to tube sound amplification technology, but also to semiconductor technology.

In the latter case, the situation is complicated due to the high currents acting here, which are tens, hundreds, and sometimes even thousands of times higher than those in lamp equipment.

The remaining elements included in the power supply circuit of this design and shown in the circuit diagram (Fig. 2) contain the SA2 switch and resistors R17, R18. Let's take a look at their purpose. With the help of SA2, the anode power supply circuit is broken. This is necessary in three cases:

• firstly, at the time of the initial inclusion of the amplifier in the network, when the cathodes of the lamps have not yet had time to warm up sufficiently. The supply of full anode voltage at this moment is fraught with a breakdown in the lamp and / or destruction of the cathode;
• secondly, it is necessary to use the SA2 switch, and this must be done, at the moment of transition from one final stage circuit to another. Removing the anode power drastically reduces the intensity of transients, which is guaranteed to protect. AU from failure during this operation;
• thirdly, this element is necessary for the organization of the so-called. standby mode.

This mode is as follows. In the first seconds after the heating voltage is applied, the heater-cathode system experiences significant electrical and mechanical loads. The former are due to the low resistance of the cold filament, and the latter are due to thermal deformations that occur during the heating of the cathode.

Of course, turning the filament on and off negatively affects the durability of the lamp. Therefore, during listening breaks lasting up to several hours, it is better not to turn off the amplifier.

On the other hand, keeping the apparatus fully prepared for 2–3 hours is unacceptable both for economic reasons (unreasonably increased power consumption and, again, reduced lamp life due to cathode wear) and for safety reasons.

Therefore, with not very long pauses in work, only high anode voltage is removed. Resistors R17, R18 in standby mode form an anode voltage divider.

Its function is related to the fact that the operation of the lamp with the heat on, but without the current selection, is considered a more difficult mode than the nominal one and can lead to the so-called. cathode poisoning.

To eliminate this "scourge" it is enough to apply a voltage to the electrodes of the lamp, which is 7-15% of the nominal. There are no special specific requirements for R17, R18 themselves.

The power supply for initial experiments can be a simple semiconductor rectifier with a capacitive filter.

It should provide an output current of at least 120 mA in the mono version of the device at a voltage of 290 V. In the future, it is desirable to assemble a power supply with a 4-fold power reserve.

Advice: A CLC filter is best for smoothing ripples, and it is useful to increase the output capacitance to 1000-1500 microfarads per channel.

In the case of building a rectifier on semiconductor devices, preference should be given to high-frequency diodes with a large crystal area. The valves themselves can be shunted with mica capacitors with a capacity of several thousand picofarads. It is even better to assemble a kenotron rectifier. Through the filament circuit, one channel of the amplifier consumes a current of about 1.5 A, although a margin of up to 1.8-2 A, of course, does not hurt.

Lamp heater supply circuit diagrams are standard, with the use of conventional anti-background measures. In the ideal case, this is the use of a constant stabilized voltage.

Manufacturing of transformers

The output transformer is made on the basis of a serial "networker" type TPP-286U manufactured by the Nikolaev (Ukraine) transformer plant. The same standard sizes, structural elements and dimensions have products of the TPP 283-TPP 289 series.

All these transformers are assembled on the basis of the ShLM 25x40 magnetic core. Its design characteristics are: the cross section of the central core is 10 cm2, the average length of the magnetic field line is 16 cm, the window dimensions are 15x45 mm, the thickness of st. tapes 0.35 mm. To avoid saturation of the core under the influence of constant bias, it is assembled with a gap of 0.25 mm.

Advice: When assembling a stereo version of the amplifier, try to find transformers from the same batch or at least the same release date. This largely guarantees the identity of the electrical characteristics of the magnetic circuits.

The coil frame of the serial transformer has a width of 39 mm and a depth of 13 mm.

Before starting winding with a file, it is necessary to give it the correct geometric shape, first of all, to bring out the right corners of the frame window.

