if i make an amplifier with an op amp, and just want to use a battery or single power supply, i can connect the negative supply to earth. Does that make the output have a buzzing sound when a speaker is connected? also if i want to make an amp using an opamp with one power supply, but want to power it with + and - 12V is there a schematic i can use to make it? Cheers.
An opamp by itself doesn't have enough output current to drive a speaker. It needs to drive a complimentary pair of output transistors to boost its output current. The amplifier will have a DC output voltage if it has a single supply and will need an output coupling capacitor to block the DC from reaching the speaker.
Two of these amplifiers can be bridged for up to nearly 4 times more power to a speaker, then an output coupling capacitor is not needed. Many IC amplifiers are made for cars and are bridged. A TDA2005 is a mono bridged amplifier IC and a TDA1554 is a stereo bridged amplifier IC. They produce about 14W RMS at clipping per channel into a 4 ohm speaker with a 13.2V supply. They produce more power (20W) when driven into severe high distortion and a 14.4V supply.
Speaker buzzing? Maybe the amplifier is clipping because its volume control is set too high.
Any opamp will operate fine from a single supply voltage if its inputs are biased correctly. Like this: