Power supplies for low frequency amplifiers. Home amplifier - unch and protection unit. The key is voltage drop.

Making a good power supply for a power amplifier (VLF) or other electronic device is a very important task. The quality and stability of the entire device depends on what the power source will be.

In this publication I will talk about the manufacture of a simple transformer power supply for my homemade low-frequency power amplifier "Phoenix P-400".

Such an uncomplicated power supply can be used to power various low-frequency power amplifier circuits.

Foreword

For the future power supply unit (PSU) to the amplifier, I already had a toroidal core with a wound primary winding of ~ 220V, so the task of choosing a "pulse PSU or based on a network transformer" was not.

Switching power supplies have small dimensions and weight, high output power and high efficiency. The power supply based on the mains transformer is heavy, easy to manufacture and set up, and also does not have to deal with dangerous voltages when setting up the circuit, which is especially important for beginners like me.

toroidal transformer

Toroidal transformers, in comparison with transformers on armored cores made of Ш-shaped plates, have several advantages:

• smaller volume and weight;
• higher efficiency;
• best cooling for windings.

The primary winding already contained approximately 800 turns of 0.8 mm PELSHO wire, it was filled with paraffin and insulated with a layer of thin PTFE tape.

By measuring the approximate dimensions of the iron of the transformer, you can calculate its overall power, so you can figure out whether the core is suitable for obtaining the required power or not.

Rice. 1. Dimensions of the iron core for a toroidal transformer.

• Overall power (W) \u003d Window area (cm 2) * Cross-sectional area (cm 2)
• Window area = 3.14 * (d/2) 2
• Cross-sectional area \u003d h * ((D-d) / 2)

For example, let's calculate a transformer with iron dimensions: D=14cm, d=5cm, h=5cm.

• Window area \u003d 3.14 * (5cm / 2) * (5cm / 2) \u003d 19.625 cm 2
• Sectional area \u003d 5cm * ((14cm-5cm) / 2) \u003d 22.5 cm 2
• Overall power = 19.625 * 22.5 = 441 watts.

The overall power of the transformer I used turned out to be clearly less than I expected - somewhere around 250 watts.

Selection of voltages for secondary windings

Knowing the required voltage at the output of the rectifier after the electrolytic capacitors, it is possible to approximately calculate the required voltage at the output of the secondary winding of the transformer.

The numerical value of the direct voltage after the diode bridge and smoothing capacitors will increase by about 1.3..1.4 times, compared with the alternating voltage supplied to the input of such a rectifier.

In my case, to power the UMZCH, you need a bipolar constant voltage - 35 volts on each arm. Accordingly, an alternating voltage must be present on each secondary winding: 35 Volts / 1.4 \u003d ~ 25 Volts.

By the same principle, I made an approximate calculation of the voltage values ​​\u200b\u200bfor other secondary windings of the transformer.

Calculation of the number of turns and winding

To power the remaining electronic components of the amplifier, it was decided to wind several separate secondary windings. A wooden shuttle was made for winding coils with copper enameled wire. It can also be made from fiberglass or plastic.

Rice. 2. Shuttle for winding a toroidal transformer.

The winding was carried out with copper enameled wire, which was available:

• for 4 UMZCH power windings - a wire with a diameter of 1.5 mm;
• for other windings - 0.6 mm.

The number of turns for the secondary windings I selected experimentally, since I did not know the exact number of turns in the primary winding.

The essence of the method:

1. We wind 20 turns of any wire;
2. We connect the primary winding of the transformer to the network ~ 220V and measure the voltage on the wound 20 turns;
3. We divide the required voltage by that obtained from 20 turns - we find out how many times 20 turns are needed for winding.

For example: we need 25V, and out of 20 turns we get 5V, 25V / 5V = 5 - we need to wind 20 turns 5 times, that is, 100 turns.

The calculation of the length of the required wire was performed as follows: I wound 20 turns of wire, made a mark on it with a marker, unwound it and measured its length. I divided the required number of turns by 20, multiplied the resulting value by the length of 20 turns of wire - I got approximately the required length of wire for winding. By adding 1-2 meters of stock to the total length, you can wind the wire on the shuttle and safely cut it off.

For example: you need 100 turns of wire, the length of 20 wound turns turned out to be 1.3 meters, we find out how many times 1.3 meters need to be wound to get 100 turns - 100/20=5, we find out the total length of the wire (5 pieces of 1, 3m) - 1.3*5=6.5m. We add 1.5m for the stock and get the length - 8m.

