Circuit diagram of the input stage of the UHF on germanium transistors. Germanium transistors. Classes of operation of audio amplifiers

Instead of an epigraph:
- And who made up such bullshit? This inventor's hands should be torn off...
- So, it’s your job! Or didn't you find out?
- Holy shit, damn it!
One of the old jokes

Probably many Datagorians, if not all, watched the cartoon “Well, Just Wait” in childhood. Including the ninth episode, where the wolf tried to play the electric guitar.

Well, if it happened in a school or club VIA (rock band or some other amateur performance), it was, of course, simpler. There was some kind of apparatus there. What if at home?

I was once little different from many others. I “stuck” a guitar into a tape recorder, a Ural-112 radio (sorry, the guitar was not a Ural), an amplifier from some other tube radio, inserted into a homemade case, into amps soldered according to circuit diagrams from magazines. I was looking for details, struggling with finalizing the circuits.

Now the task has been somewhat simplified, and if you have the required amount of banknotes in your pocket, you can find the required device in any regional center in a music store. From inexpensive, “of unknown Chinese origin,” to a company with the price of an airplane. Well, or a hybrid, that is, production (sometimes quality) is China, but the appearance and bells and whistles are like those of a company. Price too.

Yes and with self-production it seems to have become easier. You can find a circuit of any quality and complexity on the Internet. There are no particular problems with radio components, at least in the stores of those very regional centers (if they have banknotes, of course). And sometimes something from the previous shortage is lying around for free.

So I decided to talk about the amplifier that I now use at home. About an amplifier made practically from scrap material. Moreover, one that was already considered hopelessly outdated at the end of the 20th century, not to mention the beginning of the 21st, when everything was done. Moreover, it is not at all for guitar purposes.

Perhaps this article will make someone more experienced in the design and construction of amplifiers laugh. Someone will consider it “instructions on what not to do.” But I’d better start in order. That is, from afar.

New life for an old board

I took it with me just in case, otherwise they would have thrown it away anyway. The block turned out to be quite working. I drew a circuit diagram on the board. It turned out something like this:

True, during the installation of the operating point, the tuning resistor R1 (the one that was on the board showed 20 Ohms when measured) fell apart. And until recently, it was periodically replaced either with a jumper, or with other equally liquid trimmers, or with a constant resistor. Now I installed a trimmer, soldered from the wreckage of some photocopier. It's holding up for now.

As it turned out later, this is a very popular scheme among Soviet tape recorder manufacturers. For a long time, with minor changes, it was used in various bobbin makers, and even in the first cassette makers.
Here is an example circuit found in Radio magazine. The same thing, only with an emitter follower at the input. And other transistors at the “end”. And all this was connected to a universal tube amplifier.

Version 1.0 or “Radio Destroyers for the National Economy”

I quickly made a power supply and microphone amplifier from spare parts found at home. I stuffed it all into an unnecessary housing from an AVU unit, found in the same warehouse. The case is flat, does not take up much space, and can be hung on the wall. I connected to all this the M-TGU microphone found in supplies, which was lying idle due to its unimportant frequency response. But this microphone has a built-in button that, when not pressed, shorts the input to ground.

Microphone "M-TGU"

A subscriber loudspeaker (radio point) was installed in the hall without a matching transformer and volume control. Screw terminals, familiar to many from school laboratory work in physics, were used as a connector for connecting the loudspeaker to the amplifier. The connectors were found in the same warehouse; I still don’t understand what they did there.

The device, although slightly noisy and moderately loud, coped with the task. And then in one of the villages of the region, during the liquidation of the legacy of communism, the broadcast radio network was dismantled. And in place of my product, a broadcast amplifier taken from there was installed. Of course, it smells like shooting sparrows out of a cannon, but you can’t argue with the authorities. On the other hand, the broadcaster has a power reserve, and my two-watt (based on the results of later measurements) amplifier worked almost at its limit, even in that small hall.

Version 1.1 or “God grant you what is not good for us”

In principle, two honest Soviet watts (one and a half at a load of 8 ohms), fed to no less honest, not even necessarily Soviet, acoustics - the power is quite sufficient to play along with an acoustic guitar in an ordinary, not very large room, with sufficient volume and not clog " vocalist" if there is one.
And taking into account the soundproofing of our apartments and neighbors, we can have a lot of fun.

The typical frequency range of most tape recorder amplifiers is even somewhat wider than what is needed for a guitar. But at that time I was not yet familiar with the opinion of the “experts” regarding its additional artificial narrowing (where did they come from in our wilderness, then still without the Internet?) Moreover, the device was not intended for concerts with orchestras and recordings in studios. And certainly not to compare with the company.

