I just hooked up my computer to my stereo amplifier. I am getting an annoying background hum.
I do not get the same hum when I use the computer speaker with woofer.
What is the fix?

SUNDOG

A 3 prong to 2 prong adapter for the AC plug at the wall. I had the same issue recently.

it's called a "ground loop".

Sunny, welcome. It's quite possible that this is a ground loop, as the other replies indicated. If whatever the "computer speaker with woofer" is, which isn't entirely clear, wasn't plugged into the power separately from the computer, then no ground loop was possible when using just it. Connecting the amplifier and computer with a cable may have formed a loop between different electrical potentials where they were plugged in, causing a current to flow.

Plugging the amplifier and computer into the same outlet(if they aren't now)may help, as would the plug adaptor which was mentioned. It's possible if this isn't a ground loop that the headphone output is simply noisy and shouldn't be used.