Otherwise, the required amount of wire may not fit. After that, it is necessary to cut to the outer surface of the bottom those slots in the cheeks of the frame through which the conclusions 1.2.a-2.6 and 3 pass. It remains to remove the burrs and slightly round the edges of the slots intended for the winding leads to avoid wire breakage.

The anode winding contains 3000 turns, divided into 6 equal sections of 500 turns. Each section of the winding I is made in 5 layers of 100 turns.

Branch 7 is made from the 1300th turn, which is used in the ultralinear mode and provides the switching coefficient p=0.43. The secondary winding consists of five single-layer sections of 32 turns, the total number of turns is 160.

Rice. 3. The layout of the windings and electrical connections between their sections.

The layout of the windings and electrical connections between their sections is shown in fig. 3. The specified ratio of the number of turns ensures optimal matching of the output lamp with a load of 8 ohms. AT

The choice of this option is not accidental, since most of the high-sensitivity acoustic systems have just such an impedance.

Note. To obtain satisfactory sound, this amplifier must be operated with an AC sensitivity of at least 92 dB/W/m.

A characteristic feature of the coil design of this output transformer is its winding with two folded wires. The implementation of the windings of signal transformers, especially input and between lamps, with a bundle of several wires folded together or with a litz wire is not a special novelty and is relatively common.

Much less often, such winding is used in powerful output transformers. Hiroyashi Kondo, the creator of the Audio Note and Kondo trademarks, and Susumu Sakuma, the founder of the “cult” Tamura company, use this technique in some of their models.

In the considered design, the use of two parallel winding wires is explained as follows:

• on the one hand, the conductor has the property of directivity, so the sound quality is affected by the "polarity" of its connection;
• on the other hand, the output transformer coil is one of the very important and labor-intensive components of tube amplifiers.

Note. At the same time, it is practically impossible to immediately guess the correct direction of the wire connection, and even more so to be absolutely sure of this. A series of such experiments is a lengthy, extremely time-consuming and expensive task.

Considering that the amplitude of the alternating voltage acting in the anode winding of the output transformer is commensurate with the value of the anode supply, and it is the small-signal circuits that are most critical to the direction of the wires, in which the direct current also acts simultaneously, it was decided to use the proposal of V. I. Goryunov. This idea was published in [Goryunov V. Letter 1, "And if in parallel?" "Radiohobby" No. 6/2000, p. 42].

An additional argument in favor of this design can be considered the fact that when using two wires, it is possible to save 7–10% of the area of ​​the core window compared to the case when one conductor is used with a cross section equal to the total but larger diameter. To perform the primary winding, the wire PETV-1 00.16 mm was selected.

Technologically, the winding of the transformer coil is as follows. First, about half is rewound from the coil with wire to the empty drum, after which you can get to work. Using this method, rather than using two ready-made bays:

• firstly, it provides a deliberate receipt of an anti-parallel inclusion;
• secondly, it guarantees the homogeneity of the chemical composition and crystal structure of the material of both conductors.

In the process of work, it is necessary to carefully monitor that the wires lie in even parallel rows and in no case cross anywhere. An example of the correct winding of the coil is shown in fig. four.

Rice. 4. An example of the correct winding of the coil.

On it, the wires that belong to one turn are highlighted with a white / black background. Between the layers of the anode winding, insulation is laid in the form of a single layer of paper 10-15 microns thick from powerful so-called. cosine capacitors. The active resistance of a correctly made primary winding is about 220 ohms between terminals 1-14.

Note. The oil with which such paper is impregnated should not be embarrassing, because it is an excellent dielectric and, moreover, it dissolves perfectly in paraffin and / or technical wax, without interfering with the normal course of the “boiling” of the coil.

Rice. Fig. 5. Layout of the winding section terminals on a standard frame from the Chamber of Commerce and Industry: a - primary; b - secondary.

The secondary winding is also carried out with a double wire of the PEV-1 brand 0.5 mm. Interwinding insulation - combined three-layer.

