Circuits of modern power amplifiers using transistors. Two ULC circuits using transistors. Single-ended amplifier with one transistor

The amplifier offered to your precious attention is easy to assemble, terribly simple to set up (it actually doesn’t require it), does not contain particularly scarce components, and at the same time has very good characteristics and can easily match the so-called hi-fi, so dearly loved by the majority of citizens .The amplifier can operate at 4 and 8 Ohm loads, can be used in a bridge connection to an 8 Ohm load, and it will deliver 200 W to the load.

Main characteristics:

Supply voltage, V................................................... ............... ±35
Current consumption in silent mode, mA.................................... 100
Input impedance, kOhm................................................... .......... 24
Sensitivity (100 W, 8 Ohm), V............................................ ...... 1.2
Output power (KG=0.04%), W.................................... ........ 80
Reproducible frequency range, Hz.................................... 10 - 30000
Signal-to-noise ratio (not weighted), dB..................... -73

The amplifier is entirely based on discrete elements, without any op-amps or other tricks. When operating at a 4 Ohm load and a 35 V supply, the amplifier develops power up to 100 W. If there is a need to connect an 8 Ohm load, the power can be increased to +/-42 V, in this case, we will get the same 100 W.It is very strongly not recommended to increase the supply voltage above 42 V, otherwise you may be left without output transistors. When operating in bridge mode, an 8-ohm load must be used, otherwise, again, we lose all hope for the survival of the output transistors. By the way, we must take into account that there is no short-circuit protection in the load, so you need to be careful.To use the amplifier in bridge mode, it is necessary to screw the MT input to the output of another amplifier, to the input of which the signal is supplied. The remaining input is connected to the common wire. Resistor R11 is used to set the quiescent current of the output transistors. Capacitor C4 determines the upper limit of the gain and you should not reduce it - you will get self-excitation at high frequencies.
All resistors are 0.25 W except for R18, R12, R13, R16, R17. The first three are 0.5 W, the last two are 5 W each. The HL1 LED is not for beauty, so there is no need to plug a super-bright diode into the circuit and bring it to the front panel. The diode should be the most common green color - this is important, since LEDs of other colors have a different voltage drop.If suddenly someone was unlucky and could not get the output transistors MJL4281 and MJL4302, they can be replaced with MJL21193 and MJL21194, respectively.It is best to take a multi-turn variable resistor R11, although a regular one will do. There is nothing critical here - it’s just more convenient to set the quiescent current.

There were already publications on Habré about DIY tube amplifiers, which were very interesting to read. There is no doubt that their sound is wonderful, but for everyday use it is easier to use a device with transistors. Transistors are more convenient because they do not require warming up before operation and are more durable. And not everyone will risk starting a tube saga with anode potentials of 400 V, but transistor transformers of a couple of tens of volts are much safer and simply more accessible.

As a circuit for reproduction, I chose a circuit from John Linsley Hood from 1969, taking the author’s parameters based on the impedance of my 8 Ohm speakers.

The classic circuit from a British engineer, published almost 50 years ago, is still one of the most reproducible and collects information about itself exclusively. positive reviews. There are many explanations for this:
- the minimum number of elements simplifies installation. It is also believed that than simpler design, those better sound;
- despite the fact that there are two output transistors, they do not need to be sorted into complementary pairs;
- an output of 10 Watts is sufficient for ordinary human dwellings, and an input sensitivity of 0.5-1 Volts agrees very well 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. Comparison with other classes will be discussed below.

Interior design

It is possible with ordinary diodes or even ready-made bridges, but then they need to be bypassed with capacitors, and the voltage drop across them is greater. After the bridges there are CRC filters consisting of two 33,000 uF capacitors and a 0.75 Ohm resistor between them. If you take a smaller capacitance and a resistor, 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 from a price-effect point of view. A powerful cement resistor is needed for the filter; at 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 remaining resistors in the circuit, 2 W of power will be quite enough.

Next we move on to the amplifier board itself. Online stores sell a lot of ready-made kits, but there are no fewer complaints about the quality of Chinese components or illiterate layouts on boards. Therefore, it is better to do it yourself, at your own discretion. I made both channels on a single breadboard so that I could later attach it to the bottom of the case. Running with test elements:

Everything except the output transistors Tr1/Tr2 is on the board itself. The output transistors are mounted on radiators, more on that below. The following remarks should be made to the author’s diagram from the original article:

Not everything needs to be soldered tightly at once. It is better to first set up resistors R1, R2 and R6 as trimmers, unsolder them after all adjustments, measure their resistance and solder the final constant resistors with the same resistance. The setup comes down 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 didn’t have enough 100 kOhm, so it’s better to take these trimmers with a reserve. Then, using R1 and R2 (maintaining their approximate ratio!) the quiescent current is set - we set the tester to measure direct current and measure this very current at the positive input point of the power supply. I had to significantly reduce the resistance of both resistors to obtain the required quiescent current. The quiescent current of an amplifier in class A is maximum and, in fact, in the absence of an input signal, all of it goes into thermal energy. For 8-ohm speakers, this current, according to the author's recommendation, should be 1.2 A at a voltage of 27 Volts, which means 32.4 Watts of heat per channel. Since setting the current can take several minutes, the output transistors must already be on cooling radiators, otherwise they will quickly overheat and die. Because they are mostly heated.

It is possible that, as an experiment, you will want to compare the sound different transistors, so they can also be easily replaced. I tried 2N3906, KT361 and BC557C at the input, there was a slight difference in favor of the latter. In the pre-weekend we tried KT630, BD139 and KT801, and settled on imported ones. Although all of the above transistors are very good, the difference may be rather subjective. At the output, I immediately installed 2N3055 (ST Microelectronics), since many people like them.

When adjusting and lowering the resistance of the amplifier, the low-frequency cutoff frequency may increase, so for the input capacitor it is better to use not 0.5 µF, but 1 or even 2 µF in a polymer film. There is still a Russian picture-scheme of an “Ultralinear Class A Amplifier” floating around the Internet, where this capacitor is generally proposed as 0.1 uF, which is fraught with a cutoff of all bass at 90 Hz:

They write that this circuit is not prone to self-excitation, but just in case, a Zobel circuit is placed between point X and ground: R 10 Ohm + C 0.1 μF.
- 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.

Metalworking and carpentry

I made the body itself from plexiglass. We immediately order cut rectangles from glaziers, make the necessary holes for fastenings in them and paint them with reverse side black paint.

The plexiglass painted on the reverse side looks very beautiful. Now all that remains is to assemble everything and enjoy the music... oh yes, during final assembly it is also important to properly distribute the ground to minimize the background. As was discovered decades before us, C3 must 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 you will not be able to hear any background, 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 get through USB and RCA. Judging by the Internet, the problem occurs frequently: in the speakers you can hear the sounds of the HDD, printer, mouse and the background power supply of the system unit. In this case, the easiest way to break the ground loop is to cover the ground connection 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 regular 47 kOhm resistor was installed between ground and the input signal. Moreover, the regulator is on the outside sound card always at hand, and every 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...

Material costs

The amplifier is capable of delivering 2kW peak power, and 1.5kW continuous, which means this amplifier is capable of burning out most speakers you know. To imagine such power in action, you can connect (which I strongly advise against doing) two 8-ohm speakers connected in series to a 220V AC network. In this case, one speaker will have 110V effective voltage at a load of 8 ohms - 1,500W. How long do you think the acoustics will work in this mode? If you still have the desire to work on this amplifier, move on...

Amplifier Description

First, let's look at the requirements to achieve 1.5kW into 4 ohms. We need 77.5V rms voltage, but we must have some margin because the supply voltage will drop under load and there will always be some voltage drop across the collector-emitter junctions and emitter resistors.

So the supply voltage should be...

VDC = VRMS * 1.414
VDC = 77.5 * 1.414 = ±109.6V DC voltage

Since we haven't taken losses into account, we need to add about 3-5V for the amplifier tip, and an additional 10V for supply voltage drop under full load.

A 2 x 90V transformer will produce a no-load voltage of ±130V (260V between the ends of the rectifier), so the power supply must be handled with extreme care

Bipolar transistors were selected as the most suitable for the final stage of the amplifier. This is primarily dictated by the supply voltage, which exceeds the limit voltage for most MOSFET transistors. This is also a lot for bipolar transistors, but MJ15004/5 or MJ21193/4 meet the maximum voltage requirement, which means we will focus on them.

P=V? / R = 65 ? / 4 = 1056W

That is, equal to the average electric heater...
Remember that when driving a resistive load with 45° phase shifts, the power dissipation almost doubles. Based on this, it follows that good cooling is vital for this amplifier. You will need good radiators and fans for forced cooling (natural convection will not help).

MJ15024/5 (or MJ21193/4) transistors in a K-3 package (iron with two terminals like KT825/827), and are designed to dissipate 250W at a temperature of 25°C. The K-3 transistor package was chosen because it has the highest power dissipation rating because the thermal resistance is lower than any other plastic packaged transistor.

