Low frequency amplifier circuits. Symmetrical ULF from available parts based on B

03.07.2023

I want to offer beginner lovers of high-quality sound reproduction one of the developed and tested ULF circuits. This design will help to make a high-quality amplifier that can be modified at minimal cost and used for research circuit solutions.

This will help in the way from simple to complex and more perfect. The description is accompanied by printed circuit board files that can be transformed to a specific case.

In the presented version, the case from Radio Engineering U-101 was used.

This power amplifier I developed and made in the last century from what could be purchased without difficulty. I wanted to make a design with the highest possible ratio of price and quality. It's not High-End, but it's not third grade either. The amplifier has high-quality sound, excellent repeatability and is easy to set up.

Schematic diagram of the amplifier

The circuit is completely symmetrical for the positive and negative half-waves of the low-frequency signal. The input stage is made on transistors VT1 - VT4. It differs from the prototype in transistors VT1 and VT4, which increase the linearity of the cascades on transistors VT2 and VT3. There are many circuit types of input stages with various advantages and disadvantages. This cascade was chosen because of its simplicity, the possibility of reducing the nonlinearity of the amplitude characteristics of transistors. With the advent of more advanced input stage circuits, it can be replaced.

The negative feedback signal (NFB) is taken from the output of the voltage amplifier and fed into the emitter circuits of transistors VT2 and VT3. The rejection of the general OOS is due to the desire to get rid of the influence on the OOS of everything superfluous, which is not the output signal of the circuit. This has its pros and cons. With this configuration, it is justified. With higher quality components, you can try with different types of feedback.

As a voltage amplifier, a cascode circuit was chosen, which has a large input resistance, low capacitance and lower non-linear distortions in comparison with the OE circuit. The disadvantage of the cascode circuit is the lower amplitude of the output signal. That's the price you pay for less distortion. If jumpers are installed, then the OE circuit can also be assembled on the printed circuit board. The power supply of the voltage amplifier from a separate voltage source was not introduced due to the desire to simplify the design of the ULF.

The output stage is a parallel amplifier that has a number of advantages over other circuits. One of the important advantages is the linearity of the circuit with a significant spread of transistor parameters, which was checked when assembling the amplifier. This cascade should have, perhaps, greater linearity, since there is no general OOS and the quality of the output signal of the amplifier depends on it very much. Amplifier supply voltage 30 V.

Amplifier design

I developed printed circuit boards for "affordable" cases from Radiotekhnika U-101 amplifiers. I placed the circuit on two parts of the printed circuit board. On the first part, which is fixed on the radiator, there is a "parallel" amplifier and a voltage amplifier. The input stage is located on the second part of the board. This board is attached to the first board with the help of corners. Such a division of the board into two parts allows for the improvement of the amplifier with minimal design changes. In addition, such an arrangement can also be used for laboratory studies of cascades.

It is necessary to assemble the amplifier in several stages. The assembly begins with a parallel amplifier and its establishment. In the second stage, the rest of the circuit is assembled and adjusted, and the final minimization of circuit distortions is carried out. When placing the transistors of the output stage on the radiator, it is necessary to remember the need for thermal contact of the cases in pairs of transistors VT9, VT14 and VT10, VT13.

Printed circuit boards are designed using the Sprint Layout 6 program, which will allow you to adjust the placement of elements on the board, i.e. customized for a specific configuration or case. See archives below.

Amplifier details

The parameters of the amplifier depend on the quality of the radio elements used and their location on the board. The applied circuit solutions make it possible to do without the selection of transistors, but it is desirable to use transistors with a cutoff amplification frequency from 5 to 200 MHz and a margin of the maximum operating voltage by more than 2 times in comparison with the cascade supply voltage.

If there is a desire and opportunity, then it is desirable to choose transistors according to the principle of "complementarity" and the same amplifying characteristics. Tried manufacturing options with the selection of transistors and without it. The variant with selected "complementary" domestic transistors showed significantly better characteristics than without selection. Only KT940 and KT9115 from domestic transistors are complementary, while the rest have conditional complementarity. There are a lot of complementary pairs among foreign transistors and information about this can be found on manufacturers' websites and in reference books.

