Speaker protection circuit. Simple and reliable speaker protection. Switch-on delay and speaker protection device

The Internet now offers a huge number of different sound amplifiers, for every taste and color, to suit any need. As you know, even the most reliable amplifiers tend to fail, for example, due to improper operating conditions, overheating or incorrect connection. In this case, there is a high probability that the high supply voltage will end up at the output of the amplifier, and, therefore, will unhinderedly end up directly on the speakers speaker system. Thus, a failed amplifier drags with it “to another world” the speaker system connected to it, which can cost much more than the amplifier itself. That is why it is highly recommended to connect the amplifier to the speakers through a special board called speaker protection.

Scheme

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Assembling the device

Protection tests

Several different device schemes are considered. designed to protect acoustic systems (AS) and implement a time delay before connecting the speakers to the output of an audio power amplifier.

Protection and turn-on delay circuit on four transistors

The above device is designed to delay the connection of loudspeakers for the duration of transient processes in the UMZCH when turning on the power and turning them off when a constant voltage of any polarity appears at its output.

Rice. 1. Schematic diagram of the device for protecting acoustic systems and switching delay, made on four transistors.

The schematic diagram of the device is shown in Fig. 1. It consists of a diode distributor (VD1 - VD6) and an electronic relay on transistors VT1 - VT4.

It is connected to the outputs of the UMZCH channels together with the loudspeakers through the contacts of relay K1. Circuits R1C1, R2C2 prevent the device from triggering audio frequency fluctuations.

If necessary, the number of controlled channels can be increased by simply connecting the corresponding number of additional circuits, similar to the circuit R1C1VD1VD2, and using an electromagnetic relay with a large number of contact groups. The constant voltage at the output of the UMZCH, at which the protection device is triggered, is determined by the voltage of the stabilizer diode VD7 and is related to it by the relation:

When the power is turned on (the voltage source can be the UMZCH power supply), capacitor C3 begins to charge (through resistor R9), so transistor VT4 is closed and relay K1 is de-energized.

As charging occurs, the voltage on the capacitor increases, transistor VT4 begins to open, and after a while (about 3 s) its emitter current increases so much that relay K1 is activated and connects the speakers to the output of the UMZCH.

Transistors VT1 - VT3 are also closed in the initial state. When a voltage of any polarity exceeding the above value appears at the output of any of the channels, transistor VT2 opens, followed by VT1, VT3. As a result, capacitor C3 is discharged through the emitter-collector section of transistor VT3 and resistor R8, transistor VT4 closes and relay K1 disconnects the speakers and the device input from the UMZCH output.

Transistor VT1, which provides positive feedback in the cascade on transistor VT2, plays the role of a “latch”, maintaining the latter in the open state even after disconnecting the device from the output of the UMZCH: without it, after the loss of voltage at the input and closing of transistor VT2, VT3, charging of capacitor C3 would begin again and after a period of time charging the speakers would reconnect to the UMZCH.

The device uses a RES-9 relay (passport RS4.524.200). Transistors KT603b (VT3,VT4) can be replaced with KT315g. A 20V power source is used to power the device.

At high voltages, due to reverse collector currents, spontaneous opening of transistors VT1, VT2 is possible. To prevent this from happening, it is necessary to reduce the resistance of resistors R5, R6. If the supply voltage is more than 30 V, the device should use transistors with a permissible collector-emitter voltage of at least.

When reducing the voltage (by replacing the D814a zener diode), care must be taken to ensure that the amplitude of the low-frequency alternating voltage at the outputs of filters R1C1, R2C2 does not reach values ​​that cause the speakers to turn off. This is not difficult to do - just increase the time constants of the named circuits (for example, increase C1, C2).

Improved protection scheme for speakers

The protection device Fig. 2 has great potential.

Rice. 2. Schematic diagram of the protection of speaker systems from output voltage surges, powered by the UMZCH power supply.

