The article describes the construction of two universal modules, protecting the speakers or headphones DC and when the cod / audio device power is turned off. Schemes are based on the cheap, easily available items. It is important to note, that they do not affect the audio quality. These devices serve as a supplement to the power amplifiers and other devices electroacoustic.
The author has developed and manufactured the device described in two different versions – to protect the outputs to headphones or speakers. Schematic diagrams of the two devices shown in Figure. 1 and rice. 2. Fuektsiya protection is to unplug the headphones or speakers via the relay at the time of inclusion, off or leakage DC. A universal, They work well with a number of power amplifiers sound.
a protection circuit composed of two functional blocks.
Consider first block unit, eliminating bumps (knock on the speakers) when turning the power – this, probably, The most important feature of this type of scheme, provides ease of use of sound equipment. The unit operates by delaying connect and quick disconnect the amplifier output from the speakers or headphones at switching on and off the main power transformer. This function is performed by the elements in the lower part of the scheme: by JP1 connector and B1 of the rectifier bridge to the transistor and electromagnetic relays. One of the secondary windings of the transformer is connected directly to connector JP1. Variable waveform is rectified in the bridge, and capacitor C4 acts as a filter. Its capacity should be chosen experimentally. It must be sufficiently low capacitance, not to cause delays in the quick disconnect, but at the same time, sufficient, the relay could safely be included.
The prototypes, which made the author, it has a capacity 47 mF. This circuit was also included LED power indicator. Place its inclusion was chosen in such a way, that the diode extinguished immediately after the power is turned off. If the LED power source is connected to the main power supply circuit, it shuts down very slowly after switching off the device (when the capacity of the filter capacitors in the tens of mf. it can last forever), and unknown, whether the unit is turned off or broken switch or somewhere there is a short circuit. Attaching to Indicator light protection scheme presented here provides its instantaneous tripping when disconnecting the power transformer at the same time opening the relay contacts.
Resistor R15 is chosen depending on the supply voltage, LED type and the expected brightness diode. Resistor R11 is an additional burden for the discharge of the capacitor C11, accelerating off transducers. Its value can be chosen by experiment, This resistance should be not less than several kOhm. Delay circuit made of the resistor R10 and the capacitor C3. The voltage across the capacitor increases slowly, opening the MOSFET channel. Finally, drain current is large enough to switch the relay contacts. 15-volt zener diode protects the valve from excessive stress (allowed UGS ± 20 AT), which can damage the transistor, if the supply voltage is too high. Silicon diode D5 accelerates the discharge of the capacitor C3 after power off, so that in the event of momentary disconnection of the transformer power supply after a delay reclosing. normally open (NO) relay contacts are connected between the amplifier output and the converter (headphones / loudspeakers). As can be seen from the diagrams, amplifier also has resistors R5 and R6, followed by capacitors C1, C2 (they form a bipolar capacitor 0,5 × C1, because C1 = C2). This low-pass filter. A non-zero voltage at the output of the filter indicates the presence of hazardous DC component, who at the best increases distortion, introduced by the speakers or headphones, and leads to a decrease in the dynamics of, at worst – damage. For two additional filter circuit using bipolar transistors included. When the absolute value of the filter output voltage is sufficiently high (1,1 … 1,3 AT), one of these circuits will be activated and via the diode of one of the optocouplers. Power for them is taken from the main amplifier power supply. LED optocoupler open and close the gate of the MOSFET to the ground, immediately transferring it to the off state and releasing the relay contacts. Resistor R9 limits the current of the condenser C3 and the discharge does not affect Q3.
Resistors R3 and R4 may be used to extend the voltage range of tolerances on the filter output, but they are not usually installed. R14 resistor can be used, when the relay is supplied a higher voltage, close to the upper limit, permission of the manufacturer. This will reduce the voltage drop across the coil. Otherwise we put the jumper. Both variants of the protective circuit are slightly different. Version with loudspeaker includes a large double relay (contacts 250 AT / 8 BUT) and wide track, providing a high current. Option Headphone includes miniature relay, because the current, flowing through headphones, small. The presence of R12 and R13 may be appropriate resistors – they are not included in the version of the speaker.
Headphone manufacturers (at least, the best class) suggest, that their products will work in series with resistors of the order 10 … 100 AT. These resistors must prevent the flow of large current, caused by a short circuit when inserting the plug in the operating device. Often developers headphone amplifiers do not use series resistors. Presented by the author printed circuit board enables their easy installation, if it was not done in the power amplifier. Otherwise, put the jumpers in their place. The author amplifier for headphones I used two metallized, 1% resistor power 0,5 W, resistance 68,1 AT (only the author had only a hand). The device works very well with both headphones with an impedance 32 In a large and, For example, 300 AT. Figure. 3 shows a security module.
It is powered with voltage 15 AT. This prototype described protection circuit, and the printed circuit board is different from the final version, in which the author turned the transistor toward the board, To prevent short circuit with the housing. He designed wider and shorter track, increasing the distance between the connectors, etc..
protection modules should be integrated into the amplifier, so the author has tried to make them as small as possible. That's why most of the components are SMD housing. Printed circuit boards are shown in Figure 4 (speaker protection) and Figure 5 (protection headphones).
Capacitors C1 and C2 still needed in experiments. You can use the usual electrolytic, but tantalum capacitors would be more sensible choice, because in this application they will have a longer service life. This refers to part of the circuit, supplied directly from a transformer winding (JP1 connector). To do this, select the voltage of the relay coil voltage, We remember some of the possibilities of its correction resistor R14. For the supply voltage is also necessary to choose a resistor R10 and a capacitor C3, responsible for the relay switching delay. When a high voltage power transistor used to activate the relay. At the current components and the supply voltage of about 15 AT (effective voltage across winding) delay is about 2 seconds. This must be done experimentally. The author recommends the use of a lot of resistance and smaller capacitors(all within a reasonable). This allows faster discharging capacitance C3 after disconnecting the power source transformer. As mentioned, discharge current flows through the diode D5. In this way, scheme will quickly ready for reintroduction.
It is also necessary to pay attention to the power source connected to a power amplifier. The voltage should not be fed from the same winding, which is fed from the delay circuit. If there is a galvanic isolation between them, after connection protection scheme it will be saved through the use of optocouplers. In the case of high-voltage amplifier may be required to adjust the values of resistors R7 and R8, which limit current optocoupler diode. Also, you must pay attention to the maximum voltage ratings of other components, such as electrolytic capacitors – eventually, protection scheme must operate reliably for many years, and the housing may also have a high temperature, to which some components may be sensitive. The author also recommends the use of proven components, reputable manufacturers (especially relays), to protection work reliably. note, that the total cable (land) Headphones should be routed separately, off board.
Author: Michal Pendzimu