As VT1, VT3, VT5, it is possible to use transistors of the KT3107 series with any letters. As VT2, VT4, VT6, it is possible to use transistors of the KT3102 series with letters that have characteristics similar to those used for transistors for another half-wave of the audio signal. If it is possible to select transistors by parameters, then it is better to do this. Almost all modern testers allow you to do this without problems. With large deviations, the time spent during tuning will be greater and the result will be more modest. For VT6 transistors KT9115A, KP960A are suitable, and for VT7 - KT940A, KP959A.

As VT9 and VT12, transistors KT817V (G), KT850A can be used, and as VT10 and VT11 - KT816V (G), KT851A. For VT13, transistors KT818V (G), KP964A are suitable, and for VT14 - KT819V (G), KP954A. Instead of zener diodes VD3 and VD4, you can use two series-connected AL307 LEDs or the like.

The scheme allows the use of other parts, but correction of printed circuit boards may be required. Capacitor C1 can have a capacity from 1 μF to 4.7 μF and must be polypropylene or another, but of high quality. On amateur radio sites you can find information about this. Connection of supply voltage, input and output signals is carried out using terminals for printed wiring.

Setting up the amplifier

When you first turn on the ULF, it should be connected through powerful ceramic resistors (10 - 100 Ohms). This will save the elements from overload and failure in the event of an installation error. On the first part of the board, the ULF quiescent current (150-250 mA) is set by resistor R23 with the load disconnected. Next, you need to establish the absence of a constant voltage at the output of the amplifier when the dummy load is connected. This is done by changing the value of one of the resistors R19 or R20.

After mounting the rest of the circuit, set the resistor R14 to the middle position. On the equivalent load, the absence of excitation of the amplifier is checked and the resistor R5 sets the absence of a constant voltage at the output of the amplifier. The amplifier can be considered configured in static mode.

To establish in dynamic mode, a serial RC circuit is connected in parallel with the equivalent load. Resistor with a power of 0.125 W and a nominal value of 1.3-4.7 kOhm. The capacitor is non-polar 1-2 microfarads. In parallel with the capacitor, we connect a microammeter (20-100 μA). Then, having applied a sinusoidal signal with a frequency of 5-8 kHz to the input of the amplifier, it is necessary to estimate the threshold saturation level of the amplifier using the oscilloscope connected to the output and an AC voltmeter. After that, we reduce the input signal to a level of 0.7 from saturation and with the resistor R14 to achieve a minimum reading of the microammeter. In some cases, to reduce distortion at high frequencies, it is necessary to carry out phase correction in advance by installing capacitor C12 (0.02-0.033 uF).

Capacitors C8 and C9 are selected according to the best transmission of a pulse signal with a frequency of 20 kHz (set if necessary). Capacitor C10 can be omitted if the circuit is stable. By changing the value of the resistor R15, the same gain is set for each of the channels of the stereo or multi-channel version. By changing the magnitude of the quiescent current of the output stage, you can try to find the most linear mode of operation.

Sound evaluation

The assembled amplifier has a very good sound. Long listening to the amplifier does not lead to "fatigue". Of course, there are better amplifiers, but many will like the circuit in terms of cost and quality. With better quality parts and their selection, even more significant results can be achieved.

Links and files

1. Korol V., "UMZCH with compensation for the nonlinearity of the amplitude characteristic" - Radio, 1989, No. 12, p. 52-54.

09-06-2017 - The scheme has been corrected, all archives have been re-uploaded.
▼ 🕗 09/06/17 ⚖️ 24.43 Kb ⇣ 17 Hello reader! My name is Igor, I'm 45, I'm a Siberian and an avid amateur electronics engineer. I came up with, created and maintain this wonderful site since 2006.
For more than 10 years, our magazine exists only at my expense.

Good! The freebie is over. If you want files and useful articles - help me!

