loudspeaker protector
This circuit automatically disconnectsa mid-range loudspeakeror a tweeter from theoutput of an amplifier when toomuch power is applied. A relaywith a normally closed (n/c)contact is used as the switchingdevice.
The relay coil voltage is derivedfrom the loudspeaker signalwith the aid of bridge rectifier,B1. A current source, T2, isused to ensure a reasonablyconstant coil current. To ensurethat the maximum coil currentdoes not depend too much onlevel of the loudspeaker signal,the voltage across R2 is limitedto about 1.2 V by an LED, D2.The maximum energizing currentis then about 66 mA. The6-V Siemens relay used herehas a coil resistance of about80 Ω. To keep the extra load onthe amplifier to a minimum, therelay receives a coil voltagewhich is a little below the nominalvalue. Remember, the circuitdraws current even whenthe relay is not energized! Fortunately,the current consumptionis negligible at signal levelsup to 5 Vpeak.
The saturation currentwhich flows through the J-FETcurrent source, T1, is about4 mA, enabling a low-currentLED to act as a visual ‘overload’indicator. Turn the volumedown when the LEDlights! A zener diode is connectedin series with the currentsource and the LED to definethe threshold at which the relaytoggles. The zener value shownhere results in an actuationlevel of about 7.5 V, or about8.5 W into 8 Ω. The loudspeakeris connected again at apower level of about 3.5W.