This may be a long standing bug that has been reported before. The jack authors seem to see it as a 'feature'. The summing of signals connected to an input port is done when that port's owner calls jack_port_get_buffer(). There is an optimisation: when there is only one output connected, you just get a copy of its buffer pointer, the sample data is not copied to separate input buffer. Now if (1) you make a loopback AND (2) your app clears its output buffers at the start of the cycle AND (3) it does this before calling jack_port_get_buffer() on its inputs, then the looped back output will appear to be empty. A workaround would be to ensure condition (3) is not true, i.e. call jack_port_get_buffer() on inputs before clearing your ouputs. But even that won't work if there is just one output connected to the loopback input: the optimisation means its data is not copied and you will see an empy buffer anyway. IMHO Jack should make a copy of an output buffer if it is looped back even if that would otherwise not be necessary. Not doing this violates assumptions that are (IMHO) very legitimately made by all authors: either that (1, strong form) input buffers do not change during a process callback, or (2, weaker form) they do not change after the call to jack_port_get_buffer(). Ciao, -- FA