Rice. 1. Prototype

Rice. 2. Amplifier circuit (click to enlarge)

Rice. 3. Power supply circuit (click to enlarge)

Rice. 4. Scheme of turn-on delay

Rice. 5. Holder

Why am I doing transistors? Here is a tube amplifier for Tango with Tamura on the bottom shelf of the rack! This is a question. In my school years, at the dawn of my amateur radio in the city in which I then lived, only lamps were available from radio components. Transistors were then just beginning to enter radio amateur use as fashionable things. Even the magazine "Radio" in those days presented transistor electronics as something special. Likewise, when I was assembling another tube structure from parts uprooted from old radios, I dreamed of someday assembling a transistor amplifier with crazy power and reproducible frequency band for those times. And although an incalculable number of years have passed since then, and over the years I have assembled amplifiers with what power, apparently it is precisely this desire that was laid down at that time that still does not leave my amateur radio soul in peace so bewitchingly luminous lamps. But enough nostalgia, let's get to the point of this story.

The feature of this amplifier, unlike all transistor sound circuits known today, is not at all abstractionist drawn transistor cascades. The trick is that in this amplifier there is no Active Voltage Amplifier usual for transistor and tube-transistor circuitry. The function of amplifying the voltage in this amplifier is performed by a passive component - a specially made step-up transformer. You will say - look at the circuits of the first transistor amplifiers - not even one transformer was used there. That's right, but in those first amplifiers, the transformers only matched the impedance of the amplifier stages between themselves and the load. And these first amplifiers with matching transformers sounded, if you don’t remember how to put it easier ... Although I remained grateful to these first transistor amplifiers, because they put doubts in me that tubes were not promising for sound. And how could there be no doubt in a direct, completely unorganized comparison - in those days, music lovers listened to music on tube amplifiers.

Not quite by the way, but in general on the topic of sound - later we were once again exactly the same as then with transistors, bred with digital sources - progress, where would it be:.

But back to the chips. So, the main feature is a special step-up transformer. In order not to frighten with this word at the very beginning of the story, I will make a reservation that this is a specially made transformer for a transistor amplifier. Not for lamp. Therefore, only a lazy person can not make it, it would be more or less decent-sounding electrical steel with a cross section of more than five or six square meters. centimeters. In our country, this was in the old days, no doubt. But about this in a separate enclosed material on the calculation of such a transformer, in which, if I have time, I will also lay out a program for calculating this transformer on any decent-sounding material and type of core. This is about homemade ones. The rest, which breaks winding any trances, you can use ready-made ones, for example, those indicated on the amplifier circuit. Our modern trances from all sorts of labs, as well as from the remaining living transformer industries involved in sound transformers, I highly do not recommend. Since I know the situation with our modern materials and, most importantly, with the brains involved in this field - according to the latest statement - the market for our sound equipment is almost zero. From foreign manufacturers, you can find, if not almost suitable transformers for this amplifier (look at the circuit diagram), then order with the desired characteristics. They, these manufacturers, as far as I know, will even be happy about this.

So, the characteristics of the step-up transformer:

• input effective voltage up to 2 Volts;
• the resistance given to the primary winding is approximately 40 ohms;
• transformation ratio 1: 5: .10, depending on the desired output power, on which, by the way, literally everything in the output transistor stage depends on this amplifier;
• resistive resistance of the secondary winding is not more than 200 ohms.

For a test, though with a not very predictable sound result, you can use a transformer from a tube headphone amplifier, put forward by the secondary winding, as a step-up. At the same time, it is worthwhile, listening to the frequency response of the amplifier, to play around with a resistor that shunts the step-up winding. The value of which cannot be lower than 5 kOhm, speaking offhand, not counting. And when calculating, you need to start from the resistance given to the primary winding of the transformer - it should be about 40 ohms.

Let's move on to the device of transistor current amplification circuits. During my amateur radio life, I tried every possible, which was only known, transistor circuitry for building current amplification stages. And only two types of the whole variety of current amplification circuits seemed to me musical. One of them is the one that is used in the circuit of this amplifier. This is an elementary classical circuit with a reduced (!) Stability of the output stage quiescent current, reduced by fundamentally disorganizing feedback on the output transistors. You can find a description of the operation of such a circuit in any textbook on transistor circuitry.

