A simple and inexpensive do-it-yourself acoustic amplifier. How to make an inexpensive audio amplifier for speakers with your own hands

Factory-made devices for amplifying the audio signal are expensive and may not be powerful enough. Looking at the photos of homemade sound amplifiers, it is obvious that they are in no way inferior in appearance to finished products. In addition, their manufacture on their own does not require special skills or large material costs.

Device base

Beginner radio amateurs first of all ask themselves: what can they use to assemble a simple sound amplifier at home? The operation of the device is based on transistors or microcircuits, or a rare option is possible - on lamps. Let's take a closer look at each of them.

Microcircuits

The TDA series microcircuit and a similar one can be purchased in stores or you can use a microcircuit from an unnecessary TV.

Using car amplifier chips with a 12-volt power supply, it is very easy to achieve high-quality sound without the use of special skills and with a minimum of parts.

Transistors

The advantages of transistors are low power consumption. The device produces excellent sound performance, is easily integrated into any equipment and does not require additional configuration. In addition, there is no need to search for and use complex microcircuits.

Lamps

Today, the outdated assembly method based on tubes provides high-quality sound, but has a number of disadvantages:

• increased energy intensity
• dimensions
• cost of components

Recommendations for properly assembling a sound amplifier with your own hands

A device for enhancing sound quality, assembled at home based on TDA series microcircuits and their analogues, generates a lot of heat. For cooling, you need a radiator grille of a suitable size, depending on the model of the microcircuit itself and the power of the amplifier. There must be a place for it in the case.

The advantage of a self-made device is its low energy consumption, which allows it to be used in cars by connecting to a battery, as well as on the road or at home using a battery. Power consumption depends on the required degree of signal amplification. Some manufactured models require a voltage of only 3 Volts.

We take a serious and responsible approach to assembling a sound amplifier in order to avoid short circuit and failure of components.

Necessary materials

During the assembly process you will need the following tools and components:

• chip
• frame
• capacitors
• power unit
• plug
• switch button
• wires
• cooling radiator
• screws
• hot glue and thermal paste
• soldering iron and rosin

Circuits and instructions for making an amplifier at home

Each circuit is unique and depends on the sound source (old or modern digital technology), power supply, and expected final dimensions. It is assembled on a printed circuit board, which will make the device compact and more convenient. During the assembly process, you cannot do without a soldering iron or soldering station.

The British John Linsley-Hood circuit is based on four transistors without microcircuits. It allows you to similarly repeat the shape of the input signal, resulting in only pure gain and a sine wave at the output.

The simplest and most common option for manufacturing a single-channel amplifier is to use a microcircuit based on it, supplemented with resistors and capacitors.

Algorithm of actions for production

• install on printed circuit board radio components, taking into account polarity
• assemble the body (providing space for additional parts, for example, a radiator grille)

It is permissible to use a ready-made case or create it yourself, as well as install the board into the speaker case.

• run the device in test mode (identify and eliminate malfunctions if they occur)
• amplifier assembly (connection to the power supply and other components)

Note!

DIY home and car amplifiers

At home, there is often a lack of powerful sound when watching movies on a laptop or listening to music on headphones. Let's look at how to properly make a sound amplifier with your own hands.

For laptop

The sound wave amplifier must take into account the power of external speakers up to 2 watts and winding resistance up to 4 ohms.

Assembly components:

• 9 volt power supply
• printed circuit board
• chip TDA 7231
• frame
• non-polar capacitor 0.1 µF - 2 pcs.
• polar capacitor 100 µF
• polar capacitor 220 µF
• polar capacitor 470 µF
• constant resistor 10 Kom m 4.7 Ohm
• two-position switch
• input socket

Manufacturing scheme

The assembly algorithm is selected depending on the selected scheme. It is necessary to consider the appropriate size of the cooling radiator so that the operating temperature inside the case does not rise above 50 degrees Celsius. When using a laptop outdoors, you need to provide holes in the case for air access.

For car radio

An amplifier for a car radio can be assembled using the common TDA8569Q microcircuit. Its characteristics:

• supply voltage 6-18 volts
• input power 25 watts per channel into 4 ohms and 40 watts per channel into 2 ohms
• frequency range 20-20000 Hz

Note!

It is imperative to provide an anti-interference filter in addition to the circuit, created by work car.

First, draw a printed circuit board, then drill holes in it. Then the board must be etched with ferric chloride. After tinning and soldering all the parts of the microcircuit. To avoid power additives, a thick layer of solder will need to be applied to the power traces. Provide a cooling system using a cooler or radiator grille.

At the end of the assembly, it is necessary to make a filter against interference from the ignition system and poor noise insulation according to the following scheme: on a ferrite ring with a diameter of 20 mm, wind a choke with a wire with a cross-section of 1-1.5 mm in 5 turns.

Assembling a device to improve sound quality at home is not difficult. The main thing is to decide on the circuit and have on hand all the components from which you can easily assemble a simple sound amplifier.

DIY sound amplifier photo

Note!