For each subsequent winding, the measurement should be repeated, since with each new winding the length of wire required per turn will increase.

To wind each pair of windings of 25 volts, two wires were laid in parallel on the shuttle at once (for 2 windings). After winding, the end of the first winding is connected to the beginning of the second - we got two secondary windings for a bipolar rectifier with a connection in the middle.

After winding each of the pairs of secondary windings to power the UMZCH circuits, they were insulated with a thin fluoroplastic tape.

Thus, 6 secondary windings were wound: four for powering the UMZCH and two more for power supplies for the rest of the electronics.

Scheme of rectifiers and voltage stabilizers

Below is a schematic diagram of the power supply for my homemade power amplifier.

Rice. 2. Schematic diagram of the power supply for a homemade bass power amplifier.

To power the low-frequency power amplifier circuits, two bipolar rectifiers are used - A1.1 and A1.2. The remaining electronic components of the amplifier will be powered by voltage stabilizers A2.1 and A2.2.

Resistors R1 and R2 are needed to discharge electrolytic capacitors when the power lines are disconnected from the power amplifier circuits.

There are 4 amplification channels in my UMZCH, they can be turned on and off in pairs using switches that switch the power lines of the UMZCH scarf using electromagnetic relays.

Resistors R1 and R2 can be excluded from the circuit if the power supply is constantly connected to the UMZCH boards, in which case the electrolytic capacities will be discharged through the UMZCH circuit.

Diodes KD213 are designed for a maximum forward current of 10A, in my case this is enough. The diode bridge D5 is designed for a current of at least 2-3A, it was assembled from 4 diodes. C5 and C6 are capacitances, each of which consists of two 10,000 microfarad capacitors at 63V.

Rice. 3. Schematic diagrams of DC voltage stabilizers on L7805, L7812, LM317 microcircuits.

Deciphering the names on the diagram:

• STAB - voltage regulator without adjustment, current not more than 1A;
• STAB+REG - adjustable voltage regulator, current not more than 1A;
• STAB+POW - adjustable voltage stabilizer, current approximately 2-3A.

When using LM317, 7805 and 7812 microcircuits, the output voltage of the stabilizer can be calculated using a simplified formula:

Uout = Vxx * (1 + R2/R1)

Vxx for chips has the following meanings:

• LM317 - 1.25;
• 7805 - 5;
• 7812 - 12.

Calculation example for LM317: R1=240R, R2=1200R, Uout = 1.25*(1+1200/240) = 7.5V.

Design

Here's how it was planned to use the voltage from the power supply:

• +36V, -36V - power amplifiers on TDA7250
• 12V - electronic volume controls, stereo processors, output power indicators, thermal control circuits, fans, backlight;
• 5V - temperature indicators, microcontroller, digital control panel.

The voltage regulator chips and transistors were mounted on small heatsinks that I removed from non-working computer power supplies. The cases were attached to the radiators through insulating gaskets.

The printed circuit board was made of two parts, each of which contains a bipolar rectifier for the UMZCH circuit and the required set of voltage regulators.

Rice. 4. One half of the power supply board.

Rice. 5. The other half of the power supply board.

Rice. 6. Ready-made power supply components for a homemade power amplifier.

Later, during debugging, I came to the conclusion that it would be much more convenient to make voltage stabilizers on separate boards. Nevertheless, the "all on one board" option is also not bad and convenient in its own way.

Also, a rectifier for UMZCH (diagram in Figure 2) can be assembled by surface mounting, and stabilizer circuits (Figure 3) in the required quantity - on separate printed circuit boards.

The connection of the electronic components of the rectifier is shown in Figure 7.

Rice. 7. Connection diagram for assembling a bipolar rectifier -36V + 36V using surface mounting.

Connections must be made using thick insulated copper conductors.

The diode bridge with 1000pF capacitors can be placed separately on the heatsink. Mounting of powerful KD213 diodes (pills) on one common radiator must be carried out through insulating thermal pads (thermal rubber or mica), since one of the diode leads has contact with its metal lining!

For a filtering circuit (electrolytic capacitors of 10000 μF, resistors and ceramic capacitors of 0.1-0.33 μF), you can quickly assemble a small panel - a printed circuit board (Figure 8).

