Power amplifier on transistors using oh. Symmetrical ULF from available parts based on B
Nikolai Troshin
A simple germanium power amplifier.
Recently, interest in power amplifiers based on germanium transistors has grown markedly. It is believed that the sound of such amplifiers is softer, reminiscent of a “tube sound”.
I present to you two simple circuits low-frequency power amplifiers on germanium transistors, which I tested some time ago.
Here, more modern circuit solutions are used than those used in the 70s, when "germanium" was in use. This made it possible to obtain decent power at good quality sound.
The circuit in the figure below is a germanium-redesigned version of the bass amplifier from my article in Radio magazine No. 8 for 1989 (pp. 51-55).
output power this amplifier is 30 watts with a speaker load of 4 ohms, and about 18 watts with a load of 8 ohms.
Amplifier supply voltage (U pit) bipolar ± 25 V;
A few words about the details:
When assembling the amplifier, it is desirable to use mica capacitors as fixed capacitors (in addition to electrolytic ones). For example CSR type, such as below picture.
MP40A transistors can be replaced with MP21, MP25, MP26 transistors. Transistors GT402G - on GT402V; GT404G - on GT404V;
Output transistors GT806 can be set to any letter indexes. I do not recommend using lower-frequency transistors such as P210, P216, P217 in this circuit, since at frequencies above 10 kHz they work rather poorly here (distortions are noticeable), apparently due to a lack of current amplification at high frequency.
The area of radiators for output transistors must be at least 200 cm2, for terminal transistors - at least 10 cm2.
It is convenient to make radiators for GT402 type transistors from a copper (brass) or aluminum plate, 0.5 mm thick, 44x26.5 mm in size.
The plate is cut along the lines, then this blank is shaped into a tube, using for this purpose any suitable cylindrical mandrel (for example, a drill).
After that, the workpiece (1) is tightly put on the transistor housing (2) and pressed with a spring ring (3), having previously bent the side mounting ears.
The ring is made of steel wire with a diameter of 0.5-1.0 mm. Instead of a ring, you can use a band of copper wire.
Now it remains to bend the side lugs from below for attaching the radiator to the transistor case and bend the notched feathers to the desired angle.
A similar radiator can also be made from a copper tube with a diameter of 8mm. We cut off a piece of 6 ... 7 cm, cut the tube along the entire length on one side. Next, we cut the tube into 4 parts by half the length and bend these parts in the form of petals and put them tightly on the transistor.
Since the diameter of the transistor body is about 8.2 mm, due to the cut along the entire length of the tube, it will fit tightly on the transistor and will be held on its body due to springy properties.
Resistors in the emitters of the output stage are either wire-wound with a power of 5 W, or of the MLT-2 type 3 Ohm, 3 pieces in parallel. I do not advise using imported film ones - they burn out instantly and imperceptibly, which leads to the failure of several transistors at once.
Customization:
Setting up an amplifier correctly assembled from serviceable elements comes down to setting the quiescent current of the output stage to 100mA with a trimmer resistor (it is convenient to control the voltage of 100mV on a 1 Ohm emitter resistor).
It is desirable to glue or press the VD1 diode to the radiator of the output transistor, which contributes to better thermal stabilization. However, if this is not done, the quiescent current of the output stage from cold 100mA to hot 300mA changes, in general, not catastrophically.
Important: before the first power-up, it is necessary to set the tuning resistor to zero resistance.
After tuning, it is desirable to remove the tuning resistor from the circuit, measure its real resistance and replace it with a constant one.
The most scarce part for assembling an amplifier according to the above scheme is the GT806 germanium output transistors. It was not so easy to acquire them even in bright Soviet times, and now it is probably even more difficult. It is much easier to find germanium transistors of types P213-P217, P210.
If for some reason you cannot purchase GT806 transistors, then another amplifier circuit is offered to your attention, where you can use just the aforementioned P213-P217, P210 as output transistors.
This scheme is a modernization of the first scheme. The output power of this amplifier is 50W into a 4 ohm load and 30W into an 8 ohm load.
