Line-Out jack full mono?

QUESTION: Is there a way to set up my line out/headphones jack so that if I plugged in a 3.5mm to RCA audio adapter then it will output all of the information to both the left and right jack on this adapter?


BACKGROUND: I'm doing some DJing and I typically use my laptop as a music source. I have an old sound system (Peavey sub woofer and satellites from the 80s) and it is designed to take only "mono" input (one channel speaker wire). However, some of the music I use is stereo dependent (I mean it uses left and right channels separately) and my ADCOM 200W 2-channel amplifier uses 2 channels for input. My current setup involves an RCA merger adapter which combines the left and right channel (output from the laptop) into one RCA that I plug into one channel of the amp, and then output to the speakers.

This setup only allows me to use one of the amp's channels, so I am losing out on 200W of potential power and I am really taxing one of the channels at a time. My goal is to use both channels of the amp, one for sub and one split for the satellites, and to have the same (full) information going to both channels on the amp.

Sorry that this is very convoluted, I am just getting my feet wet with audio systems but I am interested and interested in learning about this stuff. Also if possible I'd like to avoid any options that involve me buying a new sound card for my laptop.....

Thanks in advance!

Frank