This project is a homemade stereo amplifier with an additional headphone output. The amplifier is built on a single integrated circuit TDA2050 which is designed to be used as a hi-fi class audio amplifier. It will work in the supply voltage range from +/-4.5 to +/-25 V. About 30 W of output power, the efficiency is about 65%. However, it is worth noting that to maintain stability, the circuit gain must be at least 24 dB. The amplifier was built to fit Klipsch RB-51 bookshelf speakers. Speakers 8 ohm, sensitivity 92 dB. The amplifier can drive most line sources such as mp3 player, cd player, tuner, etc. A small TDA2050 chip can produce very good sound. Before we get started, I suggest you take a look at the datasheet, especially if you want to make some changes to fit your stereo setup.

circuit diagram

There is also a printed circuit board. I made an amplifier circuit as shown below. Only one channel is shown. The 2-pole switch is common to both channels and this allows you to switch the output from speakers to headphones. If you don't need a headphone output, you can remove the switch and resistor.

The circuit was made on a printed circuit board. To block the input current, I used a 1 uF capacitor (metallized polypropylene film). Most capacitors should be polypropylene, polyester, mylar, electrolytic capacitors I would not recommend.

power unit

Proper grounding scheme will help to get low noise level. If you want, make two stars as ground points - for signal and for power. Try to keep the signal wires as short as possible. In addition, the signal wires must be tightly twisted together. Also try to keep them away from AC sources, both power wires and transformer. Run the wires as close to the body as possible, it helps. Use a separate power supply for each channel.

Before describing nutrition, I want to say a few words about safety. This project requires a 220 V mains connection. Incorrectly selected wire size for the mains supply can cause serious injury! It is also necessary to use only suitable fuses and connect the chassis to ground.

Toroidal transformer with two secondary windings of 18 volts. For rectifiers, I used 35 A diode bridges. The original circuit uses separate diodes. Each output has a 10,000uF capacitor.

For the body, I used a chassis that was suitable in size. The transformer and boards are attached to the bottom of the top of the case. The power switch, volume control and headphone jack are located on the front of the case for easy access.

For sound input, we put standard gold-plated RCA connectors. Speaker output via 4mm banana jack. Please note that the input jacks, speaker and connection terminals are isolated from the chassis with the included nylon gaskets. Radiators are located on the rear panel of the case. Each radiator measures 50 x 90 mm. I cut a hole in the case so that the TDA2050 can be installed directly on the heatsink. Please note that the TDA2050 chip must be isolated from the ground (case), and the negative potential is on the metal tab of the TO-220. If this is not done, the microcontroller will burn out after power is applied. For insulation, you can use flint or mica blocks and do not forget the spacers for the mounting screw that secure the microcontroller to the heatsink. After installation, check to make sure there is no contact between the microcontroller, heatsink and chassis (ground). Also, to ensure good thermal contact, you need to use thermal paste.

I will not give an assessment of the sound quality, since the final opinion depends on the particular listener. To my ears, the TDA2050 produces a very good sound that can compete with the sound of various high-quality amplifiers. The amplifier has the ability to produce deep bass, crisp mids with a wide sonic amplitude, and crisp highs that aren't too sharp. Compared to 20 watts, this one works noticeably more powerfully.

In this article, we will analyze in detail the scheme DIY tube amplifier.

SE or single-ended circuits are amplifiers in which the signal is amplified by one amplifying element (tube, transistor) in series on each stage. These systems operate in pure Class A and are appreciated by many audiophiles for their good micro-dynamics and precision in detail presentation. Simplicity is also an advantage. The disadvantages of these circuits are: low power efficiency (class A), low gain, slightly higher distortion. We present here the layout of such an amplifier.

tube amplifier

Tube amplifier worth not cheap collect. But it is quite possible, and real collect your hands. Yes, what to collect, has been going for more than one year. It is in many ways better than solid-state ones, and the sound is warmer. And so, let's get started - a diagram and a photo report of a do-it-yourself tube amplifier with all the files and descriptions.

Do-it-yourself home theater on lamps

For every true connoisseur of sound, a tube amp speaks volumes, but the latest trend has been the creation of a complete multi-channel tube home theater system. Believe me, with a 32 "screen, the effect is simply amazing! We take a classic single-cycle circuit, with parallel switching of lamps at the output to increase output power. The amplifier operates in class "A", which ensures maximum sound quality. Lamps can be used for input - 6N1P, 6N2P, 6N23P; for output - 6P14P, 6P15P, 6P43P, 6P3S - shorter than rich.