The thermal stabilization of the quiescent current of the output transistors of such a circuit is made straightforwardly unpretentious and, as a result, not very effective - by thermal coupling between the output transistor and the transistor that is on the buildup of the output transistor (hereinafter - swinging transistor, driver, etc). Due to such a simplified thermal stabilization mechanism, the output stage of an amplifier based on such circuitry requires careful calculation of the thermal conditions of transistors and a somewhat more serious approach to the design of heat sinks. That is why, for this type of output transistors, I indicate specific values ​​​​of the supply voltage of the output stage of amplifying the current of this amplifier from different impedances of the connected speakers. Regarding this amplifier, I will immediately note that its first stage of amplifying the current on transistors Q1:Q4 also requires thermal stabilization of the quiescent current of the output transistors of the stage through thermal coupling between them - placing the corresponding pairs of transistors on one radiator with a dissipated thermal power of about two watts.

In practice, this thermal stabilization can be performed by placing the necessary transistors on both sides of the landing pad of each heat sink towards each other, i.e. landing transistors with mounting holes on one tightening screw on different sides of the heat sink. It is also possible to more effectively stabilize the quiescent current of the output transistors. Those. organization of a closer thermal connection between transistors. It is this design solution that I use in the current amplification output stage of this amplifier - the corresponding pairs of transistors are placed close to each other on a plate made of a material with high thermal conductivity, such as copper, which itself is already mounted on the main aluminum heat sink. Thus, we significantly increase the efficiency of the mechanism for stabilizing the quiescent current of the output transistors, while the temperature of the transistor crystals decreases by about fifteen to twenty degrees Celsius relative to the traditional way of placing transistors on a heat sink and is far from critical for semiconductors.

The copper plate on the side of the main heat sink must be tin-plated. To make life easier, in order to exclude the electrical decoupling of transistors located on the same heat sink, thermal stabilization of the quiescent current of the output transistors is also possible through thermal coupling of the swinging and output transistors of the opposite arms of the circuit. But the temperature of the crystals, at which the quiescent current of the output transistors will stabilize in this case, will be higher than that of the method I use. And if the thermal calculation of the transistor operation mode is incorrect, this temperature can approach the critical temperature for transistor crystals.

Now about the amplitude linearity of the current amplifier circuitry used in this amplifier - it is usually carried out by executing the load of swinging transistors in the form of current sources, see fig. 1. But, instead of words, the AD797 operational amplifier circuit, with the same output stage, and probably the best linearity among opamps, would be more appropriate and indicative. It was in this classic version that I used a similar output stage circuitry in my amplifiers for more than twenty years ago. A few years ago, I argued on this issue with a friend who convinced me to try the option of stabilizing the current of the swinging transistor by means of a voltage boost, similar to the well-known scheme of 87 from the Radio magazine or described in my favorite book by Tietze and Schenk of 83 on transistor circuitry.

But I took this step, taking into account something completely different, namely the great-sounding Quad 405 amplifier, which also uses a similar solution. And also realizing that capacitors for these purposes must have a high sound quality, i.e. non-resonant, linear impedance over a wide frequency band. How could I get such capacitors, compared the sound of the cascade with a current source - and once again confirmed the correctness of my own approach in designing transistor amplifiers - the fewer semiconductors stand in the way of sound, the more musical the amplifier sounds. But, for certain reasons, he actively concealed the fact of the superiority of the variant of the circuit with a voltage boost until now. I will say more, as a result of this action, I got the results that I expected.

Now let's move on to calculating the load resistors of the swing transistors, which determine the current of both the swing transistors and the output transistors. At rest, the base-collector voltage of the output transistor of the stage is applied to these resistors. With sufficient accuracy for this calculation, we can take this voltage equal to the supply voltage of the stage arm minus the voltage falling at the base-emitter of the output transistor, which is approximately equal to 0.5:0.7 Volts. Next, you need to decide how much current should flow through the output transistors. In this matter, I am not a sadomosist, and what matters to me is not any electrical idea in the form of adherence to the generally accepted "sounding" class of circuit operation, but only sufficiency in the transfer of musicality.

After much experimentation on the heat sinks used, I settled on a quiescent current of 80:150 mA, depending on the type of transistors used. Transistors of different manufacturers and models, as well as lamps, sound differently, including for each transistor model they have a certain "sounding" value of the quiescent current for a specific circuitry of the amplifier stage and heat sink with a specific value of thermal resistance. Regarding the transistors indicated in the diagram and the heat sinks I used, the value of the quiescent current of the output stage transistors was 130 mA. The same current must flow through the calculated resistances. Otherwise, applying Ohm's law, we obtain the value of the resistor that loads the swinging transistor.