It protects the speakers from output voltage surges both when turning the power on and off, in the event of a malfunction of the UMZCH and in moments of probable failure of the latter - when one or both supply voltages decrease or completely disappear, as well as when they exceed the maximum permissible values ​​(this may have place when powered from stabilized sources) and, finally, turns them off when connecting the head stereo phones. The device is powered from the same two-field source as the output stages of the UMZCH.

At the moment the power is turned on, capacitor C3 begins to charge, so transistor VT2 is open, VT3 is closed, relay K1 is de-energized and the speakers are turned off. As soon as the voltage across the capacitor reaches

The stabilization voltage of the zener diode VD9), the states of the indicated transistors are reversed, relay K1 is activated and the loudspeaker is connected to the outputs of the UMZCH channels.

The above formula is valid provided: .

Delay time at the element ratings indicated on the diagram: .

The stabilization voltage of the zener diode VD11 is selected from the condition .

When the voltage of any power source decreases by an amount greater than transistor VT3 closes and relay K1 disconnects the speakers from the UMZCH.

Zener diodes VD7 and VD9 in the base circuits of transistors VT1, VT2, respectively, are the same and are selected taking into account the following. As can be seen from the diagram, in order for transistor VT2 to open (and therefore transistor VT3 to close and release relay K1), the supply voltage must satisfy the condition:

Where and are, respectively, the voltage and minimum stabilization current of the zener diode VD9.

From here: . With the ratings and types of parts indicated in the diagram

This means that the device will turn off the speakers if the negative supply voltage increases (relative to the nominal) by 2.8 V.

Transistor VT1 opens along the circuit VD1 - R5 - VD7, identical to the circuit VD6 - R7 - VD9. This leads to the opening of transistor VT2 and the closing of transistor VT3, i.e. to turn off the speakers when the supply voltage of positive polarity increases by 8 V.

If a constant positive voltage appears at the output of the UMZCH, transistor VT2 opens with current flowing through resistor R3 (or R4), VD4 (VD5) and circuit R7VD9. The condition for opening it in this case looks like this:

If the voltage at the output of the UMZCH has a negative polarity, transistor VT1 opens through the circuit R3 (R4) - VD2 (VD3) - R5 - VD7.

To connect stereo telephones, use the XS1 socket, to which the SA1 switch is mechanically connected. When the stereo telephone plug is inserted into the socket, the switch contacts open, relay K1 releases and the speakers are disconnected from the UMZCH.

The same thing happens when you turn off the power to the UMZCH with the SB1 button (A1 is the power source). Since the collector circuit of transistor VT3 and the mains supply circuit are broken almost simultaneously, the speakers are turned off before the transition process begins and a click is not heard.

The device uses a RES-22 relay (RF passport-4.500.130). Non-polar oxide capacitors C1, C2 - K50-6. The KT815V transistor can be replaced by any other with a permissible collector-emitter voltage of more than 50 V and a maximum collector current of at least , where - is the resistance of the relay winding K1).

Instead of KS527A zener diodes, you can use KS482A, KS510A, KS512A, KS175Zh, KS182Zh, KS191Zh, etc., connecting the required number of devices to obtain the stabilization voltage selected by the above formulas. Diodes VD1 - VD6, VD8, VD10, VD12 - any low-power silicon with a reverse voltage of more than 50 V.

AC protection circuit which is powered by the AF signal

The original loudspeaker protection devices (Fig. 3) are powered by audio frequency signal voltage, which allows it to be built into a loudspeaker.

The device turns off the latter in case of power overload, as well as in the event of a constant voltage of any polarity appearing at the output of the UMZCH. The circuit uses speakers with a power of 10 W and an electrical resistance of 4 Ohms.

Rice. 3. Schematic diagram of the protection of an acoustic speaker, which is powered by an AF signal.

In the initial state, relay K1 is de-energized and the AF (audio frequency) signal from the amplifier output is supplied through contacts K1.1 to the loudspeaker. At the same time, it rectifies with a bridge VD1 - VD4, and its constant component is supplied through normally closed contacts K1.2 to a threshold device made on transistor VT1 and microcircuit DA1.