MJE340/350 in the voltage amplifier stage guarantees good linearity. But even with a current through the stage of 12mA, the power is 0.72W, so Q4, Q6, Q9 and Q10 must have heat sinks. The transistor (Q5), which determines the bias of the final stage, must be installed on a common radiator with the terminal and have reliable thermal contact.

Protection circuit against short circuit(Q7, Q8) limits the current to 12A and the power released by one transistor to about 175W, while long-term operation of the amplifier in this mode is not permissible.
1500W professional amplifier circuit.

Additional feedback elements (R6a and C3a, shown dotted) are optional. They may be necessary if self-excitation of the amplifier occurs. Reverse diodes (D9 and D10) protect the amplifier transistors from back EMF when operating an active load. 1N5404 series diodes can withstand peak current up to 200A. The rated voltage must be at least 400V.

Resistor VR1 100 ohm is used to balance the amplifier for DC current. With the component ratings shown in the diagram, the initial offset should be within ±25mV before tuning. Resistor VR2 is used to set the quiescent current of the final stage. The quiescent current is adjusted by measuring the voltage across resistor R19 or R20, which should be within 150mV.
Input stage sensitivity is 1.77V for 900W at 8 ohms, or 1800W at 4 ohms.

Power supply:

The power supply required for the amplifier requires a serious design approach. Firstly, you need a step-down transformer with a power of at least 2kW. Power filter capacitors must be rated at 150V and can withstand up to 10A ripple current. Capacitors that do not meet these requirements may simply explode when the amplifier is operating at full power.

An important detail is the bridge rectifier. Although 35A bridges seem to be able to cope with the task, the peak repeating current exceeds the bridges' ratings. I recommend using two bridges connected in parallel as shown in the diagram. The rated voltage of the bridge rectifier must be a minimum of 400V, and they must be mounted on a sufficient heat sink for cooling.
Power supply circuit for a 1500W amplifier.

The diagram shows capacitors made up of four low-voltage capacitors since they are easier to find, and the rectifier also consists of two bridges connected in parallel.

Additional voltage sources of 5V can be eliminated, while the peak power will decrease from 2048W to 1920W, which is not significant.
The P39 module is a soft start system and consists of a relay, parallel to the contacts of which resistors with a total power of 150W and a resulting resistance of 33 Ohms are connected.

The power source must provide a stable or unstable bipolar supply voltage of ±45V and a current of 5A. This ULF transistor circuit is very simple, since the output stage uses a pair of powerful complementary Darlington transistors. In accordance with the reference characteristics, these transistors can switch current up to 5A at an emitter-collector junction voltage of up to 100V.

The ULF circuit is shown in the figure below.

The signal requiring amplification through the preliminary ULF is fed to a preliminary differential amplifier stage built on composite transistors VT1 and VT2. The use of a differential circuit in the amplifier stage reduces noise effects and ensures negative feedback. The OS voltage is supplied to the base of transistor VT2 from the output of the power amplifier. DC feedback is implemented through resistor R6. The feedback on the variable component is carried out through resistor R6, but its value depends on the ratings of the chain R7-C3. But it should be borne in mind that too much increase in resistance R7 leads to excitation.

The DC operating mode is ensured by selecting resistor R6. The output stage based on Darlington transistors VT3 and VT4 operates in class AB. Diodes VD1 and VD2 are needed to stabilize the operating point of the output stage.

Transistor VT5 is designed to drive the output stage; its base receives a signal from the differential output preamp, as well as a constant bias voltage, which determines the operating mode of the output stage for direct current.

All capacitors in the circuit must be designed for a maximum DC voltage of at least 100V. It is recommended to mount the output stage transistors on radiators with an area of ​​at least 200 cm square

The considered circuit of a simple two-stage amplifier is designed for use with headphones or for use in simple devices with pre-amplifier function.

The first transistor of the amplifier is connected according to a common emitter circuit, and the second transistor is connected to a common collector. The first stage is intended for basic signal amplification in terms of voltage, and the second stage amplifies the signal in terms of power.

The low output impedance of the second stage of a two-stage amplifier, called an emitter follower, allows you to connect not only high-impedance headphones, but also other types of acoustic signal converters.

This is also a two-stage ULF circuit made on two transistors, but of opposite conductivity. Her main feature The point is that the connection between the cascades is direct. The covered OOS through resistance R3 bias voltage from the second stage passes to the base of the first transistor.

Capacitor SZ, bypasses resistor R4, reduces the negative feedback on alternating current, thereby reducing the gain of VT2. By selecting the value of resistor R3, the operating mode of the transistors is set.

