Battery powered amplifier for KT315. Amplifier stage on one transistor

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 direct current will flow through the transistor and speaker, 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, in this circuit there must be a current limiter, that is, a constant resistor, at least 1 KOhm, connected in series with an alternating one. 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”. In this case, the output of the expensive laptop will be supplied with voltage from the power source of this cheap 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).

Most audio lovers are quite categorical and are not ready to compromise 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 purchasing, 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 willing to pay a lot for the pleasure of discerning the smallest nuances of his favorite musical compositions. Hand-assembled equipment is often included in this category.

Specifications you should pay attention to:

Input and output power. The rated output power is of decisive importance, because edge values are often unreliable.
Frequency range. Varies from 20 to 20000 Hz.
Nonlinear distortion factor. Everything is simple here - the less the better. The ideal value, according to experts, is 0.1%.
Signal to noise ratio. Modern technology assumes a value of this indicator over 100 dB, which minimizes extraneous noise when listening.
Dumping factor. Reflects the output impedance of the amplifier in its relation to the nominal load impedance. In other words, a sufficient damping factor (more than 100) reduces the occurrence of unnecessary vibrations of equipment, etc.

It should be remembered: the manufacture of high-quality amplifiers is a labor-intensive and high-tech process; accordingly, too low a price with decent characteristics should alert you.

Classification

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

By power. Preliminary is 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 output signal. Together they form a complete amplifier.

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

Based on the element base, there are tube, transistor and integrated minds. The latter arose with the goal of combining the advantages and minimizing the disadvantages of the first two, for example, the sound quality of tube amplifiers and the compactness of transistor amplifiers.
Based on their operating mode, 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 amplifiers are exactly the opposite, Class AB seems to be the optimal choice, representing a compromise between signal quality and fairly high efficiency. There are also classes C, D, H and G, which arose with the use of digital technologies. There are also single-cycle and push-pull operating modes of the output stage.
Depending on the number of channels, amplifiers can be single-, double- and multi-channel. The latter are actively used in home theaters to create volumetric and realistic sound. Most often there are two-channel ones for right and left audio systems, respectively.

Attention: studying the technical components of the purchase is, of course, necessary, but often the decisive factor is simply listening to the equipment according to the principle of whether it sounds or not.

Application

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

As part of a home audio system. Obviously, the best choice is a tube two-channel single-cycle in class A, and the optimal choice can be a three-channel class AB, where one channel is designated for a subwoofer, with a Hi-fi function.
For car audio system. The most popular are four-channel AB or D class amplifiers, depending on the financial capabilities of the buyer. Cars also require a crossover function for smooth frequency control, allowing frequencies in the high or low range to be cut as needed.
In concert equipment. The quality and capabilities of professional equipment are reasonably subject to higher demands due to the large propagation space of sound signals, as well as the high need for intensity and duration of use. Thus, it is recommended to purchase an amplifier of at least class D, capable of operating almost at the limit of its power (70-80% of the declared one), preferably in a housing made of high-tech materials that protects from negative weather conditions and mechanical influences.
In studio equipment. All of the above is also true for studio equipment. We 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 household amplifier. Also noteworthy is the ability to separately adjust the volume on different channels.

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

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

Operating principle of a push-pull amplifier

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

The DC mode is chosen such that the voltage at the connection point of the emitters of the transistors of the second stage is equal to 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 across it, which is the bias voltage at the bases of the input transistors relative to their emitters - it allows you to reduce distortion of the amplified signal.

The load is connected to the amplifier via electrolytic capacitor C2. When the amplifier is driven by a dynamic head with a resistance of 8 Ohms to 10 Ohms, the capacitance of this capacitor should be at least twice as large.