Back In The USSR, or Retro rules

I don’t know what came into my head then, but I decided to collect pred “in the same traditions” as UM. That is, on germanium transistors. Most likely because I had them and had nowhere to put them. Well, so as not to do magic with the power supply - silicon pnp transistors There were not enough supplies, as well as microcircuits. And I didn’t see the point in shoving an op-amp into a place where two or three transistors could get by.

There was no Internet in our wilderness at that time, and I learned the audiophile legend that germanium sounds better than silicon from the Internet, seven years later.

I am not an audiophile (I have respect for those who make their own equipment and do not make a religion out of their hobby), and all my experience of “listening to classics from vinyl through a lamp” comes down to “Anthropov’s” records with rock classics. n roll on the Ural-112 radio.

Let no one be confused by the number 1 at the beginning of the number of this radio; in terms of the characteristics of the sound path, the device was unlikely to be third class even according to the parameters of its time.

I listened to the rest of the classics (Soviet and foreign pop and rock) for a long time, albeit on a pure Germanium tape recorder “Snezhet-202”, but from reels recorded wherever possible. I seriously doubt that I would have felt the difference if I had played them either through high-fi or high-end.
Therefore, I don’t know how right they are about the sound of germanium. But the reliability of the electronic parts of old tape recorders, players and receivers, many of which have remained operational to this day, speaks for itself. So I decided to “shake up the old days” or “shake up the old times” or...

To begin with, I decided on the requirements:
1. The amplifier is made for as pure a sound as possible. All effects are in the form of separate lotions. Therefore, pre should be as linear as possible.

2. The input impedance must be high enough so as not to depress the “top” of the guitar signal and not to “interfere” with the operation of the tone control in the case of a direct connection.

3. Several inputs with different sensitivities. Microphone (0.3 mV), guitar (10 mV, just right for an old Soviet instrument) and line input (0.5 V).

The amplifier was sometimes planned to be used as a control amplifier, to check the signal flow, when repairing other amplifiers or other audio equipment, so the presence of such inputs would not hurt.

And it would be desirable to mix the signal from the line input with the guitar signal, to connect, for example, a tape recorder with a recording of the “accompaniment” or an existing homemade “rhythm box” (that’s right - in quotes, if I ever decide to post a description of the design, it will only be for laughter).

After excavating the rubble of papers, magazines and photocopies, the following diagram was compiled:

Initially, the circuit diagram, as far as I remember, was copied from some kind of amateur tape recorder. It has an input impedance of about 3 KOhm, with “microphone” sensitivity and a margin in the output signal level, allowing you to connect it directly to a power amplifier.

For the guitar input, the sensitivity was reduced by connecting a 100 kOhm resistor in series with the input. Not the best idea, I agree, even though it was used in industrial amplifiers. But with a minimum of details, it was possible to obtain a pre-design with two inputs of different sensitivity.
Moreover, the simultaneous use of these inputs was not planned.

Other options were also considered, but there were no field-effect transistors at hand, and somehow I didn’t want to connect an emitter follower to the input with “microphone” sensitivity.

From the output, the signal went through a simple passive mixer, where it could be mixed with the line input signal, to the input of the power amplifier.

Everything was collected in the same building from AVU:

And the product was given to novice guitarists at the other end of the region, where it was successfully used for several years to annoy neighbors.

One “undocumented opportunity” was also discovered there. When connecting a guitar to a microphone input, the output was a “terrible dirty overload”, which was used with might and main to master the riffs of groups, such as the then popular “Linkinpark” or the ageless “Aria”.
Although I suspect that even punks would spit and swear for a long time from the distorted sound.

Version 1.2 or “I wanted the best…”

I got the device again and used it for its intended purpose in my free time. That is, during breaks between seasons, shifts, etc.

And when I had a little more free time, I decided to subject the amplifier to another revision. Reduce the noise of the preamplifier a little more, which was listened to on maximum volume. Well, to overcome the nutritional background, which, although it wasn’t too stressful, was there.

First, I rebuilt the power supply:

The previous power supply was the simplest and consisted of a trans, a diode bridge and a 2000 uF capacitor.

Then I made some changes to the preamplifier circuit. I replaced the transistors with less noisy ones and adjusted the modes. I am afraid that in full accordance with the proverb about “make a fool pray to God.” Apart from the tester, ears and guitar, there were no measuring instruments at hand at that time. Focused on hearing to reduce the noise level, the absence of audible distortion and maintaining the block gain within acceptable limits.

The scheme began to look like this:

The mixer circuit is crooked, but it was made to minimize signal attenuation and ensure that the regulators influence each other as little as possible. Both goals were achieved in principle.

At that time, the amplifier was used with a Chinese “three-way type” speaker from a burnt-out active speaker. She appeared in a photo in one of the previous articles. Despite the case made of fiberboard (hardboard or “cardboard”, not to be confused with chipboard) with spacers that had long fallen off and lost, and three different-sized speakers connected from the factory in parallel without any filters, I liked the sound. But that column was not mine and was subsequently returned to the owner.