Bottom and top layer of wired cable paper 0.08 mm thick. There will be no big trouble if this paper is impregnated with transformer or condenser mineral oil. The inner layer is a fluoroplastic tape 50 microns thick.

The last section of the primary winding is insulated with two layers of PTFE and one electrical cardboard with a thickness of 0.3-0.4 mm. The layout of the terminals of the winding sections on a standard frame from the Chamber of Commerce and Industry is shown in fig. 5.

The Roman numeral I indicates the initial direction of laying the wires, and II indicates the direction of rotation of the coil frame during the winding process. After winding the coil and complete assembly of the entire transformer, it should be completely impregnated with paraffin or technical wax.

Summary

When using an output transformer of the recommended design, the amplifier has the following characteristics: maximum output power 4-6 W with a non-linear distortion factor of 2.5-6%, depending on the operating mode of the final stage. The frequency range at the level of 1.5 dB is not already 40 Hz - 22 kHz, regardless of the output lamp switching circuit.

The nominal sensitivity of the device is approximately 0.11 V when the terminal stage is operating in tetrode and ultralinear modes, in the triode it decreases to 0.2-0.23 V. All parameters are given for the case when the circuit is not covered by the general OOS loop.

Preliminary adjustment of an amplifier correctly assembled from known good parts does not cause difficulties. It usually starts working right away.

It is advisable to check the lamp modes for direct current and, if necessary, correct them. It is advisable (in the presence of an oscilloscope) to make sure that there is no self-excitation of the circuit.

After that, the amplifier is allowed to "warm up" for 30-40 hours without applying a useful signal to its input. This operation can be divided into several stages; here the total operating time is more important. During this procedure, the final formation of the components that make up the circuit takes place, and it should not be neglected.

This phenomenon is explained simply: the orientation of the magnetic domains of the material of the core of the transformer and the ordering of the structure of the conductors of its coil cannot occur instantly due to the presence of "memory" in metals.

After the preliminary "warming up" of the device, the most interesting stage of work begins - finishing the product to the "highest limit" condition. Therefore, so detailed description the requirements for details, the study of the methodology for their selection, etc. is not accidental.

The example of the proposed amplifier clearly shows that, despite the apparent simplicity of the circuit, there are many "pitfalls" when building audio equipment. Those who wish can try to "play" with the modes of operation of the triodes of the preliminary stages.

By maintaining the same value of the anode supply voltage, by changing the resistance of the resistors in the cathode and anode circuits, you can get the sound of the entire device from “terry-tube” to “flat-transistor”.

Advice."Freshly baked" output transformers (this effect is especially pronounced in single-cycle devices) must be allowed to work for at least 25-30 hours, only after that they begin to "wake up".

At a certain stage of work, you will feel that each element and / or wiring has begun to “play”, you will begin to understand the influence of the materials used, you will see the dependence of the results obtained on the overall layout of the device.

Summarizing the above, we can say: a simple repetition of constructions according to the descriptions given in various literature provides the sound of only a certain “initial” level, which may be lower or higher. The use of the full potential inherent in this or that scheme depends only on your abilities, taste and intuition.

Literature: Sukhov N. E. - The best designs of ULF and subwoofers with their own hands.

This article continues our discussion of single ended power amplifiers. As you can see, the amplifier circuit is almost no different from the amplifier circuit published in my article in the Radio Amateur magazine No. 9 for 2003.

The author of the scheme, A.I. Manakov, built an amplifier on two finger lamps 6N2P and 6P43P. Many radio amateurs who repeated this amplifier were pleasantly surprised by its soft natural sound with the relative simplicity of circuitry and the low cost of components. However, the questions that have come in regularly since the publication are mainly about two things: output power and the applicability of lamps with an octal base.

Going towards the wishes of radio amateurs, and after consultations with A.I. Manakov, I propose next option amplifier.

