How to reduce noise in elementary audio circuit

I wish to hook up the line-out from a digital piano to a surplus 4 ohm 15w speaker unit. I bought a simple Amplifier module and connected it up. My plan was to roughly assemble the circuit and, if it worked ok, transfer to strip-board with RCA connector for Audio and a DC power socket and some sort of quick connector for speaker cable. When I play a note on the piano I get a good level of sound from the speaker but the quality is very poor. I guess I need to use better wiring on the input side of the amp? Circuit: Here's a photo: Battery: 12V SLA 3.3Ah Speaker: 4 Ohm 15W Amplifier: TI amp TAS2552EVM 4.5-12V, 3.5W "Music power", for 4-15 ohm speaker, 40Hz-20kHz, Source: Line-out 1/4" TR connector (left channel, set to mono). Pot: 10K Linear The speaker is a rear satellite speaker unit from an old TV that had 5.1 surround-sound. The speaker enclosure contains two sounders (approx 5" and 1"). The digital piano is powered through a mains adapter (100-240 VAC 0.2A 50-60Hz) connected using a USB cable (5V 1A). It is light so assume switched mode. Notes sound rather buzzy. There is mains hum in the background. What steps should I do to improve the quality of the sound?You are the best.

Get a real amplifier. A car stereo produces 17.5 watts per channel. Your amplifier produces 3.5 watts. Get a Car stereo with an aux input jack. Run the piano into the aux jack.

An amplifier with Automatic level control may help you out.