Optical isolation for audio amplifiers
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Audio amplifier power supplies have typically utilized power transformers to provide safety isolation from a wall outlet. These transformers provide ohmic disconnect between the power supply and the AC mains. The power transformer and its inherent weight and expense can be eliminated by implementing an off line rectified system for high voltage portions of the amplifier. An optical isolator circuit has been designed to allow audio signals to pass while providing the isolation necessary to protect users from a wall outlet power source short circuit. The optical isolator used to pass the input signal in this design is a Fairchild 6N136 high speed opto-isolator. An identical opto-isolator has been employed to provide negative feedback for temperature stability and to maintain the operating point necessary to pass an audio signal. The circuit passes an input signal from the now optically isolated preamplifier stage to the power amplification stage with minimum distortion. The designed isolator does not exceed 1.0% total harmonic distortion, which is quite acceptable for high quality audio applications.