Non-contact loudspeaker protection. Scheme, description

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Non-contact loudspeaker protection. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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Most modern audio frequency power amplifiers (UMZCH) are built without coupling capacitors at the output. If the amplifier malfunctions, the appearance of a constant voltage at the output of the UMZCH can damage the expensive speakers of the speaker system. To protect them from direct voltage of any polarity, I propose a simple device (Fig. 1).

As a prototype, the protection device circuit of the amplifier "405" f. "Quad" was taken, but measures were taken to prevent spontaneous opening of the triac at a high slew rate of the output voltage, which happens in modern UMZCH. Additionally, a light indication of a fuse blown on a flashing LED when the protection is triggered has been introduced.

Non-contact loudspeaker protection. Loudspeaker protector circuit diagram
Fig.1. Loudspeaker protector circuit diagram

When a constant voltage of any polarity of more than 3 ... 4 V appears at the output of the UMZCH, the voltage at the terminals of capacitors C2, C3 increases sharply. The current flowing through the resistor R4, one of the diodes VD5, VD6 and one of the transistors VT1, VT2 opens the triac VS1. An open triac shunts the UMZCH output until the FU1 fuse blows. When it burns out, the HL1 LED starts flashing. Elements C1, L1 are designed to prevent unauthorized opening of the triac due to interference.

The fuse is selected based on the maximum output power of the amplifier and the impedance of the speaker system.

The device can use resistors of types C1-4, C2-23, MLT and others of appropriate power. Capacitor C1 - ceramic, types K10-7, K10-17, KM-5. Oxide capacitors C2, C3 - type K50-16, K50-35. Both of these capacitors can be replaced with one non-polar, while the diodes VD7, VD8 are excluded from the circuit. Diodes KD521A can be replaced with KD102 (A, B), KD103 (A, B), KD518A, 1N4148. The HL1 LED can be both blinking and permanently lit, for example, AL307, KIPD35, KIPD40.

Transistor VT1 can be replaced with KT503E, KT602, KT683, MPSA-43, 2N5550. VT2 is replaced by KT502E or any pn-p transistor from the KT6116, KT668, 2SA709, 2SA910, MPSA-93 series. As a triac VS1, KU208 with indices G, D or TS112-10, TS112-16 and others for an operating voltage of at least 100 V is suitable.

Choke L1 is wound with PEV-2 wire 00,68 mm - 75 turns on a thick paper frame with an outer diameter of 10 mm.

To check the assembled assembly, the fuse is temporarily replaced by an incandescent lamp with a voltage of 6 ... 12 V and a current of 0,16 ... 1 A. The assembly is connected to the output of a laboratory power supply with an adjustable output voltage of 2 ... voltage, according to the moment of ignition of the lamp, the protection threshold is determined. If it is no more than 10 V, then the node is suitable for use with automotive UMZCH. With the capacitance of capacitors C4, C2 indicated on the diagram, the delay in the operation of the protection will be about 3 s. If necessary, the delay time can be reduced to 1 ... 0,2 s by taking these smaller capacitors.

Structurally, this block can be placed both inside the UMZCH and in the AU. If the unit is installed inside the acoustic system, if the speaker contains a filler made of combustible material, such as cotton wool, it should not come into contact with the parts of the protection unit.

For a multi-channel amplifier, the corresponding number of blocks is assembled.

Literature

1. D. Ataev, V. Bolotnikov. Functional units of high-quality sound reproduction amplifiers. - Radio and communications, 1989, p.111.

Author: A. Butov, village of Kurba, Yaroslavl region; Publication: radioradar.net

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