Rice. 8. An example of a panel with slots made of fiberglass for mounting rectifier smoothing filters.

To make such a panel, you need a rectangular piece of fiberglass. Using a homemade cutter (Figure 9), made from a hacksaw blade for metal, we cut the copper foil along the entire length, then we cut one of the resulting parts perpendicularly in half.

Rice. 9. Homemade cutter from a hacksaw blade, made on a grinder.

After that, we outline and drill holes for parts and fasteners, clean the copper surface with thin sandpaper and tin it with flux and solder. We solder the parts and connect to the circuit.

Conclusion

Here is such an uncomplicated power supply was made for a future homemade audio frequency power amplifier. It remains to supplement it with a soft start circuit and a standby mode.

UPD: Yuri Glushnev sent a printed circuit board for assembling two stabilizers with voltages + 22V and + 12V. It contains two STAB + POW circuits (Fig. 3) on LM317, 7812 microcircuits and TIP42 transistors.

Rice. 10. Printed circuit board of voltage stabilizers for + 22V and + 12V.

Download - (63 KB).

Another PCB designed for the STAB + REG adjustable voltage regulator circuit based on the LM317:

Rice. 11. Printed circuit board for an adjustable voltage regulator based on the LM317 chip.

Switching power supplies are widely used in modern electronic equipment. The attention of readers is invited to a switching power supply with a power of 800 W. It differs from those described earlier by the use of field-effect transistors and a transformer with a primary winding with an average output in the converter. The first provides higher efficiency and reduced high-frequency noise, and the second - half the current through the key transistors and eliminates the need for an isolation transformer in their gate circuits.

The disadvantage of such a circuit design is the high voltage on the halves of the primary winding, which requires the use of transistors with the appropriate allowable voltage. True, unlike a bridge converter, in this case, two transistors are enough instead of four, which slightly simplifies the design and increases the efficiency of the device. In the proposed UPS, a push-pull converter with a transformer is used, the primary winding of which has an average output. It has high efficiency, low ripple and weakly radiates interference into the surrounding space. The author uses it to power a two-channel powered version of the UMZCH.

The input voltage of the UPS is 180...240 V, the rated output voltage (at the input 220 V) is 2x50 V, the maximum load power is 800 W, the operating frequency of the converter is 90 kHz. The schematic diagram of the UPS is shown in fig. 4.47. As you can see, this is a converter with external excitation without output voltage stabilization. At the input of the device, a high-frequency filter CI, LI, C2 is included, which prevents interference from entering the network. After passing it, the mains voltage is rectified by the diode bridge VD1 ... VD4, the ripples are smoothed out by the capacitor C3. Rectified DC voltage (about 310 V) is used to power the high-frequency converter.

The converter control device is made on DD1...DD3 microcircuits. It is powered by a separate stabilized source, consisting of a step-down transformer T1, a rectifier VD5 and a voltage regulator on transistors VT1, VT2 and a zener diode VD6. On the elements DD1.1, DDI.2, a master oscillator is assembled that generates pulses with a repetition rate of about 360 kHz.

This is followed by a frequency divider by 4, made on the triggers of the DD2 chip. With the help of elements DD3.1, DD3.2 additional pauses are created between pulses. A pause is nothing more than a logical 0 level at the outputs of these elements, which appears when there is a logical 1 level at the outputs of the DDI.2 element and triggers DD2.1 and DD2.2. The low-level voltage at the output of DD3.1 (DD3.2) blocks DD1.3 (DD1.4) in the "closed" state (at the output - logic level 1). The duration of the pause is equal to 1/3 of the duration of the voltage pulse at pins 1 DD3.1 and 13 DD3.2, which is quite enough to close the key transistor. From the outputs of the elements DD1.3 and DDI.4, the finally generated pulses are fed to transistor switches (VT5, VT6), which, through resistors RIO, R11, control the gates of powerful field-effect transistors VT9, VT10 (see Fig. 4.48).

Pulses from the direct and inverse outputs of the DD2.2 trigger are fed to the inputs of a device made on transistors VT3, VT4, VT7, VT8. Opening in turn, VT3 and VT7, VT4 and VT8 create conditions for the rapid discharge of the input capacitances of the key transistors VT9, VT10, i.e. their fast closing. Resistors of relatively high resistance R10 and R11 are included in the gate circuit of transistors VT9 and VT10. Together with the capacitance of the gates, they form low-pass filters that reduce the level of harmonics when the keys open.

