Unch on three transistors kt315. A simple low frequency amplifier

To anyone who finds it difficult to choose the first circuit for assembly, I want to recommend this 1-transistor amplifier. The circuit is very simple, and can be made both by surface and printed wiring.

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 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:

You can download the printed circuit board of an amplifier on a single transistor for the sprint-layout program in the My files section of the site.

You can evaluate the sound quality of this amplifier by watching the video of its work on our channel.

This amplifier can be built into any low-power equipment with low-voltage power supply: receivers, walkie-talkies, Hearing Aids and other similar equipment.

Specifications:
Maximum output power(Load 8Ω, 1kHz) = 0.3W
Rated supply voltage (0.3W, 8 ohms) = 3V
THD+N (at maximum output power, 1kHz) = 1 - 1.5%

Schematic diagram of the amplifier:

Device and principle of operation

The amplifier consists of two nodes: the input stage on the transistor T1 and the output push-pull on the transistors T2 - T5. The signal amplified by transistor T1 enters the load R1 and the output stage. The output stage transistors form two so-called "shoulders" of the output stage. Transistors in these "shoulders" of a different structure, which is a prerequisite for this amplifier. Since the KT315 transistor opens with a positive and KT361 with a negative voltage, the “shoulders” of the output stage formed by them amplify only that half-wave of the signal coming from the T1 transistor, which “opens” the transistors that form them. It turns out like this: T3 and T4 amplify the positive half-waves of the signal, T2 and T5 are negative. At the connection point of the emitters of transistors T4 and T5, the signal is combined and fed to the load. Since this amplifier is characterized by step-type distortions, which will inevitably appear during the operation of this amplifier, resistor R2 is turned on to attenuate them. This resistor creates a small bias voltage at the bases of the transistors and attenuates signal distortion.

This amplifier requires careful tuning, namely:
By selecting the resistor R1, the initial quiescent current of the transistors is set (the current flowing through the transistors in the absence of a signal). By selecting this resistor, it is necessary to set the quiescent current at the level of 5 - 7 mA.
By selecting the resistance of the resistor R5, it is necessary to set the voltage at the connection point of the output stage transistors equal to half the supply voltage, that is, 1.5 V.

Possible additions

If the device to which the amplifier is connected does not have a tone control or the signal taken from it is weak, you can assemble a preamplifier.

If there is no need for a tone control, then it can be excluded from the circuit.
On the resistor R4, a passive timbre control HF - LF is assembled with one resistor. Resistor R3 - volume control. All amplification of the signal falls on the transistor. Do not be confused by the lack of a capacitor between the resistor R3 and the collector of the transistor. Everything works and so.
Used parts and possible replacement.

Number		Possible replacement
		KT3102 a - e, KT312, 315, 316.
		KT361 a - e.
		KT315 a - e.
		KT815, 817 a - c.
		KT816, 814 a - c.

This amplifier was assembled by surface mounting, so there is no printed circuit board file. Although drawing a signet for this amplifier is not at all difficult.

The easy-to-use amplifier is made on transistors of different structures and has a voltage gain of about 10. The maximum input voltage can be about 0.1 V.

The principle of operation of a push-pull amplifier

The first stage is assembled on a transistor VT1, the second - on VT2 and VT3 of different structures. The first stage produces a voltage amplification of the audio frequency signal, with both half-waves being the same. The second one amplifies the current signal, however, the cascade on the transistor VT2 operates with positive half-waves, and on the transistor VT3 - with negative ones.

The DC mode is chosen so that the voltage at the junction point of the emitters of the transistors of the second stage is about half the voltage of the power source. This mode is achieved by turning on the feedback resistor R2. The collector current of the input transistor, passing through the diode VD1, leads to a voltage drop on it, which is the bias voltage at the bases of the input transistors relative to their emitters - it allows you to reduce the distortion of the amplified signal.

The load is connected to the amplifier through an electrolytic capacitor C2. When operating the amplifier on a dynamic head with a resistance of 8 ohms to 10 ohms, the capacitance of this capacitor must be at least twice as large.

Schematic assembly photo

Take a look at the load connection of the first amplifier stage, which is the resistor R4. Its top terminal is connected to the lower load terminal. This is the so-called “voltage boost” circuit, due to which a small value of the positive audio frequency enters the base circuit of the output transistors. feedback, equalizing the operating conditions of transistors.

