Push-pull audio amplifier system with muting

Claim 1

In an audio amplifying system including a pair of amplifiers connected in push-pull for driving a loud speaker coil, the combination therewith for amplifying and muting the speaker comprising: a pair of current-differencing operational amplifiers characterized by inverting and non-inverting current responsive inputs to provide an output voltage proportional to the difference in the input currents, each of said operational amplifiers having a feedback circuit including a resistor coupled between the output and inverting input, means for coupling a first of said operational amplifiers to a first of said amplifiers connected in push-pull and for coupling the second of said operational amplifiers to the second of said amplifiers connected in push-pull, means for biasing said operational amplifiers to provide in the quiescent state a DC output voltage therefrom of a value sufficient to equally forward bias the first and second amplifiers connected in push-pull and to provide with the value of said feedback resistors substantially equal currents to the inverting and non-inverting inputs of said operational amplifiers whereby said operational amplifiers are biased into the linear region, phase splitter means having one output coupled to the inverting input of said first operational amplifier and having a second output terminal coupled to the inverting input of said second operational amplifier and being responsive to audio signals at the input for providing first power divided audio signals of a first phase and magnitude to the inverting input of said first operational amplifier and for providing second power divided audio signal of equal magnitude and 180° out-of-phase with said first power divided signal to the inverting input of said second operational amplifier whereby the first and second power divided audio signals are amplified and applied separately to said first and second amplifiers connected in push-pull, and muting means responsive to a squelching signal for simultaneously applying sufficient DC current to the inverting input terminals of both current differencing operational amplifiers to decrease the DC output level therefrom to a muting level below the forward bias level of said first and second amplifiers connected in push-pull to simultaneously bias said first and second amplifiers off and mute the speaker.