AC protection circuit when turned off. Protection of acoustic systems from direct voltage brig. Notes on the diagram

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 to be the best, 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

List of radioelements

Designation	Type	Denomination	Quantity	Note	Shop	My notepad
VT1, VT2	Bipolar transistor	2N5551	2	BC546-BC548 or KT3102		To notepad
VT3, VT4	Bipolar transistor	BDX53	2	BD875 or KT972		To notepad
VT5	Bipolar transistor	BD135	1			To notepad
VD1-VD4	Rectifier diode	1N4148	4			To notepad
VD5	Zener diode	1N4742	1			To notepad
VD6	Zener diode	1N4743A	1			To notepad
C1, C2		47 µF	2			To notepad
C3-C5	Electrolytic capacitor	220 µF	3			To notepad
R1, R5	Resistor	1 kOhm	2			To notepad
R2, R6, R13	Resistor	1.5 kOhm	3			To notepad
R3, R7	Resistor	4.3 kOhm	2			To notepad
R4, R8	Resistor

The universal speaker protection unit is made of small-sized parts and can be built into any amplifier that does not have such protection. The peculiarity of this unit is the use of built-in mains power, reliable electromagnetic relays and LED indication of the appearance of constant voltage at the output of the amplifier. The device provides stable latency and protection even after a momentary loss of mains voltage.

It is known that when power is applied to the amplifier, a loud click (pop) may occur in the speaker system. To eliminate this phenomenon, it is necessary to connect the load to the output of the UMZCH with a certain delay sufficient to complete all transient processes (usually 1...3 s). When the power is turned off, the speaker should turn off until the storage capacitors of the amplifier power filter are noticeably discharged (by more than 20%). Otherwise, the shutdown process may also create unpleasant sounds or clicks.

The presented module implements the functions of silently turning on and off the amplifier (actually the speaker), and also allows you to protect the speaker's LF heads when a constant voltage appears at the output of the UMZCH associated with its emergency operation or failure.

Specifications

Supply voltage, V........190...264

Protection response voltage, V................0.6...0.7

Turn-on/restart delay time, s...........2.5...3

Protection response time (U input = 2 V), s, no more than 1.4

Protection response time (U input = 20 V), s, no more than 0.25

Module switch-off time, s, no more...................0.25

Power consumption, W, no more...................2.5

Maximum switching current, A...................12

There are no questions with the implementation of delay and protection of the speakers. But how to implement a quick shutdown of the speaker in the event of a loss of (relatively short-term) mains voltage, but sufficient for a transient process and a click to occur? There are two reasonable options: using information about the presence of alternating voltage in one of the existing secondary windings of the transformer feeding the UMZCH (as implemented in the μRS1237 microcircuit), or using a separate power transformer (or from an additional winding of the UMZCH transformer) for the protection unit. The first option imposes certain restrictions, narrowing the versatility of the module. The second allows you to use a smoothing capacitor of small capacity in powering the device, thanks to which the protection unit is guaranteed to turn off the speaker faster than the capacitors in the UMZCH power supply are discharged.

Obviously, the second option is more reliable and easier to implement, allowing you to connect the module to almost any amplifier. The disadvantage of this solution is the higher cost due to the use of an additional power supply, but versatility and reliability prevail here.

The device diagram is shown in Fig. 1. Its inputs must be connected to the outputs of the stereo UMZCH channels, and the outputs - to the loads (AC) of the corresponding channels. The common wire of the module, speaker speakers (or crossover) is connected directly to the common wire of the amplifier.

Rice. 1. Device diagram

When supply voltage is applied, capacitor C6 slowly charges through resistor R10 to 1.9 V (determined by the ratio of the resistance of resistors R10 and R11), which is enough to turn on transistor VT4. Relays K1, K2 are activated, and the load is connected to the amplifier.

When a constant voltage of more than ±0.6...0.7 V occurs at any of the device inputs (contacts X2a, X3), the corresponding transistor opens (VT1 - for voltage of positive polarity, VT2 - of negative polarity), including the emitting diode of optocoupler U1 or U2. The illuminated phototransistor of the optocoupler through resistor R8 discharges capacitor C6, and field-effect transistor VT4 closes, de-energizing the relay. The glow of the HL1 LED indicates that the speaker is turned off and the UMZCH is malfunctioning. Resistor R8 limits the discharge current of capacitor C6, and resistor divider R4R5 provides an artificial midpoint of the supply voltage.

Most of these protection and delay devices for turning on the speakers have an unpleasant drawback - the absence of a delay when restarting in a short period of time after the power is turned off. An example of such a situation is a short-term loss of electricity in the network. This drawback does not allow obtaining the proper level of protection for the speakers and all equipment in general where such a unit is used. To eliminate this drawback, elements R9, C5, VT3 were introduced. This circuit is briefly activated when the supply voltage disappears and reappears, discharging capacitor C6, which ensures the normal subsequent start of the protection unit. The use of field-effect transistor VT4 with a reduced opening voltage (approximately 1.5 V) provides a lower charge voltage for C6, and the restart time is almost equal to the time of the first turn on. While maintaining constant charging and discharging times of capacitor C6, its capacity can be significantly reduced by correspondingly increasing the resistance of resistors R8-R11. It is not recommended to increase the capacitance of capacitor C1 - it determines the shutdown speed of the protection unit.

At a rated mains voltage of 230 V and a room temperature of 25 o C, the DA1 stabilizer heats up to 50...52 o C. When tested at a maximum alternating voltage of 274 V (limited by the capabilities of LATR), the heating of the stabilizer was 64...65 o C - all in within normal limits. If we exclude resistor R1, then the lower permissible limit of the unit’s power supply will drop to 170 V, but at the same time the heating of DA1 will increase by an average of 10...12 o C. It is clear that this change is only advisable for areas where the network voltage is always below the nominal .

If we imagine a situation where both channels of the UMZCH fail, and in the first channel a voltage of one polarity is formed at the output, and in the second - of the opposite polarity, equal in magnitude to the voltage at the output of the first channel (with a difference of less than 0.6...0 .7 V), then after summing through resistors R2 and R3, the result is a voltage that is not enough to open transistor VT1 or VT2. That is, the protection system will not work, and this is a disadvantage (it can be overcome by changing the resistance of one of these resistors by ±10%). But the probability of such an event is negligible and is rather an example of a hypothetical failure simulation.

The printed circuit board (Fig. 2), having dimensions of 66x45 mm, is made on foil fiberglass and is designed for installation of transistors in SOT-23 packages, resistors of size 0805 (except for resistors R1 and R13 - 1206), capacitors C2, C5 of size 0805 and diode VD2 in SMA package. In the photo fig. Figure 3 shows the mounted board from the soldering side of the surface mount parts.

Rice. 2. Printed circuit board

Rice. 3. Mounted board from solder side of surface mount parts

A low-power transformer TPK-2 with a 12 V secondary winding is used as T1. The diode bridge can be any of the DB103S-DB107S or MB2S-MB6S series, for which two seats are provided on the printed circuit board. Diode VD2 - any with a forward current of 1 A and a reverse permissible voltage of at least 200 V.

The relay windings should have a current consumption of no more than 30 mA (high sensitivity) at a voltage of 12 V. It would be possible to use one relay with two pairs of contacts, but the author was unable to find one for a switching current of more than 8...10 A. Advantages of these in the TRU-12VDC-SB-CL relay diagram is that they have AgCdO (silver-cadmium oxide) coating on the contacts, resistant to mechanical wear, and a maximum switching current of 12 A. They can be replaced with more affordable SRD (T73) 12VDC relays -L-S-C from SONGLE, allowing switching current up to 10 A.

Almost any optocouplers U1, U2 can be used with the appropriate structure, for example, PS2501, PC817. LED HL1 - any, preferably red, for example, from the AL307 series or others.

Transistors VT1-VT3 can be replaced by any other low-power transistors of the appropriate structure and size. It is possible to use MMBT5551, MMBT4401 (VT1, VT3) and MMBT5401, MMBT4403 (VT2).

As a replacement for the n-channel field-effect transistor (FET) VT4 with a low Gate Threshold Voltage, we can recommend NTR4003N, IRLML2502. If such replacements are not available, then it is permissible to use another n-channel FET with an insulated gate, focusing on an open channel resistance of no more than 3...5 Ohms, a maximum drain-source voltage of at least 20 V and a maximum drain current of at least 300 mA . In this case, the following changes will need to be made to the circuit: R8 = 75 Ohm, R10 = R11 = 68 kOhm, C6 = 47 µF at 16 V. But remember that the delay time for a quick restart will decrease slightly. Since the threshold switching level for different PTs can differ significantly, it may be necessary to adjust the relay switching delay time by selecting a pair of resistors R10, R11 from the condition of their equality.

Fuse link FU1 can be used for a current of 0.16 or 0.25 A, for example, the domestic VP4-10 0.2 A, which has small dimensions and flexible leads for mounting on a board. Terminal blocks X1-X3 - DG127, XY304 series or similar. As can be seen from the diagram, the central contact in X1 is not used. This is done in order to increase the gap between the mains power conductors.

The assembled device (its photo in Fig. 4) does not require adjustment and works immediately after power is applied. Its design has been repeated many times, and its high reliability is confirmed by long-term operation.

Rice. 4. Assembled device

In Fig. Figure 5 shows a diagram that allows eliminating a small-sized transformer. As an example, a simplified circuit of a UMZCH power supply with a voltage of +/-30 V is shown. At the same time, both the circuit and the method of connecting the module to the amplifier are slightly changed.

Rice. 5. A circuit that eliminates a small-sized transformer

The module has bipolar power supply through damping resistors R8, R9, so the formation of an artificial midpoint is not required (resistors R4, R5 in Fig. 2). For greater efficiency, the relays are connected in series and a capacitor (C4) is added as a power filter.

Components VD1, R5, C3 contain a half-wave rectifier, the voltage from which is supplied to optocoupler U3. In the initial state, due to resistor R10, transistor VT3 is in saturation mode, shunting capacitor C5 until voltage appears on the emitting diode of optocoupler U3, after which VT3 closes and C5 begins to slowly charge, opening transistor VT4. In this case, the total delay time for connecting the load reaches 2...2.5 s.

When the amplifier is turned off, capacitor C3 quickly discharges, de-energizing optocoupler U3. Transistor VT3 opens and discharges capacitor C5, as a result of which the relays with the load are turned off. Thus, a quick shutdown mechanism is implemented with a total time of no more than 0.3...0.5 s.

The subsequent start of switching occurs with a discharged capacitor C5, therefore, in contrast to the circuit in Fig. 2, its forced discharge is not required.

As VT4, you can use an n-channel PT with a threshold opening voltage of 2...5 V and a maximum drain current of at least 1 A, for example, IRF510-IRF540, IRF610-IRF640. Rectifier diode VD1 - any with a reverse voltage of at least 100 V and a forward current of 100 mA: SF12-SF16, 1 N4002-1N4007, etc. When using relays with windings that consume a current of 50 mA, it is necessary to change the values ​​of resistors R8, R9 to 330 Ohm.

Note: To increase the reliability of operation, a resistor with a resistance of 50...100 kOhm must be installed between the base and emitter of transistor VT3 (Fig. 1).

Literature

1. Ataev D.I., Bolotnikov V.A. Functional units of high-quality sound reproduction amplifiers. - M.: Radio and communication, 1989, p. 120.

2.UPC1237. Protector IC for stereo power amplifier. - URL: http://www.unisonic.com. tw/datasheet/UPCI 237.pdf (03/21/16).

Publication date: 10.07.2016

Readers' opinions

• Rymkin / 02/05/2019 - 03:06
  Hello! Is it possible to use a 15 volt transformer? There is a typo in the article, “You can replace them with more affordable relays SRD (T73) 12VDC-L-S-C from SONGLE, allowing a switching current of up to 10 A.”, in fact, the relay brand is SRD (T73) 12VDC-SL-C.

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 easily end up directly on the speakers of the 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

One of the options for such protection is shown in the diagram above. The protection works as follows: the signal from the amplifier output is supplied to the IN input, and the speakers are connected to the OUT output. The negative of the amplifier is connected to the negative of the protection circuit and goes directly to the speakers. In the normal state, when the amplifier is working and power is supplied to the protection board, relay Rel 1 closes the input of the board to the output and the signal goes directly from the amplifier to the speakers. But as soon as a constant voltage of at least 2-3 volts appears at the input, the protection is triggered, the relay is turned off, thereby disconnecting the amplifier from the speakers. The circuit is not critical to resistor values ​​and allows for variation. Transistor T1 can be used 2N5551, 2N5833, BC547, KT3102 or any other low-power npn transistor. T2 must be composite with a high gain, for example, BDX53 or KT829G. The LED in the diagram serves to indicate the relay status. When it is on, the relay is on, the signal goes directly from the amplifier to the speakers. In addition to protection against DC voltage, the circuit provides a delay in connecting the speaker system. After applying the supply voltage, the relay does not turn on immediately, but after 2-3 seconds, this is necessary in order to avoid clicks in the speakers when the amplifier is turned on. The supply voltage of the circuit is 12 volts. Any relay can be used with a winding supply voltage of 12 volts and a maximum current through the contacts of at least 10 amperes. The S1 latching button is located on the wires; it is needed to force the relay to turn off, just in case. If this is not required, you can simply short-circuit the tracks on the PCB.

(downloads: 492)

Assembling the device

Amplifiers are most often designed for two channels, left and right, so the protection circuit must be repeated twice for each channel. For convenience, the board is laid out so that it already provides for the assembly of two identical circuits at once. The printed circuit board is manufactured using the LUT method, its dimensions are 100 x 35 mm.

After drilling the holes, it is advisable to tin the paths. Now you can start soldering the parts. Particular attention should be paid to the pinout of the transistors; it is very important not to confuse it and solder the transistors on the right side. As usual, small parts are soldered first - resistors, diodes, capacitors, and only then transistors, terminal blocks, and, last but not least, massive relays. To connect all wires, you can use terminal blocks, the places for which are provided on the board. After soldering is completed, you need to wash off the remaining flux from the tracks and check the correct installation.

Protection tests

Now that the board is completely ready, we can begin testing. We supply power to the circuit (12 volts), after two seconds the relay should simultaneously click and the LEDs should turn on. Now we take some kind of constant voltage source, for example, a battery, and connect it between the minus of the circuit and the input. The relay should turn off immediately. We remove the battery and the relay turns on again. You can connect a battery by changing its polarity; the circuit operates regardless of what polarity the voltage appears at its input. We perform the same manipulations with the second circuit located on the same board. The protection threshold is approximately 2 volts. Now that the protection board has been tested, you can connect it to the amplifier and not be afraid that the speakers in expensive speakers will deteriorate due to the amplifier breaking down. Happy assembly.

Device for protection against failure of speakers of acoustic systems

Often, when we turn on the amplifier, we hear an unpleasant “pop” in the speakers of our acoustics. If the volume control was close to maximum volume, then we risk burning out the speakers in our speakers. In order to protect the speakers and your own ears from the “pop” of transients at the moment of switching on, it is necessary to either make specific decisions in the circuit design of the amplifier’s output stage itself, or simply ensure that the speakers are connected to the amplifier’s output with a short delay, sufficient for the amplifier to start silently. ..

The proposed device provides a time delay at the moment the amplifier is turned on (the delay time is adjustable from 1 to 6 seconds) and provides protection for expensive speakers in the event of failure - breakdown of output stage transistors or specialized microcircuits - audio amplifiers. In the event of a breakdown in the output stage, the speaker systems will be instantly turned off and remain unharmed.

This protection device can be used in conjunction with any stereo power amplifier with output stage supply voltages up to ±50V. The device itself is powered from a unipolar 12V power supply. The protective device is assembled on a board measuring 70x45 mm.

The connection of wires from the amplifier, to the speaker connectors and to the power source is carried out using screw terminals installed on the board. The maximum current switched by the relay is 10A. Upon request, it is possible to manufacture protection devices for currents up to 30A. This device can be retrofitted to any existing amplifier or used in a “new building”.

Cost of assembled and tested device: 160 UAH.

Assembly kit cost: 120 UAH.

Cost of printed circuit board with mask and markings: 55 UAH.

There are many options for protecting speakers from constant voltage, clicks when turned on and off. The most advanced of them are assembled on microcontrollers, control a large number of channels, and have additional functions, for example, a datagor kit

Devices based on specialized microcircuits are also convenient, functional and small-sized. Unfortunately, they are not always available and may take a long time to arrive by mail.

I became interested in which circuit of discrete elements is simple, cheap, functional and requires minimal configuration. I bring to your attention the scheme that best meets these requirements, in my opinion.
Since the article is intended mainly for beginner radio amateurs, I will try to describe in detail even simple things.

AC protection prototype - A. Kotov’s scheme

At first glance, there is a wide selection of circuits, but upon closer examination it turns out that they have disadvantages - many parts, scarce parts, low sensitivity, the need for adjustment, operation in a narrow range of supply voltages, etc.

It turned out to be the most suitable.

However, this scheme is not without its drawbacks:
- there is no quick turn off of the speakers when the amplifier is turned off,
- strictly defined supply voltage,
- all current consumed flows through the LED,
- operating mode with “torn-off base” VT10.
In addition, there is no voltage diagram or tuning recommendations, nor is there a drawing of the printed circuit board.

Improved speaker protection device circuit

These shortcomings can be easily eliminated; here is the version I modified.

The numbering of parts of A. Kotov’s scheme has been preserved and continued.
I would like to note the advantages and features of the scheme:
- the turn-on delay is optimal 4 seconds, determined by the R5C3 chain,
- circuit D5R8R9C4, when disconnected from the network, allows you to quickly de-energize the relay and turn off the speaker,
- after the protection is triggered (the relay is turned off), capacitor C3 discharges quickly and charges through resistor R5 slowly, so there will be no fast chaotic switchings,
- the device operates in a wide range of voltages, from relay response voltage (plus 2 V) to 36 V (limit for TL431),
- practically the only resistor that requires selection - R7 serves to absorb the excess voltage for the relay, the values ​​of the remaining resistors can differ several times and do not require replacement in a wide range of supply voltages,
- all elements, except TL431, operate at very low currents, which ensures high reliability,
- the use of TL431 ensures the key operating mode of the relay,
- the voltages on the capacitors except C4 are very small, no more than 2.5 V, which allows the use of capacitors for low voltages, so I tested the option with single polar capacitors C1 and C2 for low voltage,
- any LED is suitable (preferably bright) since the current through it is set by a resistor,
- the sensitivity is very high (about 1 V), it is better to coarse it; for this purpose, the board has pads for SMD resistors (in gray in the diagram).

Own power supply

If you power the ultrasound from the main power supply of the amplifier (like A. Kotov), ​​when you turn off the network, the relay will not release immediately due to the large capacities of the power supply and a click, crackle, etc. is possible. Here, due to the very small capacity, C4 = 1 -4.7 µF the relay releases immediately.

You can take a variable from the transformer of the main ULF power supply, then you may have to change the divider R8R9 to reduce the voltage.

For the “versatility” of this circuit, you need a power supply with a low-power transformer with a low voltage of the secondary winding. I used a ~230/12 V transformer with a power of 2 VA. The power supply is made on a board of the same width as the protection unit; it is convenient to place them on one board.

The presence of a separate power supply allows you to use the protection unit with any amplifier, including a prototype one, which is especially convenient since the speakers are exposed to increased danger in this case.

Applied parts and setup

A 12VDC relay “OMRON G2R-2” is installed in a transparent case. This was not done by chance - although it has larger dimensions than similar ones in a non-separable opaque case, it can be opened and the contacts can be cleaned. I recommend that when using a non-separable relay, carefully cut its body in advance so that the cover can be removed and replaced. I especially recommend it in the case of a used relay.

Sealed relays are typically smaller in size and can be easily installed with minimal modification to the circuit board. Since I placed the relays and clamps with screw terminals quite tightly, when repeating the board, you need to make sure that the sizes of the clamps are identical, otherwise slightly adjust the printed circuit board. You can do without clamps; it’s even more reliable, but it’s inconvenient, especially when setting up amplifier layouts.

If there are no installation errors and serviceable parts, the circuit starts working immediately, you just need to calculate the current limiting resistor through the relay winding.
For example, +18 V power supply, 12 V relay with a resistance of 280 Ohms. Relay operating current 12 V/280 Ohm = 43 mA.
It is necessary to extinguish 18V - 12V - 2V (voltage drop across an open TL431) = 4 Volts.
4 V / 43 mA = 100 ohms. The resistor power is 43 mA x 4 V = 170 mW, i.e. you need a resistor of 0.25 W and higher. This resistor is “standing” on the board; this is done so that you can install resistors of different sizes and with a power reserve of up to 2 W.

All diodes, except for the relay shunting the winding, are almost any low-power ones, you just need to remember that the stripe marking on the body of the KD522 and other Soviet diodes is the opposite of the imported marking.

If there are problems with operation, first of all you need to check the correct installation of parts, especially diodes, transistors and TL431. Then check the quality of the soldering (my diode leads were poorly soldered), to do this you need to rinse the board thoroughly and inspect the soldering with a magnifying glass (or a good eye).
Then check the DC modes; the voltages at the bases of the transistors must correspond to those indicated in the diagram ± 0.1 V.

Since among novice amateurs there is a passion for gigantomania and amplifiers with a power of hundreds of watts and with a supply voltage of the amplifiers of the order of ± 50 V, we must remember that the greater the power of the amplifier, the greater the currents flow through the relay contacts; at high voltages, the likelihood of an arc between open circuits increases. relay contacts.

In this case, any relay with one group of contacts can be installed on this board; this relay will be intermediate and control another, more powerful relay with contacts designed for a higher current and with an increased distance between open contacts. It will be possible to connect wires of a larger cross-section to this powerful relay.

The versatility of this protection unit with its own power supply lies in the fact that it can be connected to the outputs of a bridge (usually high-power) amplifier. The common wire is connected not to the common wire of the amplifier, but to one output of the amplifier, and one input of the protection unit to the second output of the bridge amplifier.

When installing a protection unit in a finished structure, there is no need for a separate power supply (for a regular, non-bridge amplifier).

Total

I made two copies - with regular resistors and SMD, the board allows you to do this. The impressions from the devices are very good. The length of the board can be reduced by 1...2 cm, especially with SMD resistors, but I prefer wide traces that allow parts to be soldered repeatedly and are forgiving of displacement when drilling holes; sufficient gaps between tracks.

We must not forget that such a device protects only the LF heads from constant voltages and all heads from transient processes in the amplifier, including when the amplifiers fail, and does not protect the HF heads during overloads and excitation of the amplifiers. At the same time, this circuit solution allows you to connect overheating, limitation (clipping), and excitation sensors for the safety of all speaker heads.

In addition (which is used in a number of amplifiers), you can control the connection to the amplifier output of one or more pairs of speakers using a switch on the front panel of the amplifier, without the need to pass high-current signal circuits through this switch.