GB1520156A - Loudspeakers - Google Patents

Abstract

1520156 Loudspeaker circuits BRITISH BROADCASTING CORP 2 March 1977 [2 March 1976] 08340/76 Heading H4J In a loudspeaking arrangement comprising a loudspeaker coil 12a (Fig. 2) coupled to a signal input 18, the magnitude of the resistance of the coil 12a is used to modify the signal applied to the coil 12a or to another loudspeaker coil 14a. Damage caused to the loudspeaker 12 by overheating of the coil 12a may thereby be eliminated. The output of an input amplifier is applied to a crossover comprising low pass filter 22 and high pass filter 24. The filter outputs are applied to a preamplifier 26, 32, a power amplifier 28, 34 and an A.C. blocking capacitor 30, 36. A D.C. voltage source 40 and resistor 42 constitutes a constant current source to coil 12a and thus the corresponding voltage drop depends upon the resistance of coil 12a. A second low pass filter 46 has a cut off frequency of about 1 Hz and selects only D.C. voltage caused by the current from source 40 through coil 12a. After amplification this D.C. voltage is applied to a voltage comparator 50 where it is compared with a predetermined voltage defined by a Zener diode. After further amplification the signal is applied to a control input of the voltage controlled attenuator 30. Amplifiers 48 and 52 are adjusted so that the attenuation of the signal to the H.F. unit by attenuator 38 matches the attenuation of the output of the L.F. unit due to increase in the coil temperature. The constant current source comprising battery 40 and resistor 42 may be replaced by a constant voltage source. The control circuit output may be used to increase amplification in the L.F. signal path rather than decrease the amplification in the H.F. path and further control circuit may attenuate the L.F. signal for changes in the temperature of the H.F. coil. Preferably an additional resistor 62 between the D.C. voltage source 40 and the H.F. coil 14a is used to sense the resistance of the H.F. coil 14a. The voltage across coil 14a is applied to a low pass filter 64 (cut off frequency 5 Hz) through an amplifier 66 to a voltage comparator 68. This is set at a higher voltage than comparator 50 and senses when the applied voltage indicates that the temperature of coil 14a is dangerously high. An amplified signal is then applied to attenuator 38 to provide drastic reduction in or cut-off of the signal to coil 14a. Similar protection is provided for L.F. coil 12a by a voltage comparator 72 and amplifier 74 the output signal of which is applied to a voltage controlled amplifier 76. Comparator 50 responds gradually. Comparator 72 responds dramatically and may be used without or separately from the control function provided by comparator 50.