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Low Noise Transformer Input Pre-Amp Design - A Solution that Eliminates CMID

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This paper examines more closely the advantages of input transformer-op amp configurations, especially those implemented in low-noise designs. Usual transformer input stage topology works in non-inverting architecture, since it allows the transformer to work with optimum loading, to maximize the signal-to-noise ratio. However, this configuration is subject to Common-Mode voltage Induced Distortion - CMID. The susceptibility is further increased if the amplifier source impedances is not perfectly matched. This is illustrated by tests on popular audio op amps. Advanced transformer input stage topology proposed in this work completely prevents this kind of distortion. Noise performance remains unaffected, yet listening tests in practical application confirm the sound to be more pleasant.

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