Now the sound is produced by an even more non-guitar speaker from an old player, with one 8GDSH-2 speaker (4 Ohms).

I completely agree with the review of similar speakers in one Datagor article. Naturally, you shouldn’t expect miracles from such acoustic design.
So if I manage to get a more suitable speaker, or one or three more 8GDSH-2/4GD-35 (which is less realistic), I will think about making a new speaker. Though Lately group radiators in guitar acoustics do not seem to be welcome. Just like in ordinary speakers “for music”, although that is where they are used with might and main.
In the meantime, this one will do just fine for the home.

Somehow, out of curiosity, I connected to this amplifier various speakers that were at hand: 10MAS-1, 15AC-220, unidentified, from music centers, so in terms of acoustics there is always room for experimentation.
The amplifier sounded quite normal. He gave out his honest two Watts. The background was almost undetectable. The noise of the input stage, although it was audible at maximum volume, was comparable to the noise level of many second- and third-class tape recorders. In general, the sound suited me quite well until time was freed up for another bout of experimentation.

The previous ones were done in a hurry, when adjusters “from the center” came to the communication center for a couple of days with a generator and an oscilloscope. Why did he stay after work, hastily scattering his “household” on the windowsill?

Data in general outline confirmed. But something came to light that I had not noticed then - a noticeable asymmetry of the output signal. Capacitor decoupling of the sound card input eliminates the influence of the DC component (for example, if the capacitor is faulty at the output of the PA), even if the DC component is present. So this most common option had to be discarded immediately.

“During the initial check,” it turned out that the pre-terminal transistor in the upper arm (MP40A) has a gain factor that is almost half that of a similar transistor in the lower arm (MP37A).

Of course, I understand that in those days it was necessary to pursue a plan without paying attention to the little things. And I also knew that third grade was far from being a fountain of hi-fi. I just didn’t suspect that everything was so neglected. Of course, the “departure” of parameters from “antiquity” should not be discounted, but not by the same amount. In addition, more often I have seen it the other way around - with n-p-n transistors.

Throughout the amateur radio literature of those times, it was written about the pairwise selection of transistors for the arms of push-pull PAs. Even if they are made for pocket receivers. Although the amateur usually didn’t have much to choose from - whatever he found he put in, as long as it suited the nutrition.

It makes a sound - it's already good. And besides your own ears, there is still nothing to check the sound quality with. Oscilloscope? But where can I get it? Therefore, there is no point in assembling a generator. There is still nothing to look at the signal shape. Calibrate the frequency regulator scale too.
Unless you use that generator as a probe to monitor the signal and measure levels.

I myself once used children’s Faemi keys for this purpose, not really bothering with the rectangular signal shape and frequencies different from the generally accepted ones. If this affected the accuracy of the measurements, I think it was not much more than the input resistance of the Ts20-05 tester at limits of less than 1 Volt.

The industry didn’t really bother with this issue either, despite the possibility of selecting parts and the availability of measuring instruments that an amateur could only dream of (many still continue to dream of).

I didn’t check the terminal transistors P214A, so as not to get even more upset, especially since their “strategic reserve” remained at the other end of the area.

I was pleased that by replacing the MP40A with an MP42B with characteristics closer to the MP37A and selecting the emitter resistor at the “thirty-seventh” (R12), it was more or less possible to level out the sine.

By the way, the distortions described above are practically unnoticeable to my ears, which are not spoiled by hi-fi. But the slightest distortion of the “smoothness” of the sinusoid (kinks, etc.) noticeably adds “dirt” to the sound.

Before the advent of the oscilloscope, I had to struggle for a long time with one amplifier, the right channel of which was noticeably fussy. It was especially noticeable when playing music with a predominance of acoustic instruments and clean sound. On all sorts of “overloaded” styles, this was not so audible. For a more accurate assessment, a guitar was connected to the input and the sound of two simultaneously sounding strings was clearly dirty (in the past I often used such a “dual-frequency generator” to assess distortion by ear).

The oscilloscope immediately showed the presence of “step” distortion. More precisely, there was not even a step, but only a hint of it, due to a faulty tuning resistor.

Since the device was disassembled anyway, I decided to experiment a little more, to test one old idea.

Version 1.3 or “That’s it for now”

I clarified the requirements for the updated scheme:
1) Germanium transistors.
2) Sensitivity 10 mV.
3) Based on the previous point and the sensitivity of the PA, the voltage gain is 10 times.
4) Input resistance is the maximum that can be squeezed out.
In principle, nothing is impossible.

It should be noted that in magazines and other newly published literature at that time, silicon and IC + op-amp were already ruling the roost. Schemes for MP and GT were found less and less often, usually in various publications such as “To Help the Radio Circle” and in the section for beginners of “Radio” magazine. Although from there they have already begun to be replaced by red KT315s.

