Portable sound reinforcement system with universal power supply. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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In some cases, the use of a megaphone for sound amplification is clearly not enough due to its limited capabilities and low quality, and leaving with a sound system with a mixing console is an obvious "brute force" of funds. In such cases, the described sound amplification system with a number of interesting operational advantages will be useful.

For a sound reinforcement system designed for traveling work (meetings, lectures, etc.), it is not necessary to have a large capacity. More important are the dimensions and weight, as well as the ability to power both from the mains and from batteries.

Experience has shown that in most cases, a two-channel amplifier output of 20 watts per channel is sufficient for normal operation. Loudspeakers with high sensitivity (return) with an output power of UMZCH of 20 W can sound an open space at a distance of up to 100 m.

Now there are many microcircuits on sale that allow, with a small number of attachments, to assemble a two-channel UMZCH with an output power of up to 22 W per channel (TDA1556Q, TDA1554Q, TDA1555Q, TDA1558Q) or 40 W (TDA8560Q, TDA8563Q). They are designed for a supply voltage of 14 V and allow operation from a car battery. All kinds of protection circuits make these microcircuits reliable in operation, and only surges in the supply voltage can lead to their failure.

Since the mains voltage in rural areas is very unstable and even simply turns off, it is advisable that such amplifiers have universal power supply: from the mains and from the battery. The low power of the amplifier allows you to choose a relatively light mains transformer and work not only from a car battery, but also from a small (152x65x98 mm) sealed 7 Ah battery. The average current consumed by the amplifier is about 1 A, and such a battery provides several hours of autonomous operation of the equipment. It is unlikely that the equipment will be light, but it is quite possible to place everything except the speakers in a regular case.

On fig. 1, and a diagram of one of the amplifier options is shown.

(click to enlarge)

A feature of the TDA1555Q chip is the presence of a distortion detector in it, which works as follows. When distortion appears at the output of the amplifier more than 2 ... 5%, a voltage of about 15 mV appears at pin 24. Since the output distortion exceeds 2% with an input signal level of only 20 dB above the permissible value, their value can be limited using the voltage from the distortion detector for an adjustable voltage divider with an electronically controlled field-effect transistor VT1.

To obtain a sufficient control voltage, it is easiest to use an op amp comparator with a "unipolar" supply, that is, with the lowest output voltage close to the negative supply voltage. Suitable, for example, op-amp chips LM358N or LM324N. In the variant of quad op-amps, two of them can be used in the AGC circuit, and the rest in the preliminary stages: tone block, bandpass filter or microphone amplifier.

Of course, a conventional auto-regulator can provide better sound quality, but when using various speaker systems, their nominal impedance can be 2, 4 or 8 ohms. In these cases, the permissible value of the output voltage changes accordingly. Typically, power amplifiers for such cases are equipped with overload indicators, and here, along with the indication, the level of distortion is limited. The R37C31 chain is designed to eliminate clicks when the amplifier is turned on.

The more powerful TDA8560Q chip (Fig. 1b) has a special "diagnostic voltage output" (pin 12); it is connected to the supply voltage through a 10 kΩ resistor. When the amplitude of the output signal is limited or in the event of a short circuit at the output of the microcircuit, the voltage at pin 12 decreases to 0,6 V. This voltage change can be used to control the divider by connecting the bias circuit shown in the diagram of three resistors and a KD522B diode with the gate of a type VT1 field effect transistor KP103K or KP103M. In this case, the DA2.4 comparator and the electronic key on VT2 are not needed. And if an op amp with a "unipolar" power supply is not required, quad op amps of the K1401UD4, TL084, TL074, LF444 and similar microcircuits can provide higher sound quality.

Of course, the field-effect transistor also introduces some non-linear distortion at a high signal level, but their level and spectrum are significantly less than those that arise as a result of clipping.