I will not dwell on the calculation of the details of the voltage boost circuit, due to the elementarity of such a task, I will only say that the capacitor value indicated on the amplifier circuit is sufficient for the effective operation of the voltage boost circuit in the required frequency band with the values ​​\u0b\u100bof the quiescent currents of the output transistors indicated by me. I also do not recommend using a capacitor with a higher rating, based on elementary considerations for the operation of capacitors on alternating current. Further, in order not to complicate life once again, we take the value of each resistor of the voltage boost circuit equal to half the value of the load resistance of the swinging transistor. The next question is about the supply voltage of the current amplification output stage of this amplifier. This question for this amplifier output stage circuitry is the most important. The stability of the cascade and its sound depend on it. In order not to delve into these difficult jungles, I will focus on the fact that empirically, on transistors with a power dissipation of about XNUMX W, the following dependence was obtained for the output stage of this amplifier:

Based on these values, we obtain the values ​​​​of each of the four resistors of the voltage boost circuits for a load of 4 ohms equal to 100 ohms. For the second load, I provide the opportunity to practice calculating resistors on my own.

After that, according to known formulas, you need to calculate the power value of these resistors. That's all, the calculation of the amplifier is completed.

Let's get down to the most important - constructive. Before that, another small digression. I believe that the design in transistor audio technology affects the sound of the amplifier to a much greater extent than in tube technology. Speaking now about sound, I certainly mean the subtle moments of sound available to audiophiles and advanced music lovers who also hear these moments, but treat them philosophically.

So, the design of this amplifier. First, no printed circuit boards. Only hinged mounting, soldering points are organized either on the terminals of transistors, or on mounting petals, riveted on separate boards of insulating material. Once again I repeat - observe the soldering points and input / output of conductors, which are indicated on the circuit diagram of the amplifier, this determines the sound of the amplifier to a large extent when using sounding components. Otherwise, you will not recoup some of the money spent on the purchase of high-quality radio components. Quality conductors are also included in the sounding components for this amplifier. You can use Cardas mounting wires, you can use our old wires made of soft dark red untinned copper without insulation. You organize the insulation later, after desoldering, for example with electrical paper, and where it is reasonably necessary.

Secondly, each channel of the amplifier is assembled by a separate design, including decoupled power supply, including a power transformer. And structurally, the current amplification stages are also not combined. The first stage is assembled on a separate circuit board, the output stage is made as a separate three-dimensional structure, the main bearing body part of which is shown in Fig. 5. This part, with a larger area, is attached to the amplifier's own chassis through vibration decoupling. The holes of this body part are designed to accommodate capacitors C5 and C6. On top of this part, with an air gap of 1 cm, the heat sinks of the output transistors are attached, with the transistor mounting pads facing each other. The heat sinks of the output transistors were designed specifically for this amplifier and are non-blackened air radiators with an effective area of ​​490 cm ^ 2 made of aluminum, with eight fins 4 mm thick and 45 mm long on one side. The transistor mounting pad has a width of 80mm, a height of 50mm and a thickness of 10mm. All the remaining components of the output stage are located between these heatsinks and, as I have already mentioned, they are soldered directly on the terminals of the transistors and the mounting plate with petals, which is fixed in the middle between the heatsinks on the main case of the output stage.

Now attention! I will dwell in more detail on capacitors C5 and C6. Holes in the housing part of the output stage are designed to accommodate them, see fig. 5. I tell you how it should happen. We take a thin (0.05 mm) copper foil and wrap the capacitors with an interference fit several times. On top of the copper we put a couple of layers of thin fiberglass also in tension. Already on it we wind the amount of wire calculated for a power of 10 W and a voltage of 15..30 Volts from any material with high resistivity and organize the conclusions of the resulting heating element. From above, we again put a couple of layers of thin fiberglass into the tightness and one layer of thin copper foil also into the tightness. Layers of copper foil are electrically connected to the amplifier case. This design must be done very carefully, and so that it does not have its own resonances, it must be impregnated with any viscous, non-drying organosilicon liquid. After that, we insert this assembly into the hole of the body part and fill the remaining space with silicone sealant. I do not specify the exact design of the heater, because if you cannot calculate it yourself and organize its operation, then I do not advise you to take on the manufacture of this amplifier at all. The temperature on the surface of the capacitors C5 and C6, which this heater must provide, is 50-60 degrees Celsius for the first production brand ELNA CERAFINE. For capacitors of other brands, you should select this temperature by ear. I may give an explanation for this approach in the design of transistor amplifiers in the description of my new audio transistor amplifier, which is completely replete with such esotericism. If his time comes. But for the heater. If you do not use automatic temperature monitoring, it would be better to power the heater with alternating current, taking it from the power transformer of the channel. If there is automation, then from a separate power transformer, on which in this case you can hang up the power of the loudspeaker turn-on delay circuit.