Most of the germanium circuits from those sources were not much more complex than those used to describe the operation of the amplifier stage (two resistors and two capacitors per transistor). Often without indicating transistor modes, configuration recommendations, and some equally important characteristics of the units. In principle, for a beginner, the very fact of the operation of the first assembled circuits is more important. When you gain experience, you can start making improvements.

I repeat that I did not see anything particularly difficult in finding a suitable scheme. Moreover, there were several in mind that were suitable at first glance.

The fourth point can be completely solved by using an emitter follower at the input. At this level of input signal, I am no longer against its use. The third point will provide almost any transistor stage using a circuit with a common emitter, even without any particular difficulties in selecting a transistor based on the gain.

In general, I got down to business and... it began!

I almost wrote a bunch of text about the progress of work and overcoming the difficulties that arose, input-output resistances, modes and other coordination of cascades. But then I thought and decided - who needs it? Experienced radio amateurs have once gone through all this, so they already know. And for beginners, a lot of not very coherent text from the teapot, with elements of “alchemy,” will also not have much practical value. Yes, and in size it would be a separate article, which can and will be written someday. If not by me, then by a comrade who knows the “materiel” better.

I will leave only one well-known conclusion: the choice of isolation (and all other) capacitors must be approached as carefully as possible. I’m not talking about using exclusively audiophile capacitors with a price “for a very big amateur.”
I mean that the compliance of the capacitance with the value indicated on the case (and required for the circuit) and the leakage of those that are going to be soldered into the circuit need to be checked. Otherwise, it may suddenly turn out that a transistor of some cascade works best if the bias circuits are removed. Or, out of the blue, completely new regulators will “crunch.” Or is it worth replacing the capacitor and carefully adjusted modes according DC, and sometimes alternatingly, will go wrong.

In general, the result of all my “dancing with a tambourine” was this scheme.

At first I wanted to install a volume control between the stages, instead of R4. That's why I chose a two-stage circuit with capacitor decoupling. Only a suitable variable resistor was not found, so this is still in the plans.

Tests have shown that the characteristics almost meet the original requirements.
The noises with the closed entrance went somewhere to the limit of audibility. The output signal was enough to drive the PA, even taking into account the drop on the mixer. The sound was also quite satisfactory.

All that's left to do is assemble the block on the board, install it in the case, and I'll be happy. Old board was already made familiar to simple circuits"non-printed installation":

For some reason, this time I decided to make a normal (as possible) signet. Probably because I found a suitable piece of foil PCB. I quickly sketched out the location of the holes and paths on paper. I marked holes on the foil, drilled them, drew paths, etched them, and soldered the parts. It turned out something like this:

The bad habit of compacting the installation as much as possible took its toll. It seems that I haven’t “embedded” additional blocks into factory products for a long time. Children's dreams of something radio-controlled and flying remained in that very childhood. And I’m still trying to make the fee as low as possible. Although it’s not necessary, it seems.

Plus a second, no less bad habit: I just can’t bring myself to cut off the conclusions of the details “to the furthest point.” Too often at one time it was necessary to build them up on parts soldered from factory boards made according to all the rules.

The power supply has been slightly modified, taking into account the higher supply voltage of the new control unit:

During final assembly, I re-soldered the interconnect wiring. The previous one was done mostly hastily and contained a bunch of extra wires, which I didn’t understand right away. Even before this, the background from the speaker could only be heard in complete silence. So I don’t know whether the new wiring (in particular the ground) had a significant impact on the background/noise level.

This is what it looks like from the inside:

Fun experiments on the final version

Sine wave from the generator built into the “analyzer”. Signal at the generator output (linear output of an external sound card):

Signal at the PA load (Uout close to maximum):

Basically nothing unpredictable. I did not expect superior performance from this product. There is no noticeable distortion of the shape - and that’s good. Unless you can still “conjure” with the power supply.

For checks during the work, a home-made generator, hastily soldered, was used. He gave a slightly more optimistic picture:

Unlike the pictures above, here we used the built-in sound card. The higher noise level is immediately noticeable. And conclusions regarding its use suggest themselves. True, this is not relevant to the topic of the article.

And this is what a rectangular signal looks like, or rather the signal from the output of the “Faemi” tool described in my recent article.

An external sound card was used for testing. I won’t show you what the built-in one does with the signal so as not to scare anyone.
Nothing unexpected either. Trimming along the “bottoms” and “tops”. To complete the picture, it would be possible to remove the frequency response, but why? The amplifier was not made for high-fi, but for a guitar.

Conclusion

Evil tongues claim that guitars in that country, which has been gone for some twenty years, were made for anything other than music.
And they play on this... only losers and beggars who are unable to buy something more correct.

Maybe they are right in some ways, but I think that even the coolest and most branded instrument is unlikely to make me a cool musician. Whether I want to strum for myself or for friends - my instruments cope with this task quite well. Moreover, over the years that I have been using them, I have adjusted them to my hands and my hands have gotten used to them. I have already posted the sound of one of my “balalaikas” in previous articles.