For the same purpose, elements VD9 ... VD12, R16, R17, C12, C13 were introduced. The primary winding of the transformer T2 is included in the drain circuits of transistors VT9, VT10. The output voltage rectifiers are made according to the bridge circuit on VD13...VD20 diodes, which somewhat reduces the efficiency of the device, but significantly (more than five times) reduces the ripple level at the UPS output. It is important to note that the shape of the oscillations, almost rectangular at maximum load, smoothly turns into close to sinusoidal when the power is reduced to 10 ... 20 W, which has a positive effect on the noise level of the UMZCH fed from this unit at low volume. The rectified voltage of the winding IV of the transformer T2 is used to power the fans.

The device uses capacitors K73-17 (C1, C2, C4), K50-17 (C3), MBM (C12, C13), K73-16 (C14 ... C21, C24, C25), K50-35 (C5. ..C7), KM (the rest). Instead of those indicated in the diagram, it is permissible to use microcircuits of the K176, K564 series. Diodes D246 (VD1 ... VD4) are replaceable with any others rated for a forward current of at least 5 A and a reverse voltage of at least 350 V (KD202K, KD202M, KD202R, KD206B, D247B), or a diode rectifier bridge with the same parameters, diodes KD2997A (VD13 ... VD20) - on KD2997B, KD2999B, zener diode D810 (VD6) - on D814V. As VT1, you can use any transistors of the KT817, KT819 series, as VT2 ... VT4 and VT5, VT6 - respectively, any of the KT315, KT503, KT3102 and KT361, KT502, KT3107 series, in place of VT9, VT10 - KP707V1, KP707E1 . Transistors KT3102ZH (VT7, VT8) are not recommended to be replaced.

Transformer T1 - TS-10-1 or any other with a secondary voltage of 11 ... 13 V at a load current of at least 150 mA. The mains filter coil L1 is wound on a ferrite (M2000NM1) ring of size K31x18.5x7 with PEV-1-1.0 wire (2x25 turns), transformer T2 is wound on three ferrite rings glued together of the same brand, but size K45x28x12. Winding I contains 2x42 turns of wire PEV-2-1.0 (wound in two wires), windings II and III - 7 turns each (in five wires PEV-2-0.8), winding IV - 2 turns PEV-2- 0.8. Three layers of insulation made of PTFE tape are laid between the windings.

The magnetic circuits of the chokes L2, L3 are ferrite (1500NMZ) rods with a diameter of 6 and a length of 25 mm (trimmers from armored cores B48). The windings contain 12 turns of PEV-1-1.5 wire. Transistors VT9, VT10 are installed on heat sinks with fans used to cool Pentium microprocessors (similar nodes from 486 processors are also suitable). Diodes VD13...VD20 are mounted on heat sinks with a surface area of ​​about 200 cm2.

When installing the UPS, you should strive to ensure that all connections are as short as possible, and in the power section use a wire of the largest possible cross-section. It is desirable to enclose the UPS in a metal shield and connect it to the 0 V terminal of the source output, as shown in fig. 4.49. The common wire of the power unit must not be connected to the screen. Since the UPS is not equipped with a short circuit and overload protection device, 10 A fuses must be included in the power circuit. The described UPS practically does not need to be adjusted. It is only important to correctly phase the halves of the primary winding of the transformer T2.

If the parts are in good condition and there are no errors in the installation, the unit starts working immediately after being connected to the network. If necessary, the frequency of the converter is adjusted by selecting the resistor R3. To increase the reliability of the UPS, it is desirable to operate it with a UMZCH, which provides through blowing by a fan.

The audio frequency amplifier (UHF), or low frequency amplifier (ULF) is one of the most common electronic devices. We all receive sound information using one or another type of ULF. Not everyone knows, but low-frequency amplifiers are also used in measuring technology, flaw detection, automation, telemechanics, analog computing and other areas of electronics.

Although, of course, the main application of ULF is to convey a sound signal to our ears with the help of acoustic systems that convert electrical vibrations into acoustic ones. And the amplifier should do this as accurately as possible. Only in this case we get the pleasure that our favorite music, sounds and speech give us.

From the appearance of Thomas Edison's phonograph in 1877 to the present day, scientists and engineers have struggled to improve the basic parameters of ULF: primarily for the reliability of the transmission of sound signals, as well as for consumer characteristics, such as power consumption, dimensions, ease of manufacture, adjustment and use.

