You're in luck, I recently did the same thing. The best info and diagram I was able to find is here http://www.epanorama.net/circuits/speaker_to_line.html
This is exactly what you will find inside a lot of subwoofer plate amps that accept speaker level input. Its also exactly what's inside a lot of the many line out converters you can buy. That's the easiest by far, buy a LOC. But they look tacky and use really cheap connectors.
So what I did instead was modify my Melody I880 to use one set of input RCAs as a line out. First check to make sure your amp runs everything to common ground. You'll be able to see that but its nice to confirm with a meter. So leave all existing grounds in place.
Unsolder the existing input wires from the RCA. Solder the 1k resistors across the RCA line out. Solder the 10k resistors to the RCA positive. Solder each 10k to the positive of each speaker out.
This last step might be the hardest. The speaker terminals and wire tend to be pretty beefy, making it hard to get enough heat into the part for a good solder joint.
This method works great. Costs about a buck fifty. Looks stock. Sounds great. And can be put back to stock in a few minutes. Not to mention, mod, way cool!
You can also find this same thing written up here https://forum.audiogon.com/discussions/a-diy-swarm-based-distributed-bass-array-for-sota-bass
This is exactly what you will find inside a lot of subwoofer plate amps that accept speaker level input. Its also exactly what's inside a lot of the many line out converters you can buy. That's the easiest by far, buy a LOC. But they look tacky and use really cheap connectors.
So what I did instead was modify my Melody I880 to use one set of input RCAs as a line out. First check to make sure your amp runs everything to common ground. You'll be able to see that but its nice to confirm with a meter. So leave all existing grounds in place.
Unsolder the existing input wires from the RCA. Solder the 1k resistors across the RCA line out. Solder the 10k resistors to the RCA positive. Solder each 10k to the positive of each speaker out.
This last step might be the hardest. The speaker terminals and wire tend to be pretty beefy, making it hard to get enough heat into the part for a good solder joint.
This method works great. Costs about a buck fifty. Looks stock. Sounds great. And can be put back to stock in a few minutes. Not to mention, mod, way cool!
You can also find this same thing written up here https://forum.audiogon.com/discussions/a-diy-swarm-based-distributed-bass-array-for-sota-bass