If any of the respected Datagorians finds blunders in the diagrams and text or opportunities for improvement that I missed, please point your finger. Let's get better!
The most reasonable advice - “throw away all this old nafik and solder on microcircuits or lamps” will be considered, but is unlikely to be implemented. Unless when creating a completely different design.

P.S.

Typical mistakes when designing germanium amplifiers occur due to the desire to get a wide bandwidth, low distortion, etc. from the amplifier.
Here is a diagram of my first germanium amplifier, designed by me in 2000.
Although the scheme is quite workable, it sound qualities leave much to be desired.

Practice has shown that the use of differential cascades, current generators, cascades with dynamic loads, current mirrors and other tricks with environmental feedback do not always lead to the desired result, and sometimes simply lead to a dead end.
Best practice results for obtaining High Quality sound, gives the use of single-ended cascades before. amplification and the use of inter-stage matching transformers.
We present to your attention a germanium amplifier with an output power of 60 W, into a load of 8 ohms. Output transistors used in the amplifier are P210A, P210Sh. Linearity 20-16000Hz.
There is practically no subjective lack of high frequencies.
With a 4-ohm load, the amplifier produces 100 watts.

Amplifier circuit using P-210 transistors.

The amplifier is powered by an unstabilized power supply with a bipolar output voltage of +40 and -40 volts.
For each channel, a separate bridge of D305 diodes is used, which are installed on small radiators.
Filter capacitors, it is advisable to use at least 10,000 microns per arm.
Power transformer data:
-iron 40 to 80. The primary winding contains 410 vit. wires 0.68. Secondary at 59 vit. 1.25 wires, wound four times (two windings - the upper and lower arms of one amplifier channel, the remaining two - the second channel)
.Additionally regarding the power transformer:
iron w 40 by 80 from the power supply of the KVN TV. After the primary winding, a copper foil screen is installed. One open turn. A lead is soldered to it which is then grounded.
You can use any iron that has a suitable cross-section.
The matching transformer is made of Sh20 by 40 iron.
The primary winding is divided into two parts and contains 480 vit.
Secondary winding contains 72 turns and is wound in two wires at the same time.
First, 240 vit of the primary is wound, then the secondary, then again 240 vit of the primary.
The diameter of the primary wire is 0.355 mm, the secondary is 0.63 mm.
The transformer is assembled into a joint, the gap is a cable paper gasket of approximately 0.25 mm.
A 120 Ohm resistor is included to ensure no self-excitation when the load is off.
Chains 250 Ohm +2 x 4.7 Ohm are used to supply the initial bias to the bases of the output transistors.
Using 4.7 Ohm trimmers, the quiescent current is set to 100mA. The resistors in the emitters of the output transistors are 0.47 Ohms, and there should be a voltage of 47 mV.
The output transistors P210 should be almost barely warm.
To accurately set the zero potential, 250 Ohm resistors must be precisely selected (in a real design they consist of four 1 kOhm 2W resistors).
To smoothly set the quiescent current, trimming resistors R18, R19 type SP5-3V 4.7 Ohm 5% are used.
Appearance amplifier at the rear, shown in the photo below.

May I know your impressions of the sound of this version of the amplifier, in comparison with the previous transformerless version on the P213-217?

Even more rich, juicy sound. I would especially like to emphasize the quality of the bass. The listening was carried out with open acoustics on 2A12 speakers.

- Jean, why exactly are P215 and P210, and not GT806/813, included in the diagram?

Carefully look at the parameters and characteristics of all these transistors, I think you will understand everything, and the question will disappear by itself.
I am clearly aware of the desire of many to make the germanium amplifier more broadband. But the reality is that many high-frequency germanium transistors are not entirely suitable for audio purposes. Of the domestic ones, I can recommend P201, P202, P203, P4, 1T403, GT402, GT404, GT703, GT705, P213-P217, P208, P210. The method of expanding the bandwidth is the use of circuits with a common base, or the use of imported transistors.
The use of circuits with transformers has made it possible to achieve excellent results on silicon. An amplifier based on 2N3055 has been developed.
I'll share it soon.

- What about the “0” at the output? With a current of 100 mA, it is hard to believe that it will be possible to keep it at an acceptable +-0.1 V during operation.
In similar circuits from 30 years ago (Grigoriev’s circuit), this is solved either by a “virtual” midpoint or by an electrolyte:

Grigoriev amplifier.

