Low hum from left speaker.

I recently purchased a new turntable, upon discovering a phono preamp or dedicated channel was necessary I cleaned my dad's old Pioneer SX450. I connected the right and the left side aux cable and ground to the ground post on the stereo. When I turn the stereo to the phono channel a very low hum is audible and there's no other sound. When the stereo is turned to the other channels they work fine and dandy. The turntable is a Pro-ject Debut III Carbon. Thanks for the time and consideration.