List of used parts

C1, C2, C3	47uF 16V
R1, R4	1 kΩ 0.25 W
R2	10 kΩ 0.25 W
R3	3 kΩ 0.25 W
VD1	KD521A
VT1, VT2	KT315B
VT3	KT361B

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 scheme, a large D.C., 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).

Most audio enthusiasts are quite categorical and not ready for compromises when choosing equipment, rightly believing that the perceived sound must be clear, strong and impressive. How to achieve this?

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Preamplifier on kt315

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Perhaps the main role in resolving this issue will be played by the choice of amplifier.
Function
The amplifier is responsible for the quality and power of sound reproduction. At the same time, when buying, you should pay attention to the following designations, which mark the introduction of high technologies in the production of audio equipment:

• Hi fi. Provides maximum purity and accuracy of sound, freeing it from extraneous noise and distortion.
• Hi end. The choice of a perfectionist who is ready to pay a lot for the pleasure of distinguishing the smallest nuances of his favorite musical compositions. Often hand-assembled equipment falls into this category.

Specifications to pay attention to:

• input and output power. The nominal value of the output power is decisive, since edge values ​​are often unreliable.
• Frequency range. Varies from 20 to 20000 Hz.
• The coefficient of non-linear distortion. It's simple - the smaller the better. The ideal value, according to experts, is 0.1%.
• Signal-to-noise ratio. Modern technology assumes a value of this indicator above 100 dB, which minimizes extraneous noise when listening.
• dumping factor. Reflects the output impedance of the amplifier in relation to the nominal load impedance. In other words, a sufficient damping factor (more than 100) reduces the occurrence of unnecessary vibrations in equipment, etc.

Remember: making quality amplifiers- a labor-intensive and high-tech process, respectively, a too low price with decent characteristics should alert you.

Classification

To understand all the variety of market offers, it is necessary to distinguish the product according to various criteria. Amplifiers can be classified:

• By power. Preliminary - a kind of intermediate link between the sound source and the final power amplifier. The power amplifier, in turn, is responsible for the strength and volume of the signal at the output. Together they form a complete amplifier.

Important: the primary conversion and signal processing takes place precisely in the preamplifiers.

• According to the element base, tube, transistor and integrated PAs are distinguished. The latter arose in order to combine the advantages and minimize the disadvantages of the first two, for example, the sound quality of tube amplifiers and the compactness of transistor ones.
• According to the mode of operation, amplifiers are divided into classes. The main classes are A, B, AB. If class A amplifiers use a lot of power, but produce high quality sound, class B is exactly the opposite, class AB seems to be the best choice, representing a compromise between signal quality and sufficiently high efficiency. There are also classes C, D, H and G, which have arisen with the use of digital technologies. There are also single-cycle and push-pull modes of operation of the output stage.
• By the number of channels, amplifiers can be one-, two- and multi-channel. The latter are actively used in home theaters to form the volume and realism of the sound. Most often there are two-channel, respectively, for the right and left audio systems.

Attention: the study of the technical components of the purchase, of course, is necessary, but often the decisive factor is the elementary listening to the equipment according to the principle of sounds or does not sound.

Application

The choice of amplifier is more justified by the purposes for which it is purchased. We list the main areas of use of audio frequency amplifiers:

1. As part of a home audio system. It's obvious that the best choice is a tube two-channel single cycle in class A, also the best choice can be a three-channel class AB, where one channel is defined for a subwoofer, with a Hi-fi function.
2. For speaker system in the car. The most popular four-channel amplifiers are AB or D class, in accordance with the financial capabilities of the buyer. In cars, the crossover function is also in demand for smooth frequency control, which allows you to cut frequencies in the high or low range as needed.
3. in concert equipment. The quality and capabilities of professional equipment are reasonably subject to higher requirements due to large space dissemination sound signals, as well as a high need for intensity and duration of use. Thus, it is recommended to purchase an amplifier with a class not lower than D, capable of operating almost at the limit of its power (70-80% of the declared one), preferably in a case made of high-tech materials that protects against negative weather conditions and mechanical influences.
4. in studio equipment. All of the above is true for studio equipment. You can add about the largest frequency reproduction range - from 10 Hz to 100 kHz in comparison with that from 20 Hz to 20 kHz in a domestic amplifier. Also noteworthy is the possibility of separate volume control on different channels.

So that for a long time to enjoy clear and high-quality sound, it is advisable to study all the variety of offers in advance and choose the option of audio equipment that best meets your needs.