Now briefly about the delay circuit - a conventional electronic time relay, the delay is due to the time constant of the capacitor power circuit located in the base of the composite transistor. An important question about the relay is that its contacts affect the sound of the amplifier. I have little experience in this matter, since I have long settled on the TKE52PDU brand relay. This relay is used in automation devices in the nuclear industry. On the delay diagram, I indicated a well-established Fyujitsu relay, it will probably be easier to find.

Well, the last. What looks like a fuzz, but is abbreviated as GA. This is the second esoteric in this amplifier. Means - anisotropic current harmonizer. My new amplifier, which I have already mentioned, is completely esoteric - rotating transformers, coherent current sources, etc. In this I stopped at number three. So, how is this harmonizer performed? Two copper lugs are rigidly fixed at a distance of 8 mm, a conductor 0.1 mm in diameter is soldered between them. I use rhodium wire exposed to 10^22 neutron flux. In the simplest case, the conductor can be copper, but in order for it to have the properties necessary for the harmonizer, it must be naturally formed, i.e. aged over 40:50 years. Such a conductor, for example, can be taken from the RF coils of old radios. The physics of this process is quite complicated for an elementary presentation, perhaps an associative-similar model can be represented as a kind of nozzle laminating the flow.

What is the sound quality of this amplifier? The sound is very clear, tube filled and lively, and very fast. I don’t have the habit of describing subtle moments in words. I'd rather tell you about the stages of the path. The first version of this line of amplifiers was a discrete amplifier with a differential stage at the input and a transistor driver in the OE, loaded by a current source - the output stage was already the same as shown in Fig. 1. OOS was present in that amplifier; in the early eighties, the struggle against measured distortions only flared up. After this amplifier, I only came across a published book by Tietze and Schenk, and I put an operational amplifier to drive this output stage, introduced anti-parasitic resistors into all the bases. But the feedback, either by mistake or by providence, was introduced from the output of the operational amplifier. In response to this, I heard such a filling sound that I began to figure out what I had done. And when I figured it out, I began to experiment with the buildup of the output stage. The scheme in fig. 1 is just from this series, closer to the middle of the 90s and this can be seen from the picture, which is of the same age. I talked about this scheme in the nineties at the FIDO conference. The last circuit using tubes in this line of amplifiers was a design from UN to 6E5P with a transformer 5K: 150 Ohm and beyond the same UT as in fig. 1. I talked about this, the last version of the hybrid, in one of the local Internet audio forums about two years ago. Well, then there was an amplifier to which this story is dedicated.

Everything about this amplifier. I also wanted to tell you about the difference between sound engineers and electronic engineers who design sound circuits, but changed my mind. Although one of my observations - how many I met such engineers, I did not note any musical ear or deep musical preferences. That's when I realized why they are so fond of evaluating the sound quality of sound equipment with all kinds of distortions, and why it is so important for them to measure these distortions with a measuring device. And the fact that the high sound quality of amplifiers is extremely weakly linked to any distortion is of little concern to these engineers. But I am not an electronics engineer, and as a physicist, truth is most important to me. Yes, this also applies to the sound quality of this amplifier.

But why am I doing transistors? Of course, it's easiest to blame Freud. But no, the answer to this is different - because in the lamps it has long been transparently clear. And where to train your brain, if not on a transistor sound? I also seem to have figured out digital technology, but oh, how I don’t want to get into vinyl matters - I’m almost satisfied with the sound of Soviet records with classics on Micro with Rega 300. Although they have drawbacks:

Therefore, I will not swear in anything.

Author: Vladimir Ul'yanov (Vladimir Ulyanov); Publication: cxem.net

See other articles Section Transistor power amplifiers.

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