This is a common problem I had with many keyboards, and I think it's a standard, somehow: when connecting the headphones to a piano, the internal speakers are disabled. And that's cool!
However, things get a little bit tricky when there is a stage setup. Let's take for instance Yamaha YDP 143. It's a digital piano which has USB output (for MIDI and audio data) and two outputs for headphones.
We want the piano speakers to be used always, even when a jack enters in the headphones output. A cable is used to connect the piano to the main interface, currently sending the audio data via the headphones output.
Is there a way to use both the headphones output and internal speakers in a stage setup when using a digital piano which doesn't have a specific "AUX OUT" output?
Taking this specific case (Yamaha YDP 143), is there any hope that we can use the USB output and some other device to stream the audio data from the USB cable (which should be shorter than 3 meters) through another long cable going to the main interface?
I believe this is a common issue, but I can't seem to find any solutions.
This is the relevant page from the manual:
Obviously, one option would be to hack into the piano and add yet another jack output coming directly from the internal speakers' cables. But I don't feel that's going to be a good idea since the warranty would be lost.