If the housing gets very hot, the aluminum block that connects the TPA3255 to the housing does not need to be any larger, as it then performs its task perfectly as a heat exchanger.
The actual housing is then too small for heat dissipation and you have to resort to additional means.
Either activated with a fan or passive with a heat sink.
As I don't like active cooling in hi-fi devices, I opted for a massive heat sink for the O-Noorus D1, which is also used in power amplifiers.
There are also cheaper solutions on AE, for example.
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