As long as the input signal voltage does not exceed the operating threshold, the transistor is closed and the voltage at pin 12 of the DA1 microcircuit is equal to the stabilization voltage of the zener diode VD6, which is greater than the voltage of the standard source of the microcircuit, which can be in the range of 1.5 ... 3 V. (The zener diode VD6 prevents breakdown emitter junction of the transistor of the differential stage of the microcircuit with reverse voltage).

At the moment when input signal reaches the device response level (the voltage on the engine of the tuning resistor R5 is about 1.5 V), the transistor VT1 opens and the voltage at pin 12 of the DA1 microcircuit becomes less than the exemplary one.

As a result, the control transistor of the microcircuit opens, relay K1 is activated and the loudspeaker is disconnected from the UMZCH, and the relay winding is connected directly to the output of the rectifier bridge VD1 - VD4.

When the rectified voltage decreases to the relay lowering voltage, the device returns to initial state. The device behaves similarly when a constant voltage appears at the output of the UMZCH.

The response threshold is set by trimming resistor R6. Capacitor C3 prevents the device from triggering when the signal briefly exceeds the trigger threshold.

The minimum signal voltage at which the device is operational is determined by the relay response voltage. In the case of using the RES-47 relay (passport RF4.500.407-04) and parts with the ratings indicated in the diagram, it does not exceed 5 V. The VD8 zener diode limits the voltage on the relay winding.

In the absence of the K142EN1A microcircuit, you can use K142EN1, K142EN2 with any letter index. The KD522B diodes can be replaced by any other with a reverse voltage of more than 40 V, a forward current of at least 100 mA and a maximum frequency (KD51A, diode assemblies of the K542 series, etc.), the KS107A stabistor - with any silicon diode, the KT3412B transistor - with any low-power silicon transistor n-p-n structures with a permissible collector-emitter voltage of at least 40 V.

When making a device to protect loudspeakers of powerful sound-reproducing devices, you should use diodes KD204A - KD204V, KD212A, KD212B, KD213A, KD213B, etc., replace the RES-47 relay with another one, with contacts that allow switching high currents, and if necessary, " "power up" the DA1 microcircuit of external transistors to provide the required current through the relay winding.

It may happen that when the device is triggered, the relay contacts will bounce. It can be prevented by connecting a capacitor with a capacity of 10...20 μF between pins 16 and 8 of the DA1 microcircuit or a resistor with a resistance of 1 kOhm between its pin 13 and the base of the transistor VT1 (thus creating a positive feedback).

Speaker protection circuit using a resistor-optocoupler

The proposed device (Fig. 4)

Rice. 4. Schematic diagram of the protection of acoustic systems using a resistor optocoupler.

provides protection for acoustic systems (AS) from damage when a constant voltage of positive or negative polarity appears at the outputs of a stereo amplifier.

The functions of the actuator protection element are performed by resistor optocoupler U1. It works as follows. When a negative or positive DC voltage appears on any of the output audio amplifiers (AUS), the input current begins to flow through the opron and the resistance of its resistor sharply decreases.

As soon as the DC voltage reaches 3-4 V (depending on the type of optocoupler), the resistance becomes so small that transistors VT1, VT2 close, relay winding K1 is de-energized and its contacts K1.1, K1.2 disconnect the speaker from the ultrasonic sounder.

Zener diodes VD1, VD2 limit the input current of the optocoupler to 18 mA. Since for D815A zener diodes a stabilization voltage variation of 15% is allowed, it is necessary to select such specimens so that the voltage applied to the optocoupler light emitter does not exceed 5.5 V.

Chokes L1, L2 limit the alternating component of the input current of the optocoupler to a value that excludes the possibility of protection operation. They are made on magnetic cores ШЛ12*12 and contain 1200 turns of PEL-0.23 wire. the active resistance of each inductor is 36 Ohms.

