Switch Mode Power Supply circuits can greatly reduce the size, weight and cost of high current, high voltage supply rails in audio power amplifiers. But since many switching regulators are made to function in the 20 to 50kHz region, they have the possibility of putting switching noise, harmonics and/or intermodulation products into the audio frequency signal path. To examine this hypothesis, a Switch Mode Power was constructed using a popular control IC and applied to an amplifier of known quality. Measurements on the output line of the amplifier revealed only a small number of harmonics, only minimally related to the switching frequency of the power suppply. These were approximately 70dB below a 1 watt reference level, and well outside the audio frequency range.
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https://aes2.org/publications/elibrary-page/?id=11862
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