The zero potential is maintained within the limit you specify. The quiescent current can be set to 50mA. Monitored with an oscilloscope until the step disappears. No more need. Further, all op-amps can easily handle a 2k load. Therefore, there are no special coordination problems with CD.
Some high-frequency germanium transistors require attention and additional study in audio circuits. 1T901A, 1T906A, 1T905A, P605-P608, 1TS609, 1T321. Try it and gain experience.
Sometimes there were sudden failures of transistors 1T806, 1T813, so I can recommend them with caution.
They need to install “fast” current protection, designed for a current greater than the maximum in a given circuit. To prevent protection from triggering in normal mode. Then they work very reliably.
I’ll add my version of Grigoriev’s scheme

Version of Grigoriev's amplifier circuit.

By selecting a resistor from the base of the input transistor, half the supply voltage is set at the point where the 10 ohm resistors connect. By selecting a resistor in parallel with the 1N4148 diode, the quiescent current is set.

- 1. In my reference books, D305 is normalized to 50V. Is it safer to use D304? I think 5A is enough.
- 2. Indicate real h21 for devices installed in this layout or their minimum required values.

You are absolutely right. If there is no need for high power. The voltage across each diode is about 30V, so there are no reliability issues. Transistors with the following parameters were used; P210 h21-40, P215 h21-100, GT402G h21-200.

Having become fed up with designs based on lamps and modern components, lately, in a nostalgic impulse, I have been toying with designs based on germanium transistors.

Having read on forums that, supposedly, due to imperfect production technology, their parameters degrade greatly over time, to check my reserves, I even purchased an L2-54 industrial meter for the parameters of transistors and low-power diodes.

I tested more than a hundred different copies of transistors and I can note with satisfaction that not a single one was rejected - all correspond to the reference data with at least one and a half times (and most often with 2-3 times) margin. So it’s not at all a sin to employ them, especially since in my youth many of them were as desirable as they were unavailable.

And we start traditionally - with ULF construction.

A number of popular amateur radio receivers to this day, for example, are made on germanium transistors and are designed to work with high-impedance headphones, which are now in short supply. Simple emitter followers recommended there for increasing output power can provide more or less decent sound only to connected low-impedance headphones (100-600 Ohms) or low-impedance load (4-16 Ohms modern headphones or speaker), connected through a transformer with a Ktr of at least 1/5 (1/25 in resistance) and still, at low levels, step-type distortion is strongly affected. You can, of course, try to install modern ULFs on ICs there, but they require positive power supply. We can go even further and transfer the designs to modern transistors, but... the “zest”, the taste of time - “nostalgia” is lost, so this is not our way.

A power amplifier with deep feedback (Fig. 1 circled in blue), connected instead of high-impedance headphones, will help to significantly improve the sound quality for a low-impedance load and ensure loud-speaking reception.

As you can see, his scheme is almost a classic of the 60-70s. Distinctive feature is a deep (more than 32 dB) feedback on direct and alternating current (through resistor R7), which ensures high linearity of amplification (at average levels of Kg less than 0.5%, at low (less than 5 mW) and maximum power(0.5 W) Kg reaches 2%). The somewhat unusual activation of the volume control ensures an increase in the depth of feedback when the volume is reduced, thanks to this it turned out to be possible to make the ULF more economical (the quiescent current of the entire ULF PPP is no more than 7 mA) with virtually no “step” distortion. Capacitor C6 limits the passband to approximately 3.5 kHz (without it it exceeds 40 kHz!), which also reduces the level of self-noise - the ULF is very quiet. The output noise level is approximately 1.2 mV! (with the left pin C1 grounded). The total Kus from the input (from the left pin C1) is approximately 8 thousand. the level of self-noise referred to the input is approximately 0.15 µV. When connected to a real signal source (LPF), due to the current component, the level of intrinsic noise referred to the input increases to 0.3-0.4 µV.

The output stage uses inexpensive and reliable GT403. The ULF is capable of delivering high power (up to 2.5 W at a 4 Ohm load), but then you will need to install transistors on radiators and/or use a more powerful one (P213, P214, etc.), but in my opinion look, 0.5 W and modern sensitive dynamics are enough “for the eyes” even when listening to music. Almost any germanium low-frequency transistors of the corresponding structure and at least 40 N21e transistors (T2, T3, T4 - MP13-16, MP39-42, and T5 - MP9-11, MP35-38) are suitable for a low-frequency amplifier. If you plan to use this ULF in PPP, then T1 needs to be low-noise (P27A, P28, MP39B). For the output stage, it is advisable to select pairs T4, T5 and T6, T7 with close (no worse than +-10%) H21e values.

Due to the deep DC OOS, ULF modes are set automatically. When you turn it on for the first time, check the quiescent current (5-7 mA) and, if necessary, achieve the required one by selecting a more successful diode. You can simplify this procedure if you use a Chinese multimeter. In diode testing mode, it passes a current of approximately 1 mA through the diode. We need a specimen with a voltage drop of about 310-320 mV.

For testing a powerful ULF was chosen diagram of a simple dual-band PPP RA3AAE. I’ve been wanting to try it for a long time, but somehow I never got around to it, but here’s the opportunity (hi!).

