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Tuesday, April 3, 2012

Loudspeaker Protection Circuit Explained

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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;.
  7. Many modern AF power output stages are capable of delivering considerable power levels in the supersonic frequency range.
  8. The relatively long periods of these networks precludes erroneous triggering of the circuit on peaks in the input signal.
  9. 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
How to make loudspeaker protection circuit:

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