Another low purity TDA amplifier

Do-it-yourself low-frequency amplifier on tda

This amplifier is well suited for assembly and for those who have recently begun to be interested in radio engineering, have mastered the technology of how to apply tracks to a board and poison it.

The amplifier is assembled on a tda7377 and ne555 chip.

Pout - maximum 20W per channel.
The output power will allow you to enjoy the tracks you like.

DIY low pass filter

Low pass filter for subwoofer circuit

We all know that a subwoofer woofer without any filters, when connected to a power amplifier, will simply work like a regular speaker, of course, perfectly reproducing low frequencies, but without low pass filters a good subwoofer is not to collect.

DIY 50W tube amplifier

Good evening to all lovers of the sound of radio tubes! There are many good sound amplifier circuits on the site, so I will publish a version of my LUNCH mono. I collected it for a long time, for almost a whole year I periodically took up the project and gradually completed it, and now, finally, it's time to submit the final version to your court. Purpose: the use of the subwoofer channel was calculated.

DIY tube guitar amplifier

Recently there was a need to assemble a simple ULF for guitar, for which the standard scheme LUNC using lamps such as 6n23p and 6p14p.

DIY hybrid ULF

By popular demand of radio amateurs, I bring an improved and more complete hybrid ULF scheme with a detailed description, parts list and power supply diagram. The lamp at the input of the hybrid ULF 6N6P circuit was replaced by 6N2P. You can also put in this node and more common in old lamps 6N23P. Field-effect transistors are interchangeable with other similar ones - with an insulated gate and a drain current of 5A and above.

Variable R1 - 50 kOhm is a high-quality variable resistor for the volume control. You can put it up to 300kOhm, nothing will worsen. Be sure to check the regulator for the absence of rustles and unpleasant friction during rotation. Ideally, you should use the ALPS WG - a Japanese company that manufactures quality regulators. Don't forget the balance control.

Tube amplifier circuit

Do-it-yourself lamp unch

Tube amplifiers are becoming more and more popular among sound lovers. They differ in quality from transistor ones, and in a more aesthetic retro style.

Pictured tube ULF not difficult assemble with your own hands.

The author decided to assemble UMZCH according to a push-pull scheme on lamps 6P6S. I must say right away that the sound is really not bad, although I haven’t listened to it for a long time and thoughtfully. The power is enough for the eyes, though it was difficult to remove the background, especially in the right channel. Assembled according to the attached diagram, only the rectifier was made on 5Ts3S, after the kenatron, the capacitor is 47 microfarads, each channel has its own D21 inductor, after each inductor there are 330 microfarads of capacitance and it still buzzes a little.

Do-it-yourself amplifier for K174UN14

This amplifier is easy to assemble, ultrasonic diagram collected on a fairly well-known chip k174un14, which is also an analogue of the imported microcircuits tda2003.

Even beginners in radio engineering can assemble this circuit. And so we look further at the characteristics and the device circuit itself sent by Aidar Galimov

Evgenia Smirnova

To send light into the depths of the human heart - this is the purpose of the artist

Content

Connecting speakers to a laptop, TV, or other music source sometimes requires amplifying the signal with a separate device. The idea of building an amplifier is a good one if you are prone to PCB work at home and have some technical skills.

How to make a sound amplifier

Getting started on assembling an amplifying device for speakers of one type or another consists of finding tools and components. The amplifier circuit on a printed circuit board is assembled using a soldering iron on a heat-resistant support. It is recommended to use special soldering stations. If DIY assembly is for circuit testing purposes or for short term use, the on-wire option will do, but you will need more space to accommodate the components. The printed circuit board guarantees compactness of the device and convenience in further use.

A cheap and common amplifier for headphones or small speakers is created on the basis of a microcircuit - a miniature control unit with a pre-wired set of commands for controlling an electrical signal. It remains to add only a few resistors and capacitors to the circuit with a microcircuit. The total cost of an amateur-class amplifier is, as a result, much lower than the price of finished professional equipment from the nearest store, but the functionality is also limited to changing the output volume of the audio signal.

Remember the features of compact single-channel amplifiers assembled by yourself based on TDA series microcircuits and their analogues. The chip generates a lot of heat during operation, so you should avoid or minimize its contact with other parts of the device. Radiator grill for heat dissipation is recommended for use. Depending on the model of the microcircuit and the power of the amplifier, the size of the required radiator increases. If the amplifier is assembled in a case, you should first plan a place for a heat sink.