Due to the long charging time of capacitor C1 through resistor R1, there is a delay in the opening of transistors VT1, VT2, the operation of relay K1 and the connection of the speaker to the amplifier.

As a result of transient processes that occur in the amplifier after it is turned on, they fade out before the device connects the speakers, so a click is not heard in them.

When the amplifier is turned on by switch 8B1, contacts 1 and 4 of the latter close, causing instantaneous closing of transistors VT1, VT2. Naturally, the speaker opens from the amplifier before the start of transient processes in it, and the click in the speaker will also not be heard.

The AC protection device is powered by a 2-pole power amplifier power supply. When selecting elements VT1, VT2, C1, R2, K1, the source voltage should be taken into account.

When using relays RES-9, RES-22, the protection device can be supplemented with an alarm system for its operation. (Fig. 5)

Rice. 5. Scheme of supplementing the AC protection device with a light alarm.

The described device was developed for a specific amplifier with a supply voltage equal to plus or minus 15 V. In this case, when the maximum voltage appears at one of the amplifier outputs, the thermal power generated at the chokes L1 or L2 does not exceed 3 W, which eliminates its significant overheating during the time during which a conclusion can be made about a malfunction of the power amplifier (PA) and a decision can be made to turn it off.

The second version of the protection circuit with optocoupler

With more high voltage power supply and there are no guarantees of timely detection of the moment of operation of the protection device, it can be assembled according to a slightly modified diagram (Fig. 6).

Rice. 6. Schematic diagram of the protection device acoustic speakers, power supply from -30 +30V.

In this case, when the protection system is triggered, the power to the power amplifier is turned off. The light emitter of the optocoupler is connected to the power supply of the amplifier by contacts K1.3 of relay K1, which allows you to keep the protection device in the “Emergency” mode.

In addition, if one of the voltages of the 2-polar power supply is missing, the protection device does not connect the PA to it and turns it off if one of these voltages disappears. Lighting of the LEDs indicates a malfunction in the amplifier or power supply.

In a device assembled according to the diagram in Fig. 3, relay K1 must have 4 groups of contacts for switching (RES-22, passport RF4.500.130). It should be noted that such a protection system scheme loses the function of preventing clicks in the speakers.

Speaker protection circuit that disconnects the AF amplifier from the network

Figure 7 shows a diagram of the AC protection device, which disconnects the amplifier from the power supply.

Rice. 7. Schematic diagram of the protection of acoustic systems, disconnecting the AF amplifier from the 220V network.

To turn on the amplifier you need to press the SB1 button. In this case, the supply voltage will be supplied to the protection device, relay K1 is activated and its contacts will block the SB1 button so that when it is released, the PA remains connected to the power source.

To turn off the amplifier, you must press the SB2 button. The principle of this device is similar to that described above. It is triggered and disconnects the amplifier from the network when DC voltage appears at one of its outputs or the supply voltage is lost.

Buttons SB1, SB2 without fixing in the pressed position KM21, KMD2-1, and relay K1-RES-32, RF passport 4.500.335-02 (or RES-22, RF passport 4.500.130).

Passive protection system for loudspeaker

The most common way to protect speaker systems from dangerous overvoltage is to disconnect them from the signal source using an electromagnetic relay.

However, it is impractical to use it in high-end speakers due to nonlinear distortions introduced into the reproduced signal. The fact is that the relay contacts have their own active resistance, which in new products ranges from 0.1 (at best) to 0.5 Ohm.

As a result, when passing through them electric current significant thermal power is dissipated on them. This causes oxidation of the metal from which the contacts are made, which in itself is a source of distortion.

In addition, during the operation of the relay, oxidation increases and the contact resistance can increase to 1 Ohm or more, which is comparable to the resistance of the speakers themselves and can reduce their output.

In another version of the protection of speakers, when a dangerous overvoltage appears on them, the outputs of the UMZCH are connected to the common wire using a thyristor until the fuse in the power circuit of the output stage trips.

