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My Hypex amplifiers gets very warm, is this normal?

Class D is “cool” in terms of how much power gets dissipated. If one were to heat-sink a class D amplifier like an equivalent class AB amp, they would remain utterly stone cold. The idea of course is to make the amplifiers more compact by reducing the heatsink. An idling UcD™ amplifier draws up to 2% of rated power. That’s 8W for a UcD400 amplifier. Whether that translates into a small or large temperature rise depends on the size of the heatsink. Please refer to the "Thermal design" application note for more information.