A little about the microphone amplifier. Experience shows that external pickups cause much more trouble than the intrinsic noise of amplifier circuits and transistors. Good results can be obtained if you put a transformer at the input or use an instrumentation amplifier with a differential input and gain control with a single resistor. The K548UN1A chip is used in microphone amplifiers in household equipment quite often; here is the option of using it in an instrumentation amplifier. Measurements of the layout parameters with the K140UD6 op-amp (instead of DA2.1) with a bipolar supply of ±12 V showed that the non-linear distortions are less than 0,1%, and the integrated noise is at the level of -65 dB. Overload margin is 34 dB. When the supply voltage is halved, the overload capacity decreases accordingly, but if there is no need to use a battery, you can use a bipolar power supply and, if necessary, make a "phantom" power source (power supply via signal wires) +48 V to power high-quality condenser microphones. Usually, due to their high cost, fear of shocks and moisture, they try not to use such microphones in field work, but for stationary conditions, this possibility is by no means superfluous [1].

The tone control in the amplifier is built using an op-amp (DA2.2). Changing the frequency response of the path may be required when using different types of microphones or to correct the timbre of the voice, or due to the acoustic situation in the hall.

For sound reinforcement in a lecture or meeting, a stereo signal is not needed; both PA channels work in parallel from one source, and when a signal (possibly stereo) is applied from a CD player or tape recorder to the amplifier input, the microphone channel is turned off by the contacts of the X2 input connector.

The power supply of the described system is somewhat unusual. With a reduced mains voltage, if there is no voltage stabilizer in the unit, nothing dangerous happens, only the maximum output power of the PA decreases, but a sharp increase in voltage can lead to failure of the microcircuit, since the allowable supply voltage of some of them is limited to 18 V, and voltage surges exceeding 250 and even 270 V are dangerous for a network transformer! You usually don’t want to carry external stabilizers with you, and they can’t always save the day. Often, a surge protection device simply turns off the equipment, not ensuring the reliability of the event. The complexity of transformer protection lies in the fact that the amplifier operates in AB mode and its current consumption fluctuates dozens of times, so its protection by simply turning on the quenching resistor fails.

The power supply circuit is shown in fig. 2.

(click to enlarge)

Transformer T1 differs from the usual one only in that an additional section is introduced in the primary winding with the number of turns equal to 10% of the main winding (for a voltage of 220 V); it is connected to the contacts of the relay K1. Relay 851N-1C-C (current through the winding is about 28 mA at a voltage of 12 V) operates confidently already from 8,5 V at a current of less than 20 mA. Its contacts are designed for switching an alternating voltage of 250 V at a current of up to 7 A. The secondary winding of the transformer T1 must provide a rectified voltage of 21 ... 22 V. A certain voltage margin compensates for possible voltage drops in the network. Full-wave rectification from two windings was chosen from the considerations that winding one winding of a toroidal transformer with a wire with a cross section of more than 1 mm2 is more difficult than winding two windings with a thinner wire. The output current of the TDA1555Q chip reaches 4, and the TDA8560Q - 7,5 A. The winding data of the T1 network transformer are not shown here, since now they can be purchased or ordered "for every taste" on the radio markets.

To protect the PA microcircuit, an integrated voltage regulator DA2 with a low voltage drop was introduced. At the same time, the voltage used to recharge the external battery stabilizes.

To bring the indication LEDs to the front panel of the amplifier, a three-level LED indicator is assembled on a separate board. When the mains voltage is low (less than 200 V), the green LED HL2 is lit. If the mains voltage is more than 200 and less than 240 V, the yellow LED HL1 lights up. When 240 V is exceeded, the red LED HL3 flashes. As comparators, four op-amps of the LM324N microcircuit are used (K1401UD2 can be used, taking into account the difference in the pinout). Op-amp DA1.4 is used as a "latch". With an increased mains voltage, relay K1 is activated and the voltage immediately decreases by 10%, i.e., the threshold of the permissible value rises to approximately 270 V. To avoid "bounce" and this "latch" is turned on; the relay will remain on even if the mains voltage drops until the amplifier is turned off. If the voltage drops so much that the green LED (HL2) will light up, you need to turn off and turn on the amplifier without fear for its safety.

Connector X7 allows you to connect an external battery. Voltage is removed from pin 3 for recharging, and voltage from an external battery is applied to pin 5. From the car battery from the "cigarette lighter" socket, you can apply voltage to the X7 connector through the cable, the diagram of which is shown in fig. 1, in.