Since the 1920s, a letter classification of electronic amplifier classes has been formed, which is still used today. Classes of amplifiers differ in the operating modes of the active electronic devices used in them - vacuum tubes, transistors, etc. The main "single-letter" classes are A, B, C, D, E, F, G, H. Class designation letters can be combined if some modes are combined. The classification is not a standard, so developers and manufacturers can use the letters quite arbitrarily.

Class D occupies a special place in the classification. The active elements of the ULF output stage of class D operate in the key (pulse) mode, unlike other classes, where the linear mode of operation of active elements is mostly used.

One of the main advantages of class D amplifiers is the coefficient of performance (COP), approaching 100%. This, in particular, leads to a decrease in the power dissipated by the active elements of the amplifier, and, as a result, to a decrease in the size of the amplifier due to a decrease in the size of the radiator. Such amplifiers impose much lower requirements on the quality of the power supply, which can be unipolar and pulsed. Another advantage can be considered the possibility of using digital signal processing methods and digital control of their functions in class D amplifiers - after all, it is digital technologies that prevail in modern electronics.

Taking into account all these trends, Master Kit offers wide range of class amplifiersD, assembled on the same TPA3116D2 chip, but having different purposes and power. And so that buyers do not waste time looking for a suitable power source, we have prepared amplifier + power supply kits optimally matched to each other.

In this review, we will look at three such kits:

1. (LF amplifier D-class 2x50W + power supply 24V / 100W / 4.5A);
2. (LF amplifier D-class 2x100W + power supply 24V / 200W / 8.8A);
3. (D-class bass amplifier 1x150W + power supply 24V / 200W / 8.8A).

First set is intended primarily for those who need minimal dimensions, stereo sound and a classic control scheme simultaneously in two channels: volume, bass and treble. It includes and .

The two-channel amplifier itself has an unprecedentedly small size: only 60 x 31 x 13 mm, not including knobs. The dimensions of the power supply are 129 x 97 x 30 mm, weight is about 340 g.

Despite its small size, the amplifier delivers honest 50 watts per channel into a 4 ohm load at a supply voltage of 21 volts!

The RC4508 chip is used as a pre-amplifier - a dual specialized operational amplifier for audio signals. It allows you to perfectly match the input of the amplifier with the signal source, has extremely low non-linear distortion and noise level.

The input signal is fed to a three-pin connector with a pin pitch of 2.54 mm, the supply voltage and speakers are connected using convenient screw connectors.

A small heatsink is installed on the TPA3116 chip using heat-conducting glue, the dissipation area of ​​which is quite enough even at maximum power.

Please note that in order to save space and reduce the size of the amplifier, there is no protection against reverse polarity of the power supply connection (polarity reversal), so be careful when applying power to the amplifier.

Given the small size and efficiency, the scope of the kit is very wide - from replacing an outdated or failed old amplifier to a very mobile sound amplification kit for scoring an event or party.

An example of the use of such an amplifier is given.

There are no mounting holes on the board, but for this you can successfully use potentiometers that have fasteners for the nut.

Second set includes two TPA3116D2 chips, each of which is connected in bridged mode and provides up to 100 watts of output power per channel, as well as with an output voltage of 24 volts and a power of 200 watts.

With this kit and two 100-watt speakers, you can sound a solid event even outdoors!

The amplifier is equipped with a volume control with a switch. A powerful Schottky diode is installed on the board to protect against polarity reversal of the power supply.

The amplifier is equipped with effective low-pass filters, installed according to the recommendations of the manufacturer of the TPA3116 chip, and together with it provide a high quality output signal.

The supply voltage and acoustic systems are connected using screw connectors.

The input signal can be either a 3-pin 2.54mm pitch connector or a standard 3.5mm audio jack.

The radiator provides sufficient cooling for both microcircuits and is pressed against their thermal pads with a screw located on the bottom of the printed circuit board.

For ease of use, the board also has a green LED that indicates power on.

The dimensions of the board, including capacitors and excluding the potentiometer knob, are 105 x 65 x 24 mm, the distances between the mounting holes are 98.6 and 58.8 mm. Power supply dimensions 215 x 115 x 30 mm, weight approx. 660 g.

Third set represents l and with an output voltage of 24 volts and a power of 200 watts.

