Loudspeaker Protection

EE JULY/AUGUST 1987 Many modern AF power output stages are capable of deliver- ing considerable power levels in the supersonic frequency range. 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 prop- erly protected, it breaks down and supplies a direct current that effectively destroys the loudspeaker. The present loudspeaker pro- tector is composed of three sections: a measuring amplifier, a detector, and a relay driver. Four channels are shown here as an example. Potential divider 111-R2 determines the sensitivity of the protection circuit, while Di-D2 protect the input of Al. Opamp A5 is set up as a low pass filter with a cut-off fre- quency of 0.5 Hz, so that it can function as a DC detector. The second section of the circuit is composed of four detectors A9 - Al2. A9 compares any negative direct voltages to a ref- erence set with R8-R9, while C3 - R7 determine the delay time. Opam...