I immediately made minor adjustments to the circuit (see Fig. 3), which I will describe here. Everything else, incl. and the setup process, see the book.

As a two-stage low-pass filter, I have traditionally used a universal tape head, which ensured increased selectivity over the adjacent channel. The low-pass filter coil has a fairly large own capacity, so it significantly loads the GPA, especially if it is wound not with PELSHO, but with a simple wire such as PEV, PEL (including tape recorder GUs). In this case, the coil’s own capacitance is so large that it is very problematic to run a GPA with a normal amplitude on diodes - many colleagues have encountered this. That is why it is better to remove the VFO signal not from the output of the coil, but from the communication coil, which eliminates all these problems and at the same time completely eliminates the contact of the VFO voltage with the ULF input. In order not to bother with winding, I found suitable ready-made coils and went ahead to testing the PPP and unexpectedly came across a serious “rake” - when switching to the 40m range, the amplitude of the VFO signal on the communication coil decreases by 2 times! Okay, I thought, maybe I have grenades, that is, coils, of the wrong system (hi!). I found the frames and rewound them strictly according to the author (see photo)

and here we must pay tribute to Vladimir Timofeevich - without additional movements he immediately hit the indicated frequency ranges- both input circuits and GPA.

But... the problem remains, which means that it is impossible to optimally configure the mixer on both ranges - if you set the optimal amplitude on one, then on the other the diodes will either be closed or almost constantly open. Only a certain average, compromise option for setting the amplitude of the VFO is possible, when the mixer will more or less work on both ranges, but with increased losses (up to 6-10 dB). The solution to the problem turned out to be straightforward - use a free switching group in the toggle switch to switch the emitter resistor, which we will use to set the optimal amplitude of the GPA on each range. To control and adjust the optimal amplitude of the GPA, we use the same method as in.

To do this, switch the left (see Fig. 3) output of diode D1 to the auxiliary capacitor 0C1. The result is a classic GPA voltage doubling rectifier. This kind of “built-in RF voltmeter” gives us the opportunity to actually directly measure the operating modes of specific diodes from a specific GPA directly in a working circuit. By connecting a multimeter to 0C1 for monitoring in the DC voltage measurement mode, selecting emitter resistors (starting with R3 on the 40m range, then R5 on the 80m range) we achieve a voltage of +0.8...+1 V - this will be the optimal voltage for diodes 1N4148, KD522, 521, etc. Here's the whole setup. We solder the diode lead back into place and remove the auxiliary circuit. Now, with an optimal operating mixer, you can optimize (increase) its connection to the input circuit (the tap is made not from 5, but from 10 turns of L2), thereby increasing the sensitivity by 6-10 dB on both ranges.

Large voltage ripples are possible along the power circuit of a powerful push-pull ULF, especially when powered by batteries. Therefore, to power the GPA, an economical parametric voltage stabilizer was used on T4, where the reverse-biased emitter junction KT315 (which was on hand) was used as a zener diode. The output voltage of the stabilizer is selected on the order of -6..-6.5V, which ensures a stable tuning frequency when the battery is discharged up to 7V. Due to the reduced supply voltage of the GPA, the number of turns of the L3 communication coil is increased to 8 turns. But with KT315 the spread in the breakdown voltage of the emitter junction is quite large - the first one that came across gave 7.5V - a bit too much, the second one gave 7V (see graphs from)

– that’s already good, using silicon KT209v as T4 I got the required -6.3v. If you don’t want to bother with selection, you can use KT316 as T5, then T4 should be germanium (MP39-42). Then it makes sense for unification and to install KT316 in the GPA (see Fig. 4), which will have a positive effect on the stability of the GPA frequency. This is exactly the option that works for me now.

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

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

Protozoa

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

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

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

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

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

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

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

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

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

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

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

On the field workers

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

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

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

Straight up

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

Schemes of a simple high-quality UMZCH

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

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

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

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

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

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

Wires for sound

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

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

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

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

Video: do-it-yourself twisted pair interconnect wires

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

Theoretical Interlude

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

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

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

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

Volume

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

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

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

Power

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

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

Dynamics

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

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

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

SOI

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

Lamps

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

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

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

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

Experimental stand

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

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

Guitarists and rockers

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

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

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

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

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

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

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

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

How to make a transformer?

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

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

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

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

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

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

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

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

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

On microcircuits

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

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

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

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

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

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

UMZCH for subwoofer

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

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

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

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

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

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

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

A little about acoustics

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

Headphone Amplifier

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

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

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

Looking through publications on the Internet, as well as videos on YouTube, one can note a steady interest in assembling relatively simple designs of radio receivers of various types (direct conversion, regenerative and others) and audio amplifiers using transistors, including germanium ones.