Another feature of assembling a sound amplifier with your own hands is the low voltage consumption. This allows you to use a simple amplifier in cars (powered by a car battery), on the road or at home (powered by a special unit or batteries). Some simplified audio amplifiers require only 3 volts of current. The power consumption depends on how much amplification of the audio signal is required. The sound amplifier from the player for standard headphones consumes about 3 watts.

A novice radio amateur is recommended to use a computer program to create and view circuit diagrams. Files for such programs may have the extension *.lay - they are created and edited in the popular virtual tool Sprint Layout. Creating a DIY circuit from scratch makes sense if you have already gained experience and want to experiment with the knowledge gained. Otherwise, look for and download ready-made files, according to which you can quickly assemble a replacement for a low-frequency amplifier for a car radio or a digital combo amplifier for a guitar.

For laptop

A do-it-yourself sound amplifier is being assembled for a laptop in one of two cases: the built-in speakers are out of order, or their volume and sound quality are not enough for your needs. You will need a simple amplifier, designed for external speakers up to 2 watts, and winding resistance up to 4 ohms. To assemble it with your own hands, in addition to standard radio amateur tools (pliers, a soldering station), you will need a printed circuit board, a TDA 7231 chip, and a 9-volt power supply. Choose your own cabinet to house the components of the amplifier.

Add the following items to the list of purchased components:

non-polar capacitor 0.1 uF - 2 pcs.;
polar capacitor 100 uF - 1 pc.;
polar capacitor 220 uF - 1 pc.;
polar capacitor 470 uF - 1 pc.;
constant resistor 10 KΩ - 1 pc.;
resistor constant 4.7 Ohm - 1 pc.;
two-position switch - 1 pc.;
speaker output jack – 1 pc.

Determine the assembly order yourself, depending on which *.lay format wiring diagram you downloaded. Choose a radiator of such a size that its thermal conductivity allows you to keep the operating temperature of the microcircuit below 50 degrees Celsius. If the device is constantly used with a laptop outdoors, it will require a homemade case with slots or holes for air circulation. You can assemble such a case with your own hands from a plastic container or the remnants of old radio equipment, fixing the board with long screws.

For DIY headphones

The simplest stereo amplifier for portable headphones should have little power, but the most important parameter will be power consumption. In an ideal example, the design is powered by AA batteries, in extreme cases, from a simple 3 Volt adapter. You will need a high-quality TDA 2822 chip or its equivalent (for example, KA 2209), an electronic circuit for assembling an amplifier with your own hands on a TDA 2822. Additionally, take the following components:

capacitors 100 uF (4 pcs.);
up to 30 cm of copper wire;
headphone jack.

A heat sink element is needed if you want to make the amplifier compact and with a closed case. You can assemble the amplifier on a finished or home-made printed circuit board or by surface mounting. The pulse transformer in the power supply can create noise, so do not use it in this variant of the amplifier. The ready amplifier will provide a pleasant and powerful sound from the player (records or radio signal), tablet or phone.

Subwoofer amplifier circuit

The low-frequency amplifier is assembled by hand on the TDA 7294 chip. It is used both to create powerful acoustics with bass in the apartment, and as an auto amplifier - in this case, however, you need to purchase a bipolar 30-35 Volt power supply. The figures below describe the location of components, as well as the rating of resistors and capacitors. This subwoofer amplifier will provide up to 100 watts of output power with prominent low frequencies.

Mini speaker amplifier

As a sound amplification device for domestic or foreign home speakers, the design described above for laptops is suitable. Stationary placement of the device will allow you to choose any power adapter available. You can ensure the miniature and acceptable appearance of an inexpensive amplifier by observing a few rules:

Ready-made high-quality printed circuit board.
Durable plastic or metal case (order from the master).
The placement of components is pre-planned.
The amplifier is soldered neatly, without extra drops of solder.
The heatsink only touches the chip.
Ready-made sockets for signal output and power input are used.

