I bought the K272X recently and just built it. When connected to my phone and headphones (I was using some Grado SR80s and also some Hifiman RE400 IEMs for testing), it is producing sound but it is incredibly quiet - barely audible at full volume on my phone, which would normally be deafening even without an amp.
Check for soldering bridges specially around the op-amp.It is a simple circuit, if you own a multimeter you should be able to do a continuity test in no time.
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