The supply voltage of this amplifier (U pit) is also bipolar and is ± 27 V;
Operating frequency range 20Hz…20kHz:
What changes have been made to this scheme;
Added two current sources to the "voltage amplifier" and one more stage to the "current amplifier".
The use of another amplification stage on rather high-frequency P605 transistors made it possible to somewhat unload the GT402-GT404 transistors and stir up the very slow P210.
It turned out pretty bad. With an input signal of 20 kHz, and with an output power of 50 W, there is practically no distortion on the load (on the oscilloscope screen).
Minimal, hardly noticeable distortion of the output signal shape with P210 type transistors occurs only at frequencies of about 20 kHz at a power of 50 watts. At frequencies below 20 kHz and powers less than 50 watts, distortion is not noticeable.
In a real musical signal, such powers at such high frequencies usually do not exist, so I did not notice any differences in the sound (by ear) of the amplifier based on GT806 transistors and on P210 transistors.
However, on transistors like GT806, if viewed with an oscilloscope, the amplifier still works better.
With a load of 8 ohms in this amplifier, it is also possible to use output transistors P216 ... P217, and even P213 ... P215. In the latter case, the amplifier supply voltage will need to be reduced to ± 23V. The output power will, of course, also drop.
An increase in power leads to an increase in output power, and I think that the amplifier circuit according to the second option has such a potential (reserve), however, I did not tempt fate with experiments.
The following radiators are required for this amplifier - for output transistors with a dissipation area of at least 300 cm2, for pre-output P605 - at least 30 cm2, and even for GT402, GT404 (with a load resistance of 4 ohms) are also needed.
For transistors GT402-404, you can do it easier;
Take copper wire(without insulation) with a diameter of 0.5-0.8; "donut" on the body of the transistor.
It will be more efficient to wind the wire not on a round, but on a rectangular mandrel, since this increases the area of contact of the wire with the transistor case and, accordingly, increases the efficiency of heat removal.
Also, to increase the efficiency of heat removal for the entire amplifier, you can reduce the area of \u200b\u200bradiators and use a 12V cooler from a computer for cooling, supplying it with a voltage of 7 ... 8V.
Transistors P605 can be replaced with P601 ... P609.
The setting of the second amplifier is similar to that described for the first circuit.
A few words about acoustic systems. It is clear that in order to get good sound, they must have the appropriate power. It is also desirable, using a sound generator, to walk at different powers over the entire frequency range. The sound should be clear, without wheezing and rattling. Especially, as my experience has shown, the high-frequency speakers of the S-90 type speakers sin with this.
If anyone has any questions about the design and assembly of amplifiers - ask, I will try to answer as much as possible.
Good luck to all of you in your work and all the best!
Readers! Remember the nickname of this author and never repeat his schemes.
Moderators! Before you ban me for insults, think that you "let an ordinary gopnik to the microphone", who should not even be allowed close to radio engineering and, moreover, to teaching beginners.
Firstly, with such a switching circuit, a large direct current will flow through the transistor and speaker, even if the variable resistor is in the right position, that is, music will be heard. And with a large current, the speaker is damaged, that is, sooner or later, it will burn out.
Secondly, in this circuit there must be a current limiter, that is, a constant resistor, at least 1 KΩ, connected in series with a variable one. Any do-it-yourselfer will turn the variable resistor regulator all the way, it will have zero resistance and a large current will go to the base of the transistor. As a result, the transistor or speaker will burn out.
A variable capacitor at the input is needed to protect the sound source (this should be explained by the author, because immediately there was a reader who removed it just like that, considering himself smarter than the author). Without it, only those players in which such protection is already installed at the output will work normally. And if it is not there, then the player's output may be damaged, especially, as I said above, if you unscrew the variable resistor "to zero". In this case, the output of an expensive laptop will be energized from the power source of this penny trinket and it can burn out. Homemade ones are very fond of removing protective resistors and capacitors, because "it works!" As a result, the circuit can work with one sound source, but not with another, and even an expensive phone or laptop can be damaged.
The variable resistor, in this circuit, should only be a trimmer, that is, it should be adjusted once and closed in the case, and not brought out with a convenient handle. This is not a volume control, but a distortion control, that is, it selects the operating mode of the transistor so that there is minimal distortion and that smoke does not come out of the speaker. Therefore, it should never be accessible from the outside. It is IMPOSSIBLE to adjust the volume by changing the mode. For this you need to "kill". If you really want to adjust the volume, it's easier to turn on another variable resistor in series with the capacitor, and now it can already be output to the amplifier case.
In general, for the simplest circuits - and in order to work right away and not to damage anything, you need to buy a TDA-type chip (for example, TDA7052, TDA7056 ... there are many examples on the Internet), and the author took a random transistor that was lying around in his desk. As a result, gullible amateurs will look for just such a transistor, although its gain is only 15, and the allowable current is as much as 8 amperes (it will burn any speaker without even noticing).
There were already publications on Habré about DIY tube amplifiers, which were very interesting to read. No doubt, they sound wonderful, but for everyday use easier to use a device on transistors. Transistors are more convenient because they do not require warming up before operation and are more durable. And not everyone dares to start a lamp saga with anode potentials under 400 V, and transistor transformers for a couple of tens of volts are much safer and simply more affordable.
I chose John Linsley Hood's 1969 circuit as the circuit to reproduce, taking the author's parameters based on the impedance of my speakers 8 ohms.
The classic scheme from a British engineer, published almost 50 years ago, is still one of the most reproducible and collects about itself exclusively positive reviews. There are many explanations for this:
- the minimum number of elements simplifies installation. It is also considered that simpler design, topics better sound;
- despite the fact that there are two output transistors, they do not need to be sorted into complementary pairs;
- output of 10 watts with a margin is enough for ordinary human dwellings, and an input sensitivity of 0.5-1 volts is very well consistent with the output of most sound cards or players;
- class A - it is also class A in Africa, if we are talking about good sound. About comparison with other classes will be a little lower. 
Internal design
The amplifier starts with power. Separation of two channels for stereo is best done from two different transformers, but I limited myself to one transformer with two secondary windings. After these windings, each channel exists on its own, so we must not forget to multiply by two everything mentioned below. On the breadboard we make bridges on Schottky diodes for the rectifier.
It is possible on ordinary diodes or even ready-made bridges, but then they need to be shunted with capacitors, and the voltage drop across them is greater. After the bridges, there are CRC filters of two 33,000 microfarad capacitors and a 0.75 ohm resistor between them. If you take both the capacitance and the resistor less, then the CRC filter will become cheaper and heat up less, but the ripple will increase, which is not comme il faut. These parameters, IMHO, are reasonable in terms of price-effect. A powerful cement resistor is needed in the filter, with a quiescent current of up to 2A it will dissipate 3 W of heat, so it is better to take it with a margin of 5-10 W. For the rest of the resistors in the power circuit, 2 W will be enough.
Next, we move on to the amplifier board itself. A lot of ready-made kits are sold in online stores, but there are no less complaints about the quality of Chinese components or illiterate layouts on the boards. Therefore, it is better to do it yourself, under your own “loose”. I made both channels on a single breadboard, so that later I can attach it to the bottom of the case. Run with test items:
Everything except the output transistors Tr1/Tr2 is located on the board itself. Output transistors are mounted on radiators, more on that below. To the author's scheme from the original article, you need to make the following remarks:
Not everything needs to be soldered right away. It is better to put resistors R1, R2 and R6 first with trimmers, after all the adjustments, unsolder them, measure their resistance and solder the final fixed resistors with the same resistance. The setting is reduced to the following operations. First, using R6, it is set so that the voltage between X and zero is exactly half of the voltage + V and zero. In one of the channels, I lacked 100 kOhm, so it's better to take these trimmers with a margin. Then, with the help of R1 and R2 (keeping their approximate ratio!) the quiescent current is set - we put the tester for measurement direct current and measure this very current at the input point of the plus supply. I had to significantly reduce the resistance of both resistors to get the desired quiescent current. The quiescent current of the amplifier in class A is maximum and, in fact, in the absence of an input signal, everything goes into thermal energy. For 8 ohm speakers, this current, according to the author's recommendation, should be 1.2 A at 27 volts, which means 32.4 watts of heat per channel. Since it can take several minutes for the current to be applied, the output transistors must already be on cooling heatsinks or they will quickly overheat and die. Because they get hot most of the time.
It is possible that, as an experiment, you will want to compare the sound of different transistors, so you can also leave the possibility of a convenient replacement for them. I tried on the input 2N3906, KT361 and BC557C, there was a slight difference in favor of the latter. In the pre-weekend, we tried KT630, BD139 and KT801, settled on imported ones. Although all of the above transistors are very good, and the difference can be rather subjective. At the output, I immediately put 2N3055 (ST Microelectronics), since many people like them.
When adjusting and lowering the resistance of the amplifier, the cutoff frequency of the low frequencies may increase, so for the capacitor at the input it is better to use not 0.5 microfarads, but 1 or even 2 microfarads in a polymer film. The Russian picture-scheme “Ultralinear Class A Amplifier” is still circulating on the Web, where this capacitor is generally proposed as 0.1 microfarads, which is fraught with a cutoff of all basses at 90 Hz:
They write that this circuit is not prone to self-excitation, but just in case, a Zobel circuit is placed between the X point and the ground: R 10 Ohm + C 0.1 microfarad.
- fuses, they can and should be installed both on the transformer and on the power input of the circuit.
- it would be very appropriate to use thermal paste for maximum contact between the transistor and the heatsink.
Locksmith and carpentry
Now about the traditionally most difficult part in DIY - the case. The dimensions of the case are set by radiators, and in class A they should be large, remember about 30 watts of heat on each side. At first, I underestimated this power and made a case with average radiators 800cm² per channel. However, with a set quiescent current of 1.2A, they heated up to 100 ° C in just 5 minutes, and it became clear that something more powerful was needed. That is, you need to either install larger radiators, or use coolers. I didn’t want to make a quadcopter, so I bought giant handsome HS 135-250s with an area of 2500 cm² for each transistor. As practice has shown, such a measure turned out to be a little redundant, but now the amplifier can be safely touched by hands - the temperature is only 40 ° C even in rest mode. Drilling holes in the radiators for fasteners and transistors became a problem - the originally purchased Chinese metal drills were drilled extremely slowly, it would take at least half an hour for each hole. Cobalt drills with a sharpening angle of 135 ° from a well-known German manufacturer came to the rescue - each hole is passed in a few seconds!I made the body out of Plexiglas. We immediately order cut rectangles from glaziers, make the necessary holes for fastenings in them and paint the reverse side with black paint.
The plexiglass painted on the back looks very nice. Now it remains only to collect everything and enjoy the music ... oh yes, with final assembly it is also important to properly dilute the ground to minimize the background. As it was found out decades before us, C3 needs to be connected to the signal ground, i.e. to the minus of the input-input, and all other minuses can be sent to the "star" near the filter capacitors. If everything is done correctly, then no background can be heard, even if you bring your ear to the speaker at maximum volume. Another "ground" feature that is typical for sound cards that are not galvanically isolated from the computer is interference from the motherboard, which can creep through USB and RCA. Judging by the Internet, the problem is common: in the speakers you can hear the sounds of the HDD, printer, mouse and the background of the system unit's power supply. In this case, the easiest way is to break the ground loop by taping the ground on the amplifier plug with electrical tape. There is nothing to fear here, because. there will be a second ground loop through the computer.
I didn’t make a volume control on the amplifier, because I couldn’t get any high-quality ALPS, and I didn’t like the rustling of Chinese potentiometers. Instead, a conventional 47 kΩ resistor was installed between the “ground” and the “signal” of the input. Moreover, the regulator of the external sound card always at hand, and each program also has a slider. Only the vinyl player does not have a volume control, so to listen to it, I attached an external potentiometer to the connecting cable.
I can guess this container in 5 seconds...