This fairly lightweight power amplifier audio frequency(UMZCH) can be soldered on just two transistors. When supply voltage is 42V DC output power The amplifier reaches 0.25 W into a 4 ohm load. Current consumption is only 23 mA. The amplifier operates in single-cycle “A” mode.

Low frequency voltage from the signal source approaches the volume control R1. Next, through the protective resistor R3 and capacitor C1, the signal appears at the base of the bipolar transistor VT1 connected according to a circuit with a common emitter. The amplified signal through R8 is fed to the gate of a powerful field effect transistor VT2, connected according to a circuit with a common source and its load, serves as the primary winding of a step-down transformer. A dynamic head or acoustic system can be connected to the secondary winding of the transformer.

In both transistor stages there is local negative feedback on direct and alternating current, as well as by a common OOS circuit.

If the gate voltage of a field-effect transistor increases, the drain-source resistance of its channel decreases and the voltage at its drain decreases. This also affects the signal level entering the bipolar transistor, which reduces the gate-source voltage.

Together with local negative feedback circuits, the operating modes of both transistors are thus stabilized even in the case minor change supply voltage. The gain depends on the ratio of the resistances of resistors R10 and R7. Zener diode VD1 is designed to prevent failure of the field-effect transistor. The amplifier stage at VT1 is powered through the RC filter R12C4. Capacitor C5 is blocking in the power supply circuit.

The amplifier can be assembled on printed circuit board dimensions 80×50 mm, all elements are located on it except the step-down transformer and dynamic head

The amplifier circuit is adjusted at the supply voltage at which it will operate. For fine tuning It is recommended to use an oscilloscope, the probe of which is connected to the drain terminal of the field-effect transistor. By applying a sinusoidal signal with a frequency of 100 ... 4000 Hz to the amplifier input, by adjusting the tuning resistor R5, we ensure that there is no noticeable distortion of the sinusoid with the signal amplitude swing at the transistor drain terminal being as large as possible.

The output power of the field-effect transistor amplifier is small, only 0.25 W, the supply voltage is from 42V to 60V. Dynamic head resistance is 4 ohms.

The audio signal through variable resistance R1, then R3 and separating capacitance C1 is supplied to the amplifier stage at bipolar transistor according to a common emitter circuit. Next, from this transistor the amplified signal passes through resistance R10 to the field-effect transistor.

The primary winding of the transformer is the load for the field-effect transistor, and secondary winding A four ohm dynamic head is connected. By the ratio of resistances R10 and R7 we set the degree of voltage amplification. In order to protect the unipolar transistor, a zener diode VD1 was added to the circuit.

All part values ​​are shown in the diagram. The transformer can be used like TVK110LM or TVK110L2, from the frame scanning unit of an old TV or similar.

I came across this circuit in an old issue of a radio magazine, the impressions from it were most pleasant, firstly, the circuit is so simple that even a novice radio amateur can assemble it, and secondly, provided that the components are working and the assembly is correct, it does not require adjustment.

If you are interested in this circuit, then you can find the rest of the details on its assembly in Radio Magazine No. 8 for 1982.

Readers! Remember this author's nickname and never repeat his schemes.
Moderators! Before you ban me for insulting me, think that you “allowed an ordinary gopnik to the microphone, who should not even be allowed close to radio engineering and, especially, to teaching beginners.

Firstly, with such a connection scheme, a large D.C., even if the variable resistor is in the desired 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, this circuit must have a current limiter, that is constant resistor, at least 1 KOhm, connected in series with the variable. Any homemade product will turn the variable resistor knob all the way, it will have zero resistance and a large current will flow 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 (the author should explain this, because there was immediately a reader who removed it just like that, considering himself smarter than the author). Without it, only those players that already have similar protection 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 turn the variable resistor “to zero”. At the same time, on the way out expensive laptop voltage will be supplied from the power source of this penny trinket and it may burn out. Homemade people love to remove protective resistors and capacitors, because “it works!” As a result, the circuit may 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 tuning, that is, it should be adjusted once and closed in the housing, 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 so that no smoke comes out of the speaker. Therefore it should under no circumstances be accessible from the outside. You CANNOT adjust the volume by changing the mode. This is something to kill for. If you really want to regulate the volume, it’s easier to connect another variable resistor in series with the capacitor and now it can be output to the amplifier body.

In general, for the simplest circuits - and to make it work right away and not to damage anything, you need to buy a TDA type microcircuit (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 permissible current is as much as 8 amperes (it will burn out any speaker without even noticing).