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 receives extremely positive reviews. There are many explanations for this:
- the minimum number of elements simplifies installation. It is also believed that the simpler the design, the better the 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

An amplifier starts with power. It is best to separate two channels for stereo using 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 a breadboard we make bridges using Schottky diodes for the rectifier.

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 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 of different transistors, so you can also leave the possibility of convenient replacement for them. 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 “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

Now about the traditionally most difficult part in DIY - the body. The dimensions of the case are determined by radiators, and in class A they must be large, remember about 30 watts of heat on each side. At first, I underestimated this power and made a case with average radiators of 800 cm² per channel. However, with the quiescent current set to 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 135-250 with an area of ​​2500 cm² for each transistor. As practice has shown, this measure turned out to be a little excessive, but now the amplifier can be easily touched with your hands - the temperature is only 40°C even in rest mode. Drilling holes in the radiators for mounts and transistors became a bit of a problem - the initially purchased Chinese metal drills were drilled extremely slowly, each hole would have taken at least half an hour. 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 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 won’t 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 on an external sound card is 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...

Finally, you can start listening. The sound source is Foobar2000 → ASIO → external Asus Xonar U7. Microlab Pro3 speakers. The main advantage of these speakers is a separate block of its own amplifier on the LM4766 chip, which can be immediately removed somewhere away. An amplifier from a Panasonic mini-system with a proud Hi-Fi inscription or an amplifier from the Soviet Vega-109 player sounded much more interesting with this acoustics. Both of the above devices operate in class AB. JLH, presented in the article, beat all the above-mentioned comrades by 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 was clearly more detailed and transparent. It's quite easy, for example, to hear the difference between MP3 256kbps and FLAC. I used to think that the lossless effect was more like a placebo, but now my opinion has changed. Likewise, it has become much more pleasant to listen to files uncompressed from loudness war - dynamic range less than 5 dB is not ice at all. Linsley-Hood is worth the investment of time and money, because a similar brand amp will cost much more.

Material costs

Transformer 2200 rub.
Output transistors (6 pcs. with a reserve) 900 rub.
Filter capacitors (4 pcs) 2700 rub.
“Rassypukha” (resistors, small capacitors and transistors, diodes) ~ 2000 rub.
Radiators 1800 rub.
Plexiglas 650 rub.
Paint 250 rub.
Connectors 600 rub.
Boards, wires, silver solder, etc. ~1000 rub.
TOTAL ~12100 rub.

Circuit of a simple transistor audio amplifier, 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 - a total of five transistors for the entire circuit! This design of the UMZCH can be freely used, for example, as part of a home music center or to drive a subwoofer installed in a car or at a disco.

The main attractiveness of this audio power amplifier lies in the ease of its assembly even by novice radio amateurs; there is no need for any special configuration, and there are no problems in purchasing components at an affordable price. The mind diagram presented here has electrical characteristics with high linearity of operation in the frequency range from 20Hz to 20000Hz. p>

When choosing or self-production transformer for the power supply, the following factor must be taken into account: - the transformer must have sufficient power reserve, for example: 300 W per channel, in the case of a two-channel version, then naturally the power doubles. You can use a separate transformer for each, and if you use a stereo version of the amplifier, then you will generally get a “dual mono” type device, which will naturally increase the efficiency of sound amplification.

The effective voltage in the secondary windings of the transformer should be ~34v AC, then the constant voltage after the rectifier will be in the region of 48v - 50v. In each power supply arm, it is necessary to install a fuse designed for an operating current of 6A, respectively, for stereo when operating on one power supply - 12A.

An article about how you can assemble an amplifier with your own hands that has the sound of a factory one in the mid-price range. Below we will describe the assembly of a complete ULF, which includes a preamplifier, an audio power amplifier, an indicator, protection, and two power supplies. All this is collected in a housing from Radiotekhnika. To enlarge the electrical diagram, click.

From many various schemes those that, in my personal opinion, are optimal in terms of price/quality ratio. I did not make any changes other than those described to the original diagrams, everything was done as it is. To power the power amplifier, I took a toroidal transformer with two identical secondary windings of 20 V each with a power of about 100 W and bolted it to a metal substrate at the bottom of the amplifier housing, having previously drilled a hole of the required diameter in it. Next to this trance we place the power amplifier rectifier. We assemble a block of 6 capacitors of 4700 μF x 50V, 3 per arm and shunt with two film capacitors 1 µF each. The preamplifier, indicator, protection and switching will operate from the original transformer.

Preamplifier with three NE5532 op-amps – the sound is excellent! For some reason I didn’t find a linearity mode for the frequency response, a hormonal coefficient for these specifiers in the datasheet, but there is data that is 0.007%. It’s bad that there is no loudness compensation and its implementation is possible, again, with a special resistor. This is exactly the tone block that will go into my complete amplifier. I couldn’t find the board, so I had to develop it myself. Can .