Photo of circuit assembly

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

List of parts used

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

KT315 is a legendary domestic transistor, copies of which are present in large quantities in every radio amateur. Not surprising - after all, this is the very first mass-produced silicon transistor; you can find it in almost any Soviet device. By the beginning of the 90s, more than 7 billion of them were manufactured. By modern standards, the KT315 is far from an ideal transistor in terms of its parameters, because new, cheaper and more advanced semiconductor devices have been invented and have been produced for a long time. But, nevertheless, sometimes you want to take a handful of old transistors out of the back drawer and assemble something simple with them, for example, an amplifier.

Scheme

The circuit is special in that it does not contain any other active elements except KT315 transistors. This circuit will be an excellent choice not only for antique lovers, but also for those who do not have the opportunity to get other transistors. Resistor values are not very critical and can vary within 20-30%, the same with capacitors. It is advisable to select transistors for this circuit with a high gain, in this case the maximum volume of the amplifier will increase. In this case, it is necessary to comply with the condition - both transistors of the output stage must have the same letter index. The circuit starts working with a voltage of 5 volts, the most optimal power supply is 9 volts. The current consumption is approximately 20 mA and is almost independent of the volume level. It should also be taken into account that to reproduce a stereo signal, the circuit must be repeated twice.

Amplifier assembly

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The circuit is assembled on a printed circuit board measuring 50x40 mm, which already contains both channels. First of all, using laser-iron technology we produce the board itself. Below are some photos of the process.

Once the board is ready, you can start soldering the parts. First of all, resistors are installed on the board, then capacitors with transistors. The terminals of KT315 transistors, unlike the terminals of modern parts, are thin flat strips that come off very easily from the body, so you should not apply too much force to them.

After installing the parts on the board, you need to check the adjacent tracks for short circuits and check that the transistors are installed correctly - after all, they can easily be soldered on the wrong side. The base terminal of the KT315 is on the right when looking at the front side of the transistor. Now all that remains is to connect the board with the speakers and sound source using wires, apply power and the amplifier is ready.

First launch and tests

The amplifier can work with speakers with a resistance of 4-8 Ohms, and you can also connect headphones to its output, which do not have enough power from the standard signal source. The signal source can be, for example, a telephone, player or computer. Before connecting one of the supply wires for the first time, you need to turn on the milliammeter and measure the current consumed; it should not exceed 100 mA in total for both channels. If it exceeds, then it is necessary to reduce the supply voltage. Due to its low consumption, this amplifier can even be powered from the crown. The power of the resulting amplifier is approximately 0.1 watt - not much, but quite enough for quiet listening to music indoors. Happy building!

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 (8 Ohm load, 1 kHz) = 0.3 W
Rated supply voltage (0.3W, 8 Ohm) = 3V
THD+N (at maximum output power, 1kHz) = 1 – 1.5%

Amplifier circuit diagram:

Design and principle of operation

The amplifier consists of two units: an input stage on transistor T1 and a push-pull output stage on transistors T2 - T5. The signal amplified by transistor T1 goes to load R1 and the output stage. The output stage transistors form two so-called “arms” of the output stage. The transistors in these “arms” have different structures, which is a prerequisite for this amplifier. Since transistor KT315 opens with a positive voltage, and KT361 with a negative voltage, the “arms” of the output stage formed by them amplify only that half-wave of the signal coming from transistor T1, which “opens” the transistors that form them. It turns out like this: T3 and T4 enhance the positive half-waves of the signal, T2 and T5 negative. At the junction point of the emitters of transistors T4 and T5, the signal is combined and supplied to the load. Since this amplifier is characterized by step-type distortions, which will inevitably appear during 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 reduces signal distortion.

This amplifier requires careful tuning, namely:
By selecting 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 5 - 7 mA.
By selecting the resistance of 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 pre-amplifier.

If there is no need for a tone control, then it can be excluded from the circuit.
A passive HF-LF tone control with one resistor is assembled on resistor R4. Resistor R3 is a volume control. All signal amplification falls on the transistor. Don't be confused by the absence of a capacitor between resistor R3 and the collector of the transistor. Everything works just like that.
Parts used and possible replacement.

Number		Possible replacement
		KT3102 a – d, KT312, 315, 316.
		KT361 a – f.
		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.