The diagram of one channel of the amplifier, as well as the power supply for both channels, is shown in the figure.

Rice. one. circuit diagram one channel of the amplifier, as well as a power supply for both channels

There are two main differences, these are increased output power, about 4 W per channel, and kenotron power, which has a beneficial effect on the sound.

The input signal goes to the dual variable resistor, which is the volume control. I used ALPS, but due to its high cost, you can use any, preferably a wire resistor, group "B" (logarithmic dependence). Two separate volume controls can be applied, one for each channel.

One of the best (from my point of view) domestic small-signal triodes, 6H9C, was chosen as the preliminary stage lamp. Both halves of the lamp are connected in parallel. This achieves a decrease in the internal resistance of the lamp, which entails an improvement in load capacity and signal-to-noise ratio. The setting of the cascade consists in setting the voltage on the cathode of the 6H9C lamp in the range of 1.3-1.5 volts, by selecting the resistor R3. Resistor R4 is chosen for the best sound quality. If you want to use another triode, for example 6H8C, then the resistance of the resistor R4 will be 20-25 kohm, in which case you will again have to select the resistor R3. The 6H8S tube sounds more analytical, it has a lower gain (21 versus 70 for 6N9S), but maybe someone will like this sound more. The choice is yours.

The output stage is made on a 6P13S beam tetrode connected by a triode. It is the triode inclusion that is most optimal in terms of sound quality. The output stage has no features. The only thing that has to be done is to select, using the resistor R8, the current through the lamp within 60-65 ma. This resistor can be made up of two resistors connected in parallel, for example, 1 kΩ 2 watts each. If you wish, you can use the common lamp 6P3S or 6P7S. The quiescent current of the output stage in this case should be in the range of 70-75 ma. However, I want to note that in this case the power will decrease to 2 watts (when using 6P3S) and the total harmonic coefficient of the amplifier will increase. I tried the 6P7S lamp and I want to note that it sounds good. When it is used, the resistor in the automatic bias circuit is selected within 220-230 ohm 2W, and the resistor between the second grid and the anode is within 150-230 ohm 2W. Quiescent current in this case will be about 70 ma. The output power of the amplifier in this case will be about 3W per channel.

Now for the details. The sound of the amplifier as a whole depends on the quality of the coupling capacitor C3. I used Jensen, and from domestic ones, you can use K71, K78, K73, K40U-9, K40U-2, K42U-2 for the corresponding voltage from 250V.

Capacitors of constant capacity, shunting electrolytic, in circuits of automatic displacement of lamps - film. Bypassing electrolytes with fixed capacitors improves sound transmission in the high frequency region.

The capacitance of these capacitors can be one or two orders of magnitude less than the capacitance of the electrolytic one. Electrolytic shunt capacitors in power circuits can be used K73; K77, and the electrolytes themselves in the filters of the power supply - Teapo, Samsung, etc. In automatic lamp bias circuits, try to use electrolytic capacitors best quality, such as Black Gate. When using them, it is possible to abandon shunt capacitances altogether.

Output transformer TW6SE of the Moscow firm "Audioinstrument". By visiting the company's website at www.audioinstr.h1.ru, you can view and order the lamps, transformers, chokes, lamp panels, etc. you are interested in.

Fixed resistors P1-71 with a tolerance of 1-2%. You can use the aircraft, as well as the more common type C2-33N or MLT, corresponding to the power.

There are a number of questions regarding the cap put on the anode of the 6P13S lamp. There are many suggestions in the amateur radio literature about this. I have been successfully using spark plug wire tips from any car in my designs for a long time and with success. Due to the design of the tip, the contact is tight and reliable, and, importantly, you can change its inner diameter, since it is different for different lamps. If the tip does not solder well, use flux for soldering steel or non-ferrous metals.

The power supply is made on the kenotron 5Ts3S (5Ts4S, 5U4G). The use of kenotron power, in comparison with diodes, makes the sound of the amplifier warmer and more coherent.