DIY tube sound amplifier

Tube audio amplifiers are expensive devices, provided that you purchase all the components with your own funds. Older radio amateurs sometimes keep collections of lamps and other parts. Assembling a tube amplifier at home is relatively easy if you are willing to spend a few days searching the internet for detailed schematics. The sound amplifier circuit in each case is unique and depends on the sound source (old tape recorder, modern digital technology), power source, estimated dimensions and other parameters.

Transistor audio amplifier

Assembling a sound preamplifier with your own hands without the use of complex microcircuits is possible on transistors. The amplifier on germanium transistors is easily built into modern audio systems, it does not require additional configuration. The disadvantage of transistor circuits is the larger size of the assembled boards. Dependence on the "purity" of the background is also unpleasant - you will need a shielded cable, or an additional noise and ripple suppression circuit from the network.

Video: do-it-yourself sound power amplifier

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High input impedance and shallow feedback are the main secret of warm tube sound. It's not a secret for anyone that it is on the tubes that the most high-quality and expensive amplifiers are realized, which belong to the HI-End category. Let's understand what a quality amplifier is? The low-frequency power amplifier has the right to be called high-quality, which completely repeats the shape of the input signal at the output, without distorting it, of course, the output signal is already amplified. On the network you can find several circuits of really high-quality amplifiers that have the right to be classified as HI-End and tube circuitry is not at all necessary. To obtain maximum quality, you need an amplifier whose output stage operates in pure class A. The maximum linearity of the circuit gives a minimum amount of distortion at the output, therefore, in the structure of high-quality amplifiers, special attention is paid to this factor. Lamp circuits are good, but not always available even for self-assembly, and industrial lamp UMZCH from branded manufacturers cost from several thousand to several tens of thousands of US dollars - this price is certainly not affordable for many.
The question arises - is it possible to achieve similar results from transistor circuits? The answer will be at the end of the article.

There are a lot of linear and super-linear low-frequency power amplifier circuits, but the circuit that will be considered today is a high-quality ultra-linear circuit, which is implemented on only 4 transistors. The circuit was created back in 1969 by British sound engineer John Linsley-Hood. The author is the creator of several more high-quality circuits, in particular class A. Some experts call this amplifier the highest quality among transistor ULFs, and I was convinced of this a year ago.

The first version of such an amplifier was presented on. A successful attempt to implement the circuit made me create a two-channel ULF according to the same scheme, assemble everything in a case and use it for personal needs.

Circuit Features

Despite its simplicity, the scheme has several features. Proper operation may be affected by improper board layout, poor component placement, improper power supply, etc.
It is the power supply that is a particularly important factor - I strongly advise against powering this amplifier from all kinds of power supplies, the best option is a battery or a power supply with a battery connected in parallel.
The power of the amplifier is 10 watts with a power supply of 16 volts into a load of 4 ohms. The circuit itself can be adapted for 4, 8 and 16 ohm heads.
I created a stereo version of the amplifier, both channels are located on the same board.

The second one is designed to build up the output stage, I installed KT801 (it was rather difficult to get it.
In the output stage itself, I installed powerful bipolar reverse conduction switches - KT803 got undoubtedly high sound quality with them, although I experimented with many transistors - KT805, 819, 808, even put powerful composite ones - KT827, with it the power is much higher, but the sound cannot be compared with KT803, although this is just my subjective opinion.

Input capacitor with a capacity of 0.1-0.33uF, you need to use film capacitors with minimal leakage, preferably from well-known manufacturers, the same with the output electrolytic capacitor.
If the circuit is designed for a load of 4 ohms, then you should not increase the supply voltage above 16-18 volts.
I decided not to install a sound regulator, it, in turn, also affects the sound, but it is advisable to put a 47k resistor in parallel with the input and minus.
The board itself is a breadboard. I had to tinker with the board for a long time, since the track lines also had some effect on the sound quality in general. This amplifier has a very wide frequency response, from 30 Hz to 1 MHz.

Setup is easy. To do this, you need a variable resistor to achieve half the supply voltage at the output. For a more precise setting, it is worth using a multi-turn variable resistor. We connect one probe of the multimeter to the minus power supply, put the other to the output line, i.e. to the plus of the electrolyte at the output, thus, slowly rotating the variable, we achieve half the power at the output.