Load impedance detector for audio power amplifiers

A power supply for a single-ended push-pull amplifier having a pair of first and second transistors of opposite conductivity types, said transistors having emitters interconnected by a pair of series-connected first and second resistors, and a loudspeaker having an impedance connected to a first node between said first and second resistors, comprising:

• a transformer having a primary winding connectable in use to a voltage source and a secondary winding having a first tap for generating a high voltage and a second tap for generating a low voltage;
• a smoothing capacitor for supplying a gradually varying voltage to the collectors of said first and second transistors;
• first, gate-triggered unidirectionally conducting means connected from said first tap to said smoothing capacitor;
• second, unidirectionally conducting means connected from said second tap to said smoothing capacitor;
• and
• an impedance detector for generating a disabling signal when said impedance is different from a preset value, the load impedance detector comprising:
• third and fourth resistors connected in series between the emitter of said first transistor and ground to define a second node therebetween;
• differential means coupled to said first and second nodes to form a bridge circuit with said first, third and fourth resistors and said impedance so that the differential means becomes operative when said impedance develops a potential higher than a predetermined threshold, the differential means detecting a voltage difference between said first and second nodes and generating therefrom an output signal as said disabling signal in response to said detected voltage difference being higher than a predetermined value;
• and
• means for normally applying an enabling signal to the gate of said first unidirectionally conducting means and inhibiting said enabling signal in response to said disabling signal.