The amplifier provides up to 150 watts of output power into a 4 ohm load. The main application of this amplifier is the construction of a high-quality and energy-efficient subwoofer.

Compared to many other dedicated subwoofer amplifiers, the MP3116btl is excellent at driving fairly large diameter woofers. This is confirmed by customer reviews of the considered ULF. The sound is rich and bright.

The radiator, which occupies most of the PCB area, provides efficient cooling of the TPA3116.

To match the input signal at the input of the amplifier, the NE5532 chip is used - a two-channel low-noise specialized operational amplifier. It has minimal non-linear distortion and a wide bandwidth.

The input also has an input signal amplitude control with a slot for a screwdriver. It allows you to adjust the volume of the subwoofer to the volume of the main channels.

To protect against polarity reversal of the supply voltage, a Schottky diode is installed on the board.

Power and speakers are connected using screw connectors.

The dimensions of the amplifier board are 73 x 77 x 16 mm, the distance between the mounting holes is 69.4 and 57.2 mm. Power supply dimensions 215 x 115 x 30 mm, weight approx. 660 g.

All kits include switching power supplies from MEAN WELL.

Founded in 1982, the company is the leading manufacturer of switching power supplies in the world. Currently, MEAN WELL Corporation consists of five financially independent partner companies in Taiwan, China, the United States and Europe.

MEAN WELL products are characterized by high quality, low failure rate and long service life.

Switching power supplies, developed on a modern element base, meet the highest requirements for the quality of the output DC voltage and differ from conventional linear power supplies in their low weight and high efficiency, as well as the presence of protection against overload and short circuit at the output.

The power supplies LRS-100-24 and LRS-200-24 used in the presented kits have an LED power indicator and a potentiometer for fine adjustment of the output voltage. Before connecting the amplifier, check the output voltage, and if necessary, set its level to 24 volts using a potentiometer.

The applied sources use passive cooling, so they are completely silent.

It should be noted that all the considered amplifiers can be successfully used to design sound reproducing systems for cars, motorcycles and even bicycles. When the amplifiers are powered by 12 volts, the output power will be somewhat less, but the sound quality will not suffer, and the high efficiency makes it possible to efficiently power the ULF from autonomous power sources.

We also draw your attention to the fact that all the devices discussed in this review can be purchased separately and as part of other kits on the site.

It would seem that it could be easier to connect the amplifier to power supply and enjoy your favorite music?

However, if we recall that the amplifier essentially modulates the voltage of the power supply according to the law of the input signal, it becomes clear that the design and installation issues power supply should be approached very responsibly.

Otherwise, mistakes and miscalculations made at the same time can spoil (in terms of sound) any, even the most high-quality and expensive amplifier.

Stabilizer or filter?

Surprisingly, most power amplifiers are powered by simple circuits with a transformer, a rectifier, and a smoothing capacitor. Although most electronic devices today use stabilized power supplies. The reason for this is that it is cheaper and easier to design an amplifier that has a high ripple rejection ratio than it is to build a relatively powerful regulator. Today, the level of ripple suppression of a typical amplifier is about 60dB for a frequency of 100Hz, which practically corresponds to the parameters of a voltage regulator. The use of direct current sources, differential stages, separate filters in the power circuits of the stages and other circuitry techniques in the amplifier stages makes it possible to achieve even greater values.

Food output stages most often made unstabilized. Due to the presence in them of 100% negative feedback, unity gain, the presence of LLCOS, the penetration of the background and ripple of the supply voltage to the output is prevented.

The output stage of the amplifier is essentially a voltage (power) regulator until it enters clipping (limiting) mode. Then the ripple of the supply voltage (frequency 100 Hz) modulates the output signal, which sounds just awful:

If for amplifiers with a unipolar supply only the upper half-wave of the signal is modulated, then for amplifiers with a bipolar supply, both half-waves of the signal are modulated. Most amplifiers have this effect at large signals (powers), but it is not reflected in any way in the technical characteristics. In a well-designed amplifier, clipping should not occur.

To test your amplifier (more precisely, the power supply of your amplifier), you can conduct an experiment. Apply a signal to the input of the amplifier with a frequency slightly higher than you can hear. In my case, 15 kHz is enough :(. Increase the amplitude of the input signal until the amplifier enters clipping. In this case, you will hear a hum (100 Hz) in the speakers. By its level, you can evaluate the quality of the power supply of the amplifier.

