The answer to this seems really obvious to me. I'll try to be brief in answering.

The source end is where the signal comes from. It's not the preamp/processor and it's not NOT the preamp/processor. The exact component doesn't create the definition. The signal flow creates the definition.

If you have a BD player connected to a preamp/processor connected to a power amp, the preamp/processor is the source end of the connection of the power amp. The preamp/processor is the... let's say 'sink' end of the connection of the BD Player and the preamp/processor. Many people communicate source and sink by saying the things are connected FROM the source TO the sink.

This is not so simple with two-way signals, since signals go both ways. But even with a BD Player, with its two-way HDMI connection, the player is the source. And if a signal always goes one way on its wire, though there may be two wires (such as RS232), each has a source end and a sink end.


Sometimes we see people talk about running a connection from speakers to an amp, or from a preamp/processor to a BD Player. This seems to reflect fuzzy thinking because the direction of signal flow is such an aid in understanding (and troubleshooting!!) the systems we work with. Hell, that's the reason most schematics have the sources on the left and the sinks on the right!