Assembling structures based on germanium transistors is a kind of nostalgia, because the era of germanium transistors ended 30 years ago, in fact, as did their production. Although audiophiles still argue until they are hoarse, which is better for high fidelity sound reproduction - germanium or silicon?

Let's leave lofty matters and move on to practice...

There are plans to repeat a couple of designs of simple radio receivers (direct conversion and regenerative) for reception in the short wave range. As you know, an AF amplifier is an essential component of any radio receiver. Therefore, it was decided to manufacture the ultrasonic sounder first.

The low-frequency (or audio, as you wish) amplifier will be manufactured as a separate unit, so to speak, for all occasions...

We will assemble the ultrasonic transistors using germanium transistors produced in the USSR, fortunately I have probably hundreds of different types of them. Apparently it's time to give them a second life.

For a radio receiver, a large ULF output power is not needed; up to several hundred milliwatts is enough. The search for a suitable circuit led to this design.

This scheme comes in handy. Output power -0.5 W, all transistors are germanium, and are also available, frequency response is optimized for radio receivers (limited above by a frequency of 3.5 kHz), fairly high gain.

Schematic diagram of the amplifier.

All parts necessary for assembling the amplifier are not in short supply. Transistors MP37, MP39, MP41 took the first ones that came to hand. It is recommended to select the GT403 output transistors according to their gain, but I didn’t do this - I had a couple of new ones from the same batch, so I took them. The input MP28 turned out to be a single copy, but serviceable.

All transistors were checked with an ohmmeter for serviceability. As it turned out, this is not a guarantee against malfunctions, but more on that below... I used imported electrolytic capacitors, C1-film, C5-ceramic.

In the SprintLayout program we create the PCB layout. View from the side of the printed conductors.

Actually, the printed circuit board is manufactured using LUT and etched in ferric chloride.

We solder all the necessary parts. The board of the assembled amplifier looks like this.

Since the output power of the amplifier is small, radiators for the output transistors are not needed. They are barely warm when working.

Amplifier settings.

The assembled amplifier needs some tuning.

After supplying 9V power, we measure the voltage at the control points, which are indicated in the diagram above. At the collector of transistor VT2, the voltage was minus 2.5 V when the required -3...4 V.

By selecting resistor R2 we set the required voltage.

With the pre-amplification stage on transistors VT1 and VT2 there were no problems in setting up. The situation is different with the output stage. Measuring the voltage at the midpoint (the connection point between the emitter VT6 and the collector VT7) showed a value of minus 6 V. An attempt to change the voltage by selecting resistors R7 or R8 did not lead to the desired results.

In addition, the total quiescent current of the amplifier was reduced - 4 mA instead of 5...7 mA. The culprit of the malfunction turned out to be transistor VT3. Although it was checked by the ohmmeter as working, it refused to work in the circuit. After replacing it, all modes of the amplifier transistors were set automatically according to those indicated in the diagram. The voltages on the electrodes of the transistors in my amplifier at a supply voltage of 9V are indicated in the table. The voltages were measured with a DT830B tester relative to the common wire.

The quiescent current of the amplifier is set by selecting a diode D2 of type D9. With the first diode I came across, I got a quiescent current of 5.2 mA, i.e. exactly what is needed.

To check the functionality, we apply a sinusoidal voltage of 0.3 mV with a frequency of 1000 Hz from the G3-106 audio frequency generator.
In the photo, the output voltage level is approximately 0.3V according to the dial gauge. The signal is additionally attenuated by 60 dB (1000 times) by a divider at the generator output.

We connect a load to the output of the amplifier – a resistor MON-2 with a resistance of 5.6 Ohms. We connect the oscilloscope probes in parallel to the load resistor. We observe a clean, distortion-free sinusoid.

On the oscilloscope screen, the vertical division price is -1V/div. Therefore the voltage swing is 5V. The effective voltage is 1.77V. Having these numbers we can calculate the voltage gain: The output power at a frequency of 1 kHz was:

We see that the parameters of the amplifier correspond to the declared ones.

It is clear that these measurements are not entirely accurate, because the oscilloscope does not allow you to measure voltage with high accuracy (this is not its task), but for amateur radio purposes this is not so important.

The amplifier has a high sensitivity, so when the input is not connected anywhere, noise and the background of alternating voltage can be heard quietly in the speaker.

When the input is short-circuited, all extraneous noise disappears.

Oscillogram of noise voltage at the amplifier output with a shorted input:

The vertical division value is -20 mV/div. The noise and background voltage swing is about 30 mV. Effective noise voltage is 10mV.

In other words, the amplifier is quite quiet. Although the author's article indicates a noise level of -1.2 mV. Perhaps, in my case, the not entirely successful layout of the printed circuit board played a role.

By supplying an alternating voltage of different frequencies to the input of the amplifier at a constant level and monitoring the output voltage across the load with an oscilloscope, we can take a graph of the amplitude-frequency response of a given ULF.