Warning! Be sure to turn off the tweeter of your speaker system before this experiment, otherwise it may fail.

A stabilized power supply avoids this effect and results in less distortion during prolonged overloads. However, taking into account the instability of the mains voltage, the power loss on the stabilizer itself is approximately 20%.

Another way to reduce the clipping effect is to feed the stages through separate RC filters, which also reduces power somewhat.

In serial technology, this is rarely used, since in addition to reducing power, the cost of the product also increases. In addition, the use of a stabilizer in class AB amplifiers can lead to excitation of the amplifier due to the resonance of the feedback loops of the amplifier and regulator.

Power losses can be significantly reduced if modern switching power supplies are used. Nevertheless, other problems emerge here: low reliability (the number of elements in such a power supply is much larger), high cost (for single and small-scale production), high level of RF interference.

A typical power supply circuit for an amplifier with an output power of 50W is shown in the figure:

The output voltage due to smoothing capacitors is approximately 1.4 times greater than the output voltage of the transformer.

Peak Power

Despite these shortcomings, when the amplifier is powered from unstabilized source, you can get some bonus - short-term (peak) power is higher than the power of the power supply, due to the large capacity of the filter capacitors. Experience shows that a minimum of 2000µF is required for every 10W of output power. Due to this effect, you can save on the power transformer - you can use a less powerful and, accordingly, cheap transformer. Keep in mind that measurements on a stationary signal will not reveal this effect, it appears only with short-term peaks, that is, when listening to music.

A stabilized power supply does not give such an effect.

Parallel or series stabilizer?

There is an opinion that parallel regulators are better in audio devices, since the current loop is closed in a local load-stabilizer loop (power supply is excluded), as shown in the figure:

The same effect is obtained by installing a decoupling capacitor at the output. But in this case, the lower frequency of the amplified signal limits.

Protective resistors

Every radio amateur is probably familiar with the smell of a burnt resistor. It's the smell of burning varnish, epoxy and... money. Meanwhile, a cheap resistor can save your amp!

When the author first turns on the amplifier in the power circuits, instead of fuses, he installs low-resistance (47-100 Ohm) resistors, which are several times cheaper than fuses. This has repeatedly saved expensive amplifier elements from installation errors, incorrectly set quiescent current (the regulator was set to maximum instead of minimum), reversed power polarity, and so on.

The photo shows an amplifier where the installer mixed up TIP3055 transistors with TIP2955.

The transistors were not damaged in the end. Everything ended well, but not for the resistors, and the room had to be ventilated.

The key is voltage drop.

When designing printed circuit boards for power supplies and not only, one should not forget that copper is not a superconductor. This is especially important for "ground" (common) conductors. If they are thin and form closed circuits or long circuits, then due to the current flowing through them, a voltage drop occurs and the potential at different points turns out to be different.

To minimize the potential difference, it is customary to wire the common wire (ground) in the form of a star - when each consumer has its own conductor. The term "star" should not be taken literally. The photo shows an example of such a correct wiring of a common wire:

In tube amplifiers, the resistance of the anode load of the cascades is quite high, of the order of 4 kOhm and higher, and the currents are not very large, so the resistance of the conductors does not play a significant role. In transistor amplifiers, the resistance of the cascades is significantly lower (the load generally has a resistance of 4 ohms), and the currents are much higher than in tube amplifiers. Therefore, the influence of conductors here can be very significant.

The resistance of a track on a printed circuit board is six times higher than the resistance of a piece of copper wire of the same length. The diameter is taken 0.71mm, this is a typical wire that is used when mounting tube amplifiers.

0.036 Ohm as opposed to 0.0064 Ohm! Considering that the currents in the output stages of transistor amplifiers can be a thousand times higher than the current in a tube amplifier, we find that the voltage drop across the conductors can be 6000! times more. Perhaps this is one of the reasons why transistor amps sound worse than tube amps. This also explains why PCB-assembled tube amps often sound worse than surface-mounted prototypes.

Don't forget Ohm's law! Various techniques can be used to reduce the resistance of printed conductors. For example, cover the track with a thick layer of tin or solder a tinned thick wire along the track. The options are shown in the photo:

charge impulses

To prevent the penetration of the mains background into the amplifier, measures must be taken to prevent the penetration of charge pulses of the filter capacitors into the amplifier. To do this, the tracks from the rectifier must go directly to the filter capacitors. Powerful pulses of charging current circulate through them, so nothing else can be connected to them. the power supply circuits of the amplifier must be connected to the terminals of the filter capacitors.

