Load impedance detector for audio power amplifiers

Claim 2

A load impedance detector for detecting a variation of the impedance of a load in 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, said load being connected to a first node between said first and second resistors, comprising: third and fourth resistors connected in series to the emitter of said first transistor, said third and fourth resistors defining a second node therebetween; and differential means coupled to said first and second nodes to form a bridge circuit when said first, third and fourth resistors and said load, the differential means being operative only when the potential across said load exceeds a predetermined threshold for detecting a voltage difference between said first and second nodes and generating therefrom an output signal in response to said detected voltage difference being higher than a predetermined value.

Claim 3

A load impedance detector is claimed in claim 1 wherein said differential means comprises a differential amplifier having first and second inputs coupled to said first and second nodes respectively, and means for causing said differential amplifier to become operative when the potential across said load exceeds said threshold.

Claim 4

A load impedance detector as claimed in claim 1, wherein said differential means comprises: a pair of third and fourth transistors of like conductivity types having emitters coupled together by an emitter circuit to ground and base electrodes respectively coupled to said first and second nodes, said third and fourth transistors having collectors connected by fifth and sixth resistors to a voltage source; and a seventh resistor by which said second node is connected to the emitter of said second transistor.

Claim 5

A load impedance circuit as claimed in claim 3, wherein said emitter circuit includes a diode and a resistor.

Claim 6

A load impedance detector as claimed in claim 3, wherein said differential means further comprises: a pair of fifth and sixth transistors of like conductivity types but opposite to the conductivity types of said third and fourth transistors, said fifth and sixth transistors having emitters connected by a common resistor to said voltage source and base electrodes coupled, respectively, to said fifth and sixth resistors, and having grounded collector circuits; an eighth resistor for biasing the base of said sixth transistor; and a ninth resistor connected in the collector circuit of said fifth transistor to generate said output signal thereacross.

Claim 7

A load impedance detector as claimed in claim 1, wherein said differential means comprises a differential amplifier having first and second inputs coupled to said first and second nodes, respectively, to generate a first signal corresponding to the voltage difference between said first and second nodes, a comparator for comparing the potential across said load with said threshold to generate a second signal, and a coincidence gate for detecting when said first signal coincides with said second signal to generate said output signal.

Claim 8

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.

Claim 9

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.

Claim 10

A power supply as claimed in claim 7 wherein said means for normally applying an enabling signal comprises means for triggering said first unidirectionally conducting means at intervals.

Claim 11

A power supply as claimed in claim 8, further comprising a voltage sensor for controlling the triggering intervals of said triggering means as a function of said gradually varying voltage.

Claim 12

A power supply as claimed in claim 7, wherein said differential means comprises a differential amplifier having first and second inputs coupled to said first and second nodes, respectively, and means for causing said differential amplifier to become operative when the potential across said impedance exceeds said threshold.

Claim 13

A power supply as claimed in claim 7, wherein said differential means comprises: a pair of third and fourth transistors of like conductivity types having emitters coupled together by an emitter circuit to ground and base electrodes respectively coupled to said first and second nodes, said third and fourth transistors having collectors connected by fifth and sixth resistors to a voltage source; and a seventh resistor by which said second node is connected to the emitter of said second transistor.

Claim 14

A power supply as claimed in claim 11, wherein said emitter circuit includes a diode and a resistor.

Claim 15

A power supply as claimed in claim 9, wherein said differential means further comprises: a pair of fifth and sixth transistors of like conductivity types but opposite to the conductivity types of said third and fourth transistors, said fifth and sixth transistors having emitters connected by a common resistor to said voltage source and base electrodes coupled respectively to said fifth and sixth resistors, and having grounded collector circuits; an eighth resistor for biasing the base of said sixth transistor; and a ninth resistor connected in the collector circuit of said fifth transistor to generate said disabling signal thereacross.

Claim 16

A power supply as claimed in claim 7, wherein said differential means comprises a differential amplifier having first and second inputs coupled to said first and second nodes respectively to generate a first signal corresponding to the voltage difference between said first and second nodes, a comparator for comparing the potential across said impedance with said second predetermined value to generate a second signal and a coincidence gate for detecting when said first signal coincides with said second signal to generate said disabling signal.