The power supply has an additional connector X6 ("+ Up") for powering the radio microphone receiver. Typically, wireless system receivers are connected to the network through an AC adapter, and this is the only connection possible when powered by a battery.

The AB battery pack is made remote, since it is not always needed. In addition, with its help it is possible to recharge the remote battery from the on-board network of the car while driving. The battery pack (Fig. 3) contains a charger with its own LED voltage indicator. This LED indicator is very similar to the one used in the power supply. At a voltage below 11,5V, the blinking HL5 LED is lit. If the voltage is more than 11,5 V and less than 12,2 V, the red LED HL4 is on, and if the voltage is in the range of 12,2 ... 13,4 V, yellow HL3. LED HL2 - two-color. While the battery is charging, it glows red, and the brighter, the greater the charging current. When the battery is fully charged, the green LED lights up, which indicates the need to stop charging.

For recharging a remote battery from the on-board network of a car, a current stabilizer on microcircuits, even with a small voltage drop, turned out to be unacceptable. But the use of the simplest current limiter on the KT837K transistor (with any letter index) or KT818A turned out to be quite successful. The battery charging current is limited by the selection of resistor R2, and the highest voltage supplied from the output of the power supply is set by a trimming resistor R14 (see Fig. 2). To check the voltage on the battery when the unit is off, you can use the SB2 button without fixing ("Control") to turn on the indicator. If the amplifier is powered from the unit and the SB1 switch is on, then the indicator is connected permanently.

To protect the remote battery from accidental short circuits, a self-resetting fuse Type MF - R400 D141S for a current of 4A is installed in its load circuit. Exceeding this current leads to a sharp increase in the resistance of the fuse. After the short circuit is eliminated and the normal temperature of the fuse is restored, its conductivity is restored.

As an integral stabilizer DA3 in the power supply (see Fig. 2), you can use the proprietary analogue - SD1083, and for the DA3 stabilizer in the amplifier, you can also recommend KR1158EN12V. In the amplifier, the K548UN1A chip can be replaced with an analog - LM381. As an LED indicator HL5, it is desirable to use an LED that blinks at low supply voltage.

When using an external battery pack, it is necessary to complete it with cables for connecting to the power supply unit and to the car's cigarette lighter.

It makes no sense to propose a specific design of the sound reinforcement system, since it is determined by the tasks and style of the developer. Apparently, it is advisable to place a portable sound reinforcement system in a case, and when designing a stationary version, it is better to adhere to the dimensions of a standard 19 "rack. When using a more powerful UMZCH chip (TDA8560Q), a network transformer of about 150 W will be required, and 1555 W is enough for TDA75Q. the size of the LED indicators, the use of buttons or toggle switches is determined only by the availability of components and your own preferences.

It is advisable to make an acoustic system from two loudspeakers, using light domestic or imported heads with a sensitivity of at least 92 dB, while it is quite enough to limit the sound reproduction band from below with a frequency of 100 Hz, and from above - 8 ... 10 kHz. The hulls are made of wood fiber boards and foam. The design and acoustic design (open box or phase inverter) - to the taste of the developer.

It is better to finish the surface of the cases from leatherette, and protect the corners of the cases with metal corners. The heads are protected by a metal mesh painted with decorative paint. In the manufacture of loudspeakers from several heads of the same type (with a series-parallel connection of voice coils), they are placed vertically to obtain a sharper directivity pattern in the vertical plane and increase acoustic efficiency. Loudspeaker outputs (detachable) are best located in the lower part of the case to increase the stability of the case in case of accidental jerking of the connecting cable.

Such a sound amplification system can be improved by providing for the inclusion of a radio microphone receiver to the input of the tone control. The refinement of the amplifier and the manufacture of the radio microphone are carried out taking into account the information presented in [2].

Literature

1. Kuznetsov E. Mixing console. - Radio, 2001, No. 7, p. 12 - 15.
2. Kuznetsov E. Radio microphone for lecturers. - Radio. 2002, no. 3, p. 24, 25.

Author: E.Kuznetsov, Moscow

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