The correct connection (mounting) of the power supply for an amplifier with unipolar power supply is shown in the figure:

Zoom on click

The figure shows a PCB variant:

Ripple

Most unregulated power supplies have only one smoothing capacitor after the rectifier (or several connected in parallel). To improve the quality of power, you can use a simple trick: split one container into two, and connect a small resistor of 0.2-1 ohm between them. At the same time, even two containers of a smaller denomination can be cheaper than one large one.

This gives a smoother output voltage ripple with less harmonics:

At high currents, the voltage drop across the resistor can become significant. To limit it to 0.7V, a powerful diode can be connected in parallel with the resistor. In this case, however, at the peaks of the signal, when the diode opens, the output voltage ripples will again become “hard”.

To be continued...

The article was prepared based on the materials of the journal "Practical Electronics Every Day"

Free translation: Editor-in-Chief of Radio Gazeta

Maybe someone will be interested in such a device - ULF 2x25 W built into the system unit.

Appearance of the device

Good mother, good sound card, good but passive speakers…

As a result, at the workplace (at the computer) there is no decent sound. For a long time I perverted with all sorts of external amplifiers that take up space on the table, require an additional outlet, wires, and all sorts of other inconveniences. In the end, I got tired of it, and made a built-in ULF based on the ms TDA8560Q - a 2x40 W car two-channel amplifier at a 2 Ohm load. At a 4-ohm load, the power is slightly less - 2x25 watts. The piping is a pair of electrolytes for power supply, input dividers (25 W is too much, however), 4 conders for decoupling at the input and in the power circuit, and if the fingers are really like a fan - a transistor for a “soft” start (so that there are no clicks when turned on).

All this is very conveniently located on the board of the format of a standard PCI card, which I inserted into a free slot on the mother. In order not to load the tracks of the motherboard, the power supply (on-board 12 volts) was supplied through a separate connector (as on all IDE devices - CDs, screws, and modern video cards). I had a mounting plate from an old S3-Trio video card at hand, so I didn’t have to do anything with a file at all.

I used a DRB-9 socket as an output connector (similar to a COM port connector, only a "mother"). It is not very convenient that the wires from both speakers had to be driven into one connector, but the “design” of the device turned out to be very simple.

The amplifier was connected to the output of the sound card with a conventional audio cord from the CD (I only soldered a mini-jack connector with a diameter of 3.5 mm on one side).

To cool the amplifier microcircuit, a standard radiator from an old processor, either the 486th or from the first stump (only 12 mm high), was perfect. If desired, you can even put a cooler on it (I provided a connector on the board). But, as a month of active operation showed, this is not required, the temperature of the radiator does not exceed 40-50 degrees even during long-term operation and at high power.

(drawn in SLayout-4 scriber). The scheme is a standard one from a datasheet for a microcircuit, but if necessary, I will post it additionally. The only difference is that at the input of each channel I made 6:1 dividers (5.6 kOhm and 1 kOhm), otherwise the signal level from the sound system is too high.

The ratings of all parts are drawn on the signet.

By the way, in order to install a heatsink, the small circuit had to be laid "on its back" - with a metal substrate towards the heatsink, so the microcircuit pins had to be mirrored (bent in the other direction).

If you use a mounting plate from another card (for example, from an additional COM port connector), you may have to change the location of the output connector (move it up or down on the board). As a last resort, you can use a standard blanking plate, but you will have to spend half an hour after it and cut out a hole for the output connector.

The connector for supplying power to the amplifier fell out from the board of some ancient hard drive. You can take it from a 5-inch drive or CD.

I hope that there will be no problems with the repetition of this useful contraption.

The only advice: do not forget that the current of the computer's PSU at 12 volts is only a few amperes (specifically, look at your PSU), and therefore do not try to "pump out" everything that it can give out from the TDA. The calculation is simple - 1 ampere of current consumption can provide an amplifier output power of about 5 watts per channel, respectively, 2 amperes - 2x10 watts, etc. I have a 450 W power supply in my computer, capable of delivering up to 14 amps at 12 volts, so 4-5 amps “per side” do not adversely affect the operation of the computer.

Do not be greedy, and you will have everything in chocolate!