However, this method also has significant disadvantages, since it poses a certain danger to the UMZCH itself and is associated with the need to replace fuses.

A number of foreign speakers use polycrystalline elements, specially designed to protect the HF and midrange heads, but they introduce even greater distortion into the signal and also cannot be used in high-class speakers.

The proposed passive speaker protection device is a powerful diode symmetrical audio frequency signal limiter (Fig. 8).

Rice. 8. Powerful diode symmetrical audio frequency signal limiter.

It is made in the form of a 2-terminal network, connected in parallel to the protected circuit: either the speaker as a whole, or one of its emitters, for example, the HF or midrange head. In the latter case, it is installed directly in the speaker, and in the first, it can be placed both at the output of the UMZCH and in the speaker itself.

The device works as follows. When a voltage exceeding the set limit threshold appears at its terminals, the diodes of the corresponding branch open and current begins to flow through them.

A certain thermal power is dissipated on the diodes, and the signal entering the speaker or emitter is softly limited in voltage and, accordingly, in power.

When the voltage supplied to the AC decreases below the operating threshold, the device turns off. In standby mode, the protection device does not affect the audio frequency, since in this case the diodes of both branches are closed, and their resulting capacitance is negligible.

The device should use powerful rectifier diodes with a high overload capacity, increased maximum operating frequency and a small capacity of its own. Of the most common, we can recommend KD213 with any letter index, as well as KD2994, KD2995, KD2998, KD2999.

These diodes allow the flow of direct current 10..30 A or more depending on the type, and the maximum pulse current through them can reach 100 A.

Without a heat sink, each diode is capable of dissipating electrical power of about 1 W, which corresponds to a current of about 1 A. When installed on simple plate heat sinks, the power dissipated by each diode can be increased to 20 W. In Fig. 9 shows a possible design protective devices using plate heat sinks.

Rice. 9. Possible design of protective devices using plate heat sinks.

Of the operating features of the protection device, the following must be taken into account. When the diodes open, a small current flows through them. In this case, to open each of the diodes, a voltage of 0.6...0.7 V is required, depending on its type.

With a further increase in voltage at the sockets of the protection device, the passing current increases and, accordingly, the voltage drop across the diode junctions increases. Its value can be up to 1..1.4 V in the current range up to 10...30 A.

Calculation of the protection device comes down to determining the type of diodes and their number in each branch. To do this, it is necessary to determine the power and voltage limit threshold.

Let's assume that we want to protect a dynamic head with a nominal power of 10 W and a normal impedance of 8 ohms from overload.

In this case, it is advisable to determine the voltage at a power level of about 8 W. Then a current of 1 A should flow through the head at an input voltage of 8 V.

When using KD213 diodes with a threshold voltage of 0.6 V, the number of diodes in each branch is approximately 13. In total, there are 26 diodes for 2 branches.

The technical characteristics of such a protection system will be very high. The response threshold is 8 V. The maximum level of power limitation on the protected circuit at a current through the diodes of 10 A is about 30 W. The initial power absorbed by the protection system is approximately 4+4 W, the maximum at a current of 10 A and using a heat sink is up to 130 W.

When choosing diodes, it is preferable to those that allow maximum currents of 20...30 A with a voltage drop across them of 1 V. These include: KD2994.

They are much more expensive than KD213, but have significantly better characteristics for our purposes. Thus, their threshold voltage is higher and is about 0.7 V, and the voltage drop at a current of 20 A is only 1.1 V. In addition, their housing is more convenient for mounting on a printed circuit board and attaching a heat sink.

When using KD2994 (instead of KD213) in the above calculation, their number in the branches will decrease from 13 to 11, which partly compensates for the high cost. The characteristics of the protection device will be much flatter: with a current through the diodes of 10 A, the power limitation level on the protected circuit will no longer be 30, but only 12 W. In this case, the protection system will absorb power of the order of 100+100 W.

The use of the described circuit in the high-fidelity sound reproduction path, especially if the UMZCH output stage operates in pure class A, allows you to completely get rid of the distortions introduced by conventional protection devices.

