I'm using the schematic at the FAQ section of runoffgroove.com. Sorry I can't give you direct link, I'm replying from my cellphone. It's very simple, really. Connect a 10 ohms resistor from jack's ground to speaker out negative.But instead of 10 ohms, I use 220 ohms/2w because it's still too loud for me. Works for me, but I don't know if it's technically correct.Use plastic socket. It needs to be floating. Grounding it will make VERY loud hum.You can wire it so when you insert headphone jack, it will break the connection with the speaker.Don't expect too high. This is the simplest way. The sound will be a bit dull. A cab sim will do better.Hopefully this helps.
