Methods and apparatus for connecting and conditioning audio signals
AssigneeApple Computer, Inc.
InventorLawrence F. Heyl
Life of this patent
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Abstract
Audio signal connection and conditioning circuitry employs a resistive drop across the ground input for the signal source to generate a common mode voltage on both input lines. A differential amplifier subtracts the generated common mode voltage on the ground line from the common mode voltage on the source input line, leaving only the source voltage as an output. The differential amplifier output is generated with respect to a positive, stable voltage reference instead of a floating ground.
Claims13
An apparatus for connecting and conditioning an audio signal from a floating signal source using a differential amplifier having an output and first and second inputs characterized by input and common mode input impedances, the apparatus comprising:
- an audio signal input having first and second input lines for receiving the audio signal from the floating signal source;
- a first input resistive drop, external to the differential amplifier, between the first input line of the audio signal input and a ground reference;
- and
- a second input resistive drop, external to the differential amplifier, between the first and second input lines, the first and second input lines being coupled to the first and second inputs, respectively, of the differential amplifier and the first and second input resistive drops being substantially smaller than the input and common mode input impedances of the differential amplifier.
An apparatus for connecting and conditioning an audio signal from a floating source using a differential amplifier having an output and first and second inputs characterized by input and common mode input impedances, the apparatus comprising:
- a first resistor serially connected between the first terminal of the differential amplifier and a first terminal of a first capacitor to form a first input line of an audio signal input at a second terminal of the first capacitor;
- a second resistor serially connected between the second input of the differential amplifier and a first terminal of a second capacitor to form a second input line of the audio signal input at the second terminal of the second capacitor;
- a first resistive drop, external to the differential amplifier, between the second terminal of the first capacitor and a ground reference;
- a second resistive drop, external to the differential amplifier between the second terminals of the first and second capacitors, the first and second input resistive drops being substantially smaller than the input and common mode input impedances of the differential amplifier;
- a third resistor connected in parallel with a third capacitor coupling the output of the differential amplifier to the second input of the differential amplifier;
- a stable voltage reference;
- and
- a fourth resistor connected in parallel with a fourth capacitor coupling the stable voltage reference to the first input of the differential amplifier.
A method for connecting and conditioning an audio signal using a differential amplifier having an output and first and second inputs characterized by input and common mode input impedances, the apparatus comprising:
- coupling the audio signal across first and second input lines;
- creating a first input resistive drop, external to the differential amplifier and substantially smaller than the input and common mode input impedances of the differential amplifier, between the first input line and a ground reference to transform audio noise to a common mode voltage signal on the input lines;
- creating a second resistive drop, external to the differential amplifier and substantially smaller than the input and common mode input impedances of the differential amplifier, between the first and second input lines to generate an input voltage signal on the second input line from the audio signal applied across the first and second input lines;
- coupling the common mode and input voltage signals on the first and second input lines to the first and second inputs of the differential amplifier;
- and
- subtracting out the common mode voltage signal from the input voltage signal using the differential amplifier, to provide a conditioned audio signal at an output of the differential amplifier.
Dependencies are inferred from the claim text.