Finally, you can start listening. The sound source is Foobar2000 → ASIO → external Asus Xonar U7. Speakers Microlab Pro3. The main advantage of these speakers is a separate block of their own amplifier on the LM4766 chip, which can be immediately removed somewhere far away. Much more interesting with this acoustics sounded the amplifier from the Panasonic mini-system with the proud inscription Hi-Fi or the amplifier of the Soviet player Vega-109. Both of the above devices work in class AB. The JLH presented in the article outplayed all of the above comrades in one wicket, according to the results of a blind test for 3 people. Although the difference was audible to the naked ear and without any tests, the sound is clearly more detailed and transparent. It's quite easy, for example, to hear the difference between 256kbps MP3 and FLAC. I used to think that the lossless effect is more like a placebo, but now the opinion has changed. Similarly, it became much more pleasant to listen to uncompressed from loudness war files- dynamic range less than 5 dB is not ice at all. The Linsley Hood is worth the time and money, as a similar branded amp will cost a lot more.Material costs
Transformer 2200 rub.Output transistors (6 pieces with a margin) 900 rubles.
Filter capacitors (4 pcs) 2700 r.
"Rose" (resistors, small capacitors and transistors, diodes) ~ 2000 rubles.
Radiators 1800 r.
Plexiglas 650 rub.
Paint 250 rub.
Connectors 600 rub.
Boards, wires, silver solder, etc. ~1000 r.
TOTAL ~12100 rub.
I must say right away that the assembly of this amplifier is justified only as an experiment, since the sound quality will, at best, be at the level of cheap Chinese receivers - scanners. If someone wants to build himself a low-power amplifier with more high-quality sound, using microchip TDA 2822m , you can go to the following link:
Portable speaker for a player or phone on a tda2822m chip
Amplifier test photo:
The following figure lists the required parts: 
Almost any of the medium and high power bipolar transistors can be used in the circuit. n-p-n structures, for example, KT 817. It is desirable to put a film capacitor at the input, with a capacity of 0.22 - 1 μF. An example of film capacitors in the following photo:
I bring a drawing of a printed circuit board from the program Sprint layout : 
The signal is taken from the output of an mp3 player or telephone, ground and one of the channels are used. In the following figure, you can see the wiring diagram for the Jack 3.5 plug, for connecting to a signal source:
If desired, this amplifier, like any other, can be equipped with a volume control by connecting a 50 KΩ potentiometer according to the standard scheme, 1 channel is used:
Parallel to the power supply, if the power supply after diode bridge it is not worth a large-capacity electrolytic capacitor, you need to put an electrolyte at 1000 - 2200 uF, with an operating voltage greater than the supply voltage of the circuit.
An example of such a capacitor:
Download printed circuit board amplifier on one transistor for the sprint-layout program can be found in the section of the site My files.
You can evaluate the sound quality of this amplifier by watching the video of its work on our channel.
Scheme simple amplifier sound on transistors, which is implemented on two powerful composite transistors TIP142-TIP147 installed in the output stage, two low-power BC556B in the differential path and one BD241C in the signal pre-amplification circuit - only five transistors for the entire circuit! Such a UMZCH design can be freely used, for example, as part of a home music center or to drive a subwoofer installed in a car, at a disco.
The main attraction of this sound power amplifier lies in the ease of assembly even by novice radio amateurs, there is no need for any special settings, there are no problems in purchasing components at an affordable price. The UM circuit presented here has electrical characteristics with high linearity in frequency range from 20Hz to 20000Hz. p>
When choosing or self-manufacturing transformer for the power supply, the following factor must be taken into account: - the transformer must have a sufficient power reserve, for example: 300 W per one channel, in the case of a two-channel version, then naturally the power doubles. You can use your own separate transformer for each, and if you use the stereo version of the amplifier, then you will generally get a “double mono” type device, which will naturally increase the sound amplification efficiency.
Operating voltage in secondary windings transformer should be ~ 34v change, then constant pressure after the rectifier it will turn out in the region of 48v - 50v. In each power supply arm, it is necessary to install a fuse rated for a working current of 6A, respectively, for a stereo when working on one power supply - 12A.