The power amplifier at a voltage of +/- 27 Volts and when applying a sine wave with a frequency of 1 kHz at a 4-ohm load produced 104 Watts. Of course, there are many other power amplifier circuits, but I chose this one because it is simple, cheap and the sound quality is incomparably better than that of .

Do not connect the common wire of the power amplifier power supply to the housing frame directly as a preamplifier! A low-frequency hum appears, which is precisely why the problem with the protection power supply remained unresolved, because When connecting the common protection wire to the common wire of the power amplifier, a slight hum also appears. Therefore, the protection circuit on this moment It functions only as a turn-on delay circuit; in this mode there is no unnecessary noise. As a coil in a power amplifier, a coil from Holton, Radiotekhnika’s own powerhouse, was perfect.

Tests. Excellent sound detail, good stereo panorama. As for the bass, everything is fine here too, it is clear, but not harsh. The radiators of the pre-output transistors are warm, the radiators of the output transistors are cold, this is how it should be. The power is 100 watts at 4 ohms, there is no way to measure the distortion factor, but I think it is small.

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

Frequency characteristics

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

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

Classes of operation of audio amplifiers

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

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

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

The working area of ​​a class “A” transistor amplifier is characterized by fairly small nonlinear distortions. If the incoming signal spits out higher voltage pulses, this causes the transistors to become saturated. In the output signal, higher ones begin to appear near each harmonic (up to 10 or 11). Because of this, a metallic sound appears, characteristic only of transistor amplifiers.

If the power supply is unstable, the output signal will be modeled in amplitude near the network frequency. The sound will be on the left side frequency response more tough. But the better the stabilization of the amplifier's power supply, the more complex the design of the entire device becomes. ULFs operating in class “A” have a relatively low efficiency - less than 20%. The reason is that the transistor is constantly open and current flows through it constantly.

To increase (albeit slightly) efficiency, you can use push-pull circuits. One drawback is that the half-waves of the output signal become asymmetrical. If you transfer from class “A” to “AB”, nonlinear distortions will increase by 3-4 times. But the coefficient useful action the entire circuit of the device will still increase. ULF classes “AB” and “B” characterize the increase in distortion as the signal level at the input decreases. But even if you turn up the volume, this will not help completely get rid of the shortcomings.

Work in intermediate classes

Each class has several varieties. For example, there is a class of amplifiers “A+”. In it, the input transistors (low voltage) operate in mode “A”. But high-voltage ones installed in the output stages operate either in “B” or “AB”. Such amplifiers are much more economical than those operating in class “A”. Noticeably lower number nonlinear distortion- not higher than 0.003%. Better results can be achieved using bipolar transistors. The operating principle of amplifiers based on these elements will be discussed below.

But it still exists a large number of higher harmonics in the output signal, causing the sound to become characteristically metallic. There are also amplifier circuits operating in class “AA”. In them, nonlinear distortions are even less - up to 0.0005%. But main drawback There are still transistor amplifiers - a characteristic metallic sound.

"Alternative" designs

This is not to say that they are alternative, but some specialists involved in the design and assembly of amplifiers for high-quality sound reproduction increasingly prefer tube designs. Tube amplifiers have the following advantages:

1. Very low level of nonlinear distortion in the output signal.
2. There are fewer higher harmonics than in transistor designs.

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

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

Moreover, the efficiency of such devices is quite high - about 50%. But you should not focus only on efficiency and power indicators - they do not indicate the high quality of sound reproduction by the amplifier. Much higher value have linear characteristics and their quality. Therefore, you need to pay attention primarily to them, and not to power.

Single-ended ULF circuit on a transistor

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

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

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

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

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

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

Amplifiers based on MOS transistors

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

The alternating current from the signal source is separated from the direct supply voltage by capacitor C1. The field-effect transistor amplifier must necessarily have a gate potential that will be lower than the same source characteristic. In the diagram shown, the gate is connected to the common wire via resistor R1. Its resistance is very high - resistors of 100-1000 kOhm are usually used in designs. Such a large resistance is chosen so that the input signal is not shunted.

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

ULF with transformer at the output

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

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

Push-pull audio amplifier

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

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

Transformerless ULF

A transistor-based bass amplifier made using a transformer, despite the fact that the design may have small dimensions, is still imperfect. Transformers are still heavy and bulky, so it's better to get rid of them. A circuit made on complementary semiconductor elements with various types conductivity. Most of Modern ULFs are carried out precisely according to such schemes and operate in class “B”.

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

Consequently, only positive half-waves can pass through the load. But the negative ones open the second transistor and completely turn off the first. In this case, only negative half-waves appear in the load. As a result, the signal amplified in power appears at the output of the device. Such an amplifier circuit using transistors is quite effective and can provide stable operation and high-quality sound reproduction.

ULF circuit on one transistor

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

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

Touch the amplifier input with your finger - a characteristic noise should appear. If it is not there, then most likely the structure was assembled incorrectly. Double-check all connections and element ratings. To make the demonstration more clear, connect a sound source to the ULF input - the output from the player or phone. Listen to music and evaluate the sound quality.