It is most appropriate to use the proposed system to protect relatively low-power speakers and emitters. However, if you have the appropriate means and free space in the speaker system, it can also be recommended for protecting low-frequency emitters.

True, in this case it will be necessary to increase the number of parallel-connected diode branches. So, when 2 identical diode branches are connected in parallel, the power absorbed by the protection system increases by 2 times.

Switch-on delay and speaker protection device

The schematic diagram of this device is shown in Figure 10. It consists of an input low-pass filter R1R2С1, a time relay on transistor VT1 and elements R1 - R4, C1 and a switch on transistor VT2.

At the moment the power is turned on, capacitor C1 begins to charge through resistors R1, R2. During its charging time, transistor VT1 will be open, VT2 will be closed and no current will flow through the relay winding.

Rice. 10. Circuit diagram of the switch-on delay and loudspeaker protection device, assembled on two transistors.

Resistor R3 eliminates the influence of the base current of transistor VT1 on the charging of the capacitor and increases the positive threshold of the protection device.

When the capacitor is charged, the voltage at the base of transistor VT1 will drop and it will close, and the key transistor VT2 associated with it will open and current will flow through the winding of relay K1. The relay will operate, and its closed contacts K1.1 and K1.2 will connect the speakers to the amplifier. The switch-on delay is approximately 4 s.

If a constant voltage of positive polarity appears at one of the amplifier outputs, this will lead to partial discharge of capacitor C1, opening transistor VT1 and closing transistor VT2.

As a result, the current through the relay coil will stop and its contacts will disconnect the speakers from the amplifiers. If a constant voltage of negative polarity appears at the outputs of the latter, then it will go directly through the diode VD1 to the base of the transistor VT2, close it and thus de-energize relay K1, the contacts K1.1, K1.2 of which will open and again disconnect the speakers from the amplifier. Diode VD1, VD2 limit the maximum negative voltage at the base of the input transistor VT1 at 1.3 V.

Although both in the loudspeaker protection mode and in the loudspeaker delay mode, capacitor C1 is charged through the same circuits, the response time of the protection is an order of magnitude shorter, since for this the capacitor must change its potential by only a few volts. The protection thresholds are no more than +-4 V.

A correctly manufactured device starts working immediately and does not require any configuration. Any silicon diodes can be used. It is advisable to use the remaining elements as indicated in the diagram.

Relay K1 - RES-9, passport RS4.524.200 with a winding resistance of approximately 400 Ohms. Any other relay that operates at the selected supply voltage is also suitable, but in this case you need to select resistor R4, on which the negative threshold of protection operation depends.

The device is operational when the supply voltage changes within 20...30 V. With a different supply voltage, you will need to change the resistance of resistor R4.

The disadvantage of this device is that it needs to be powered from a source with ripples of no more than 1 V, otherwise false alarms are possible.

Literature:

1. Voishillo A. - “On methods of switching on the load of low-frequency amplifiers” Radio 1979 No. 11 p. 36, 37;
2. Kornev I. “Protection of loudspeakers” Radio’1960 No. 5 p. 28;
3. Roganov V. “Loudspeaker protection device” Radio’1981 No. 11 p. 44, 45; 1982 No. 4 p. 62;
4. “Loudspeaker protection devices” Radio’1983 No. 2 p. 61;
5. Baraboshkin D. “Power amplifier protection unit” Radio’1983 No. 8 p. 62, 63;
6. Reshetnikov O. “Protection device using optocouplers” Radio’1984 No. 12 p. 53;
7. “Loudspeaker protection devices” Radio’1986 No. 10 p. 56-58.

This acoustics protection project was borrowed from one of the Portuguese sites. In addition to protection against constant voltage, the unit provides a delay in connecting speakers to the output of the power amplifier from approximately 3 to 10 seconds, while eliminating clicks when the amplifier's power is turned on. Schematic diagram:

The circuit uses relays for a voltage of 12 Volts with one group of switching contacts capable of holding a current of 6...8 Amps.

The original article contained the following images of the printed circuit board:

And the type of PCB board format:

Using the image data, we drew the protection board in the Sprint Layout program. LAY6 format looks like this:

Photo view of the printed circuit board for acoustic protection LAY6 format:

Foil fiberglass one-sided. We reduced the size of the board a little, now it is 45 x 75 mm.

A conventional parametric stabilizer is used as the power supply for the circuit, the stabilization voltage is 12 Volts. The diagram is shown below:

We hope it will not be difficult for you to calculate the value of the current-limiting resistor for the zener diode; it is indicated by an arrow in the diagram. Its rating will depend on what voltage you have after the diode bridge. The power supply can also be implemented on LM7812.

The connection of the protection and acoustics unit to the power amplifier is shown in the following image:

List of elements of the acoustic protection block circuit:

Relay 12 Volt - 2 pcs.
Transistors 2SC945 - 2 pcs.
Transistor 2SC9013 – 1 pc.
Diodes 1N4007 – 5 pcs.
Electrolytic capacitors 220 uF/ 50V – 2 pcs.
Resistors 10 kOhm – 4 pcs.
Resistor 1 kOhm – 1 pc.
Resistor 39 kOhm – 1 pc.
2 Pin connectors – optional
Trimmer resistor 220...500 kOhm – 1 pc.
Zener diode 12 Volt 1 Watt – 1 pc. (for example imported 1N4742A)

Acoustic protection unit board assembly:

A link to download an archive with a circuit diagram and a printed circuit board in LAY6 format will appear on the same page after clicking on any line of the advertising block below except for the line “Paid advertising”. File size – 0.3 Mb.

Protection of acoustic systems from constant voltage at the output of an amplifier called “Brig” (copied from an amplifier of the same name produced by Soviet industry) has been familiar to many radio amateurs for many years. Over these many years this scheme has proven itself with the best side saving hundreds and thousands of speaker systems. The circuit is reliable and simple.

The scheme I presented below is one of the variations on the theme of the “Brig” defense. The skeleton of the scheme remains the same. The changes affected only the circuit ratings and transistor models.

Circuit specifications:
Supply voltage: +27 ... +65V
Speaker connection delay time: 2 seconds
DC input sensitivity: +/- 1.5V

A wide limit of supply voltages is ensured by the use of a voltage stabilizer in the power circuit on VD5, VD6, R13 and transistor VT5. It is necessary to install a small heat sink on the VT5 transistor. If you significantly increase the heat sink area and replace transistor VT5 with BD139, you can increase the maximum supply voltage to +120V.

A composite transistor is used as a relay driver, which made it possible to eliminate the need for an additional low-power transistor and save some space on the board. Other composite transistors, for example: BD875 or KT972, can be used as a relay driver transistor (VT3 VT4). Before replacing transistors with similar ones, you should check their pinout because it does not match for all the listed transistors.

Transistors VT1 and VT2 can be replaced with BC546-BC548 or KT3102. We also do not forget about the pinout, as in the previous case.

VD3 and VD4 are necessary to avoid interference when switching relay contacts. VD1 and VD2 are necessary to protect VT1 and VT2, respectively, from breakdown of the BE junction when there is a negative voltage of less than -15V at the input of the circuit.

The circuit also provides a delay in connecting the speaker system (AS) by 1-2 seconds. This is necessary so that when the amplifier is turned on, no popping or other unpleasant sounds are heard from the speakers that accompany transient processes in the amplifier. Capacitors C3 and C4 are responsible for the delay time for connecting the speakers. The larger their capacity, the longer the delay time for connecting the acoustics. With the values ​​indicated in the diagram, the delay time is about 2 seconds.

The relay must be used with a control winding of 24V, 15mA and a current not less than the output current of the amplifier. I used a relay - Tianbo HJR-3FF-S-Z.

Photo of the finished device

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