Try it and see for yourself. Many articles have been written about kenotron power, so I will not go into details. The power transformer has four secondary windings. Two of them feed the incandescence of the lamps of the first and second amplifier channels, one feeds the kenotron, and the anode one, with a midpoint output, is designed for 300v x 2 at a current of 200 ma. As a first approximation, how many volts are on the transformer winding, so many are at the output, after the chokes and power capacitors.

Chokes can be used DR-2LM, DR-2.3-0.2 from black and white TVs, unified D 21, D 31, the data of both are on the website igdrassil.tk.

The chokes I use in this circuit are also from Audio Instrument. Their inductance is 5H, they are designed for a current of 300 ma.

The amplifier was mounted in a hinged way, the conclusions of the parts themselves and the contacts of the lamp panels were used to the maximum. The ground bus is made of a single-core copper wire with a diameter of 0.8 mm and has contact with the chassis at one point, next to the input. The wires going to the filament terminals of all lamps must be twisted together. This is to reduce the background. alternating current. Resistors R9-R12 also serve the same purpose. You also need to twist the wires from the input jack to the volume control. As these wires, I also use single-core wires, with a diameter of 0.4-0.7 mm, each of which (to protect against short circuits) is insulated with silk insulation (a thin shoelace is used).

In conclusion, I want to say that this amplifier is not just a circuit, but a really manufactured and well-proven device. I have been using it for about three months now and am very pleased with its sound. For those who think that 4 watts per channel is not enough, I will say that in a room of 16 square meters when using KEF Q1 acoustics (sensitivity 91 dB), the amplifier develops a sound pressure commensurate with the sound pressure developed by a transistor amplifier with a power of 40 watts per channel (these are the results of a subjective assessment of my friends - musicians). But the sound is different. The amplifier perfectly feels the slightest nuances of the sound of instruments or voices and, as it were, “breathes” (forgive me if the comparison is not very correct). The sound does not tire him, he wants to listen and listen.

List of radio elements

Designation	Type of	Denomination	Quantity	Note	Score	My notepad
L1 *2	radio tube	6Н9С	2			To notepad
L2 *2	radio tube	6P13S	2	6P7S		To notepad
L3	radio tube	5Ts3S	1	There are known facts that this lamp was replaced by two diodes		To notepad
C1, C4, C9 *2 C10		220uF 450V	7	C4 at 25 volts		To notepad
C2, C8 *2	Capacitor	1uF 400V	4			To notepad
C3 *2	Capacitor	0.22uF 400V	2			To notepad
C5, C6 *2	Capacitor	2.2uF	4			To notepad
C7 *2	electrolytic capacitor	470uF 50V	2			To notepad
C11	Capacitor	2uF 400V	1			To notepad
R1 *2	Variable resistor	47 kOhm	2			To notepad
R2 *2	Resistor	300 kOhm	2			To notepad
R3, R7 *2	Resistor	510 ohm	4	R7 at 2 watts. For lamp 6P7S, R7 150-220 Ohm		To notepad
R4 *2	Resistor	47-51 kOhm	2	2 W		To notepad
R5 *2	Resistor	1.3-1.5 kOhm	2	2 W		To notepad
R6 *2	Resistor

The simplest transistor amplifier can be a good tool for studying the properties of devices. The schemes and designs are quite simple, you can independently manufacture the device and check its operation, measure all parameters. Thanks to modern field-effect transistors, it is possible to make a miniature microphone amplifier literally from three elements. And connect it to a personal computer to improve the sound recording parameters. And the interlocutors during conversations will hear your speech much better and more clearly.

Frequency characteristics

Amplifiers of low (sound) frequency are available in almost all household appliances - music centers, televisions, radios, radios, and even in personal computers. But there are also high-frequency amplifiers on transistors, lamps and microcircuits. Their difference is that ULF allows you to amplify the signal only audio frequency which is perceived by the human ear. Transistor audio amplifiers allow you to reproduce signals with frequencies in the range from 20 Hz to 20,000 Hz.

Therefore, even the simplest device is able to amplify the signal in this range. And it does it as evenly as possible. The gain depends directly on the frequency of the input signal. The graph of the dependence of these quantities is almost a straight line. If, on the other hand, a signal with a frequency outside the range is applied to the input of the amplifier, the quality of work and the efficiency of the device will quickly decrease. ULF cascades are assembled, as a rule, on transistors operating in the low and medium frequency ranges.

Classes of operation of audio amplifiers

All amplifying devices are divided into several classes, depending on what degree of current flow through the cascade during the period of operation:

1. Class "A" - the current flows non-stop during the entire period of operation of the amplifying stage.
2. In the class of work "B" current flows for half the period.
3. Class "AB" indicates that the current flows through the amplifying stage for a time equal to 50-100% of the period.
4. In "C" mode electricity runs for less than half of the operating time.
5. Mode "D" ULF has been used in amateur radio practice quite recently - a little over 50 years. In most cases, these devices are implemented on the basis of digital elements and have a very high efficiency - over 90%.

The presence of distortion in various classes of low-frequency amplifiers

The working area of ​​a class "A" transistor amplifier is characterized by rather small non-linear distortions. If the incoming signal emits pulses with more than high voltage, this causes the transistors to saturate. In the output signal, higher harmonics (up to 10 or 11) begin to appear near each harmonic. Because of this, a metallic sound, characteristic only for transistor amplifiers.

With an unstable power supply, the output signal will be modeled in amplitude near the mains frequency. The sound will be on the left frequency response more rigid. But the better the power stabilization of the amplifier, the more complex the design of the entire device becomes. ULF operating in class "A" have a relatively low efficiency - less than 20%. The reason is that the transistor is constantly on and current flows through it constantly.

To increase (albeit insignificant) efficiency, you can use push-pull circuits. One disadvantage is that the half-waves of the output signal become asymmetrical. If you transfer from class "A" to "AB", the non-linear distortion will increase by 3-4 times. But the efficiency of the entire circuit of the device will still increase. ULF classes "AB" and "B" characterizes the increase in distortion with a decrease in the signal level at the input. But even if you turn up the volume, it will not help to completely get rid of the shortcomings.

Work in intermediate classes

Each class has several varieties. For example, there is a class of amplifiers "A +". In it, the transistors at the input (low-voltage) operate in the "A" mode. But high-voltage, installed in the output stages, work either in "B" or in "AB". Such amplifiers are much more economical than those operating in class "A". A noticeably smaller number of non-linear distortions - no higher than 0.003%. Better results can be achieved using bipolar transistors. The principle of operation of amplifiers on these elements will be discussed below.

But still there are a large number of higher harmonics in the output signal, which makes the sound characteristic metallic. There are also amplifier circuits that work in the "AA" class. In them, non-linear distortion is even less - up to 0.0005%. But the main drawback of transistor amplifiers is still there - a characteristic metallic sound.

"Alternative" designs

It cannot be said that they are alternative, just some specialists involved in the design and assembly of amplifiers for high-quality sound reproduction are increasingly preferring tube designs. Tube amplifiers have the following advantages:

1. Highly low value the level of non-linear distortion in the output signal.
2. There are fewer higher harmonics than in transistor designs.

But there is one huge minus that outweighs all the advantages - you must definitely install a device for coordination. The fact is that the tube cascade has a very high resistance - several thousand ohms. But the speaker winding resistance is 8 or 4 ohms. To match them, you need to install a transformer.

Of course, this is not a very big drawback - there are also transistor devices that use transformers to match the output stage and speaker system. Some experts argue that the most effective circuit is hybrid - which uses single-ended amplifiers that are not covered by negative feedback. Moreover, all these cascades operate in the ULF class "A" mode. In other words, a transistorized power amplifier is used as a repeater.

Moreover, the efficiency of such devices is quite high - about 50%. But you should not focus only on efficiency and power indicators - they do not speak of the high quality of sound reproduction by the amplifier. Much more important are the linearity of the characteristics and their quality. Therefore, you need to pay attention first of all to them, and not to power.

Scheme of a single-ended ULF on a transistor

The simplest amplifier, built according to the common emitter circuit, operates in class "A". The circuit uses a semiconductor element with an n-p-n structure. A resistance R3 is installed in the collector circuit, which limits the flowing current. The collector circuit is connected to the positive power wire, and the emitter circuit is connected to the negative. In the case of using semiconductor transistors with the structure p-n-p scheme will be exactly the same, only you need to change the polarity.

With the help of a coupling capacitor C1, it is possible to separate the AC input signal from the DC source. In this case, the capacitor is not an obstacle to the flow of alternating current along the base-emitter path. The internal resistance of the emitter-base junction, together with resistors R1 and R2, is the simplest supply voltage divider. Typically, resistor R2 has a resistance of 1-1.5 kOhm - the most typical values ​​\u200b\u200bfor such circuits. In this case, the supply voltage is divided exactly in half. And if you power the circuit with a voltage of 20 Volts, you can see that the value of the current gain h21 will be 150. It should be noted that HF ​​amplifiers on transistors are made according to similar circuits, only they work a little differently.

In this case, the emitter voltage is 9 V and the drop in the “E-B” circuit section is 0.7 V (which is typical for transistors based on silicon crystals). If we consider an amplifier based on germanium transistors, then in this case the voltage drop in the “EB” section will be 0.3 V. The current in the collector circuit will be equal to that which flows in the emitter. You can calculate by dividing the emitter voltage by the resistance R2 - 9V / 1 kOhm = 9 mA. To calculate the value of the base current, it is necessary to divide 9 mA by the gain h21 - 9mA / 150 \u003d 60 μA. ULF designs usually use bipolar transistors. The principle of its work is different from the field.

On the resistor R1, you can now calculate the drop value - this is the difference between the base and supply voltages. In this case, the base voltage can be found by the formula - the sum of the characteristics of the emitter and the "E-B" transition. When powered by a 20 Volt source: 20 - 9.7 \u003d 10.3. From here, you can calculate the resistance value R1 = 10.3V / 60 μA = 172 kOhm. The circuit contains capacitance C2, which is necessary for the implementation of the circuit through which the alternating component of the emitter current can pass.

If you do not install capacitor C2, the variable component will be very limited. Because of this, such a transistor audio amplifier will have a very low current gain h21. It is necessary to pay attention to the fact that in the above calculations the base and collector currents were assumed to be equal. Moreover, the base current was taken to be the one that flows into the circuit from the emitter. It occurs only when a bias voltage is applied to the output of the base of the transistor.

But it must be borne in mind that absolutely always, regardless of the presence of bias, the collector leakage current necessarily flows through the base circuit. In circuits with a common emitter, the leakage current is increased by at least 150 times. But usually this value is taken into account only when calculating amplifiers based on germanium transistors. In the case of using silicon, in which the current of the "K-B" circuit is very small, this value is simply neglected.

MIS transistor amplifiers

Amplifier on FETs, shown in the diagram, has many analogues. Including using bipolar transistors. Therefore, we can consider as a similar example the design of a sound amplifier assembled according to a common emitter circuit. The photo shows a circuit made according to a circuit with a common source. R-C connections are assembled on the input and output circuits so that the device operates in the class “A” amplifier mode.

Alternating current from the signal source is separated from the DC supply voltage by capacitor C1. Be sure the field-effect transistor amplifier must have a gate potential that will be lower than that of the source. In the presented diagram, the gate is connected to a common wire through a resistor R1. Its resistance is very large - resistors of 100-1000 kOhm are usually used in designs. Such a large resistance is chosen so that the signal at the input is not shunted.

This resistance almost does not pass electric current, as a result of which the potential of the gate (in the absence of a signal at the input) is the same as that of the ground. At the source, the potential is higher than that of the ground, only due to the voltage drop across the resistance R2. From this it is clear that the potential of the gate is lower than that of the source. Namely, this is required for the normal functioning of the transistor. It should be noted that C2 and R3 in this amplifier circuit have the same purpose as in the design discussed above. And the input signal is shifted relative to the output signal by 180 degrees.

ULF with output transformer

You can make such an amplifier with your own hands for home use. It is carried out according to the scheme that works in class "A". The design is the same as discussed above - with a common emitter. One feature - it is necessary to use a transformer for matching. This is a disadvantage of such a transistor audio amplifier.

The collector circuit of the transistor is loaded primary winding, which develops the output signal transmitted through the secondary to the speakers. A voltage divider is assembled on resistors R1 and R3, which allows you to select the operating point of the transistor. With the help of this circuit, a bias voltage is supplied to the base. All other components have the same purpose as the circuits discussed above.

push-pull audio amplifier

This is not to say that this is a simple transistor amplifier, since its operation is a little more complicated than that of those discussed earlier. In push-pull ULF, the input signal is split into two half-waves, different in phase. And each of these half-waves is amplified by its own cascade, made on a transistor. After each half-wave has been amplified, both signals are combined and sent to the speakers. Such complex conversions can cause signal distortion, since the dynamic and frequency properties of two, even of the same type, transistors will be different.

As a result, the sound quality at the output of the amplifier is significantly reduced. When a push-pull amplifier is operating in class “A”, it is not possible to reproduce qualitatively complex signal. The reason is that the increased current flows constantly through the arms of the amplifier, the half-waves are asymmetrical, and phase distortions occur. The sound becomes less intelligible, and when heated, signal distortion increases even more, especially at low and ultra-low frequencies.

Transformerless ULF

The low-frequency amplifier on a transistor, made using a transformer, despite the fact that the design may have small dimensions, is still imperfect. Transformers are still heavy and bulky, so it's best to get rid of them. A much more efficient circuit is made on complementary semiconductor elements with various types conductivity. Most of the modern ULFs are performed exactly according to such schemes and work in class "B".

Two powerful transistors used in the design work according to the emitter follower circuit (common collector). In this case, the input voltage is transmitted to the output without loss and amplification. If there is no signal at the input, then the transistors are on the verge of turning on, but still turned off. When a harmonic signal is applied to the input, the first transistor opens with a positive half-wave, and the second one is in the cutoff mode at this time.

Therefore, only positive half-waves can pass through the load. But negative ones open the second transistor and completely block the first one. In this case, only negative half-waves are in the load. As a result, the signal amplified in power is at the output of the device. Such a transistor amplifier circuit is quite effective and is able to provide stable work, high-quality sound reproduction.

ULF circuit on one transistor

Having studied all the above features, you can assemble an amplifier with your own hands on a simple element base. The transistor can be used domestic KT315 or any of it foreign analogue- for example BC107. As a load, you need to use headphones, the resistance of which is 2000-3000 ohms. A bias voltage must be applied to the base of the transistor through a 1 MΩ resistor and a 10 µF decoupling capacitor. The circuit can be powered from a source with a voltage of 4.5-9 Volts, current - 0.3-0.5 A.

If the resistance R1 is not connected, then there will be no current in the base and collector. But when connected, the voltage reaches a level of 0.7 V and allows a current of about 4 μA to flow. In this case, the current gain will be about 250. From here, you can make a simple calculation of the transistor amplifier and find out the collector current - it turns out to be 1 mA. Having assembled this transistor amplifier circuit, you can test it. Connect the load - headphones to the output.

Touch the input of the amplifier with your finger - a characteristic noise should appear. If it is not there, then most likely the design is assembled incorrectly. Recheck all connections and element ratings. To make the demonstration clearer, connect a sound source to the ULF input - the output from the player or phone. Listen to music and appreciate the sound quality.