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I hooked up my UcD to an oscilloscope and I see some HF sine wave at about 400 kHz. Is this normal?
Yes. The 400kHz signal you see at the output of the amplifier is the residue of the switching nature of class D amplifiers. This is completely normal and does no harm. For more information please refer to our EMI application note.
https://www.diyclassd.com/FAQ-Hypex/Technical-support/I-hooked-up-my-UcD-to-an-oscilloscope-and-I-see-some-HF-sine-wave-at-about-400-kHz.-Is-this-normal