- The stated component values result in Pmax“30 W in 8Q. When the input signals are all right, the open collector out- puts of A9‘A12 are in their high impedance state, so that the out- put voltage is +15V via Rm. When a fault condition exists at one or more of the inputs, junction R41;-R41 is pulled down to -15 V The central part in the relay driver is bistable PF1.
- When the loudspeaker can not handle that power, the voice coil is rapidly overheated, and causes a short-circuit. If the power output stage is not properly protected, it breaks down and supplies a direct current that effectively destroys the loudspeaker. The present loudspeaker protector is composed of three sections: a measuring amplifier, a detector, and a relay driver.
- The loudspeaker protector is conveniently fed from the amplifier’s symmetrical supply, but care should be taken to dimension D4; and Rv such that the indicated voltage across C44 and C45 is not exceeded. If the amplifier supply delivers less than 28 V ICG may be omitted, and the loudspeaker relay, Re, replaced with a 12 V type fed from the +15 V rail. Voltage divider Ras-R44 ShO11ld Ih€I\ be redimensioned such that the input of N4 is held at about +l3 V when R43+R44~’l00 kQ.
- Four channels are shown here as an example. Potential divider R1-R2 determines the sensitivity of the protection circuit, while D1-D2 protect the input of At. Opamp As is set up as a low pass filter with a cut—off frequency of 0.5 Hz, so that it can function as a DC detector.
- Gate N1 is a resettable power-up delay circuit which clocks PF1. The logic high level at the D (data) input is only transferred to out- put Q when the R (reset) input is logic high. It is seen that a reset pulse can originate either from the mains detector N3-N4, or from the fault detectors As—A1z.
- The second section of the circuit is composed of four detectors A9-A12. Ag compares any negative direct voltages to a ref- erence set with R¤—R9, while Ca —Rv determine the delay time. Opamp A10 has a similar function for positive direct voltages. The circuit is actuated when Vi¤Rz _O`65 >_ 15Rz RH-R2 Ra+R9 Comparators A11 and A12 function as the power limiter. Positive and negative peak voltages are rectified in Da-D4 and averaged with the aid of R- C combinations Ras-C3; and Rza-C2;.
- Many modern AF power output stages are capable of delivering considerable power levels in the supersonic frequency range.
- The relatively long periods of these networks precludes erroneous triggering of the circuit on peaks in the input signal.
- The power limiter is actuated when V1nR2\/Z _O-65 > l5Rza R1+Rz Rza+Rz9 This equation is also valid for the positive detector set up around A12
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Tuesday, April 